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Models of the Contribution of Blazars to the Anisotropy of the Extragalactic Diffuse Gamma-ray Background
Journal of Cosmology and Astroparticle Physics
Abstract
We study the relation between the measured anisotropies in the extragalactic diffuse gamma-ray background (DGRB) and the DGRB spectral intensity, and their potential origin from the unresolved blazar population.
Using a physical-evolution model for blazars with a luminosity dependent density evolution (LDDE) and an observationally-determined luminosity-dependent blazar spectral energy distribution (SED), we find that blazars can account for the observed anisotropy of the DGRB consistent with their observed source-count distribution, but are in turn constrained in contributing significantly to the observed DGRB intensity.
For the best-fit LDDE model accounting for the DGRB anisotropy and source-count distribution, blazars only contribute 5.9+2.1−1.0% (68% CL) of the DGRB intensity above 1 GeV.
Requiring a higher fraction of the DGRB intensity contribution by blazars overproduces the DGRB anisotropy, and therefore blazars in the LDDE+SED-sequence model cannot simultaneously account for the DGRB intensity as well as anisotropy.
We discuss the limitations of LDDE models.
However, these models do not require the many unjustified and observationally-inconsistent simplifying assumptions — including a single power law for all blazar spectra and a simple broken power-law model for their source-count
distribution — that are present in much previous work.
... The first detection of the IGRB anisotropy APS was reported by the Fermi LAT Collaboration in 2012, in the energy range between 1 and 50 GeV [33]. The signal was compatible with being due entirely to unresolved blazars [9,10,12]. ...
... Different populations of gamma-ray emitters are expected to induce different levels of anisotropies in the IGRB (see Refs. [5][6][7][8][9][10][11][12] among others). Thus, a measurement of the gamma-ray angular power spectrum Our analysis will also quantify the impact of the new APS measurement in constraining the properties of the blazar population, e.g., its redshift evolution and distribution in luminosity (especially at low luminosities). ...
... A positive answer would confirm the result of Refs. [9,10] (based on the original 2012 APS data [33]), according to which the IGRB APS is compatible with being due entirely to blazars. On the other hand, a negative answer would corroborate the phenomenological analysis performed in Ref. [34], establishing the need for more than one component to interpret IGRB anisotropies. ...
Recently, a new measurement of the auto- and cross-correlation angular power spectrum (APS) of the isotropic gamma-ray background was performed, based on 81 months of data of the Fermi Large-Area Telescope (LAT). Here, we fit, for the first time, the new APS data with a model describing the emission of unresolved blazars. These sources are expected to dominate the anisotropy signal. The model we employ in our analysis reproduces well the blazars resolved by Fermi LAT. When considering the APS obtained by masking the sources in the 3FGL catalogue, we find that unresolved blazars under-produce the measured APS below $\sim$1 GeV. Contrary to past results, this suggests the presence of a new contribution to the low-energy APS, with a significance of, at least, 5$\sigma$. The excess can be ascribed to a new class of faint gamma-ray emitters. If we consider the APS obtained by masking the sources in the 2FGL catalogue, there is no under-production of the APS below 1 GeV, but the new source class is still preferred over the blazars-only scenario (with a significance larger than 10$\sigma$). The properties of the new source class and the level of anisotropies induced in the isotropic gamma-ray background are the same, independent of the APS data used. In particular, the new gamma-ray emitters must have a soft energy spectrum, with a spectral index ranging, approximately, from 2.7 to 3.2. This complicates their interpretation in terms of known sources, since, normally, star-forming and radio galaxies are observed with a harder spectrum. The new source class identified here is also expected to contribute significantly to the intensity of the isotropic gamma-ray background.
... It is expected to be isotropic on large angular scales but it can still contain anisotropies on small angular scales. Indeed, the contribution to the IGRB from unresolved sources imprints anisotropies in the diffuse emission which can be used to infer the properties of the contributing sources (see Refs. [6][7][8][9][10][11][12][13][14] among others). For example, the detection of a significant angular power in Ref. ...
... [1] determined an upper limit to the contribution of unresolved blazars [10,11,15] to the IGRB. Additional tools to reconstruct the nature of the IGRB are the study of its cross-correlation with catalogs of resolved galaxies [16][17][18][19][20][21], with gravitational lensing cosmic shear [22] and with lensing of the cosmic microwave background radiation [23]. ...
... 10 The fit is performed with Minuit2 v5.34.14, http://lcgapp.cern.ch/project/cls/workpackages/mathlibs/minuit/index.html. 11 If the auto-and cross-APS are interpreted as produced by a population of unresolved sources, they can be expressed in terms of the 3-dimensional power spectrum of the density field associated with the sources of the gamma-ray emission [5,44,45]. The latter determines the dependence on multipole, hence the shape of the auto-and cross-APS. ...
The isotropic gamma-ray background arises from the contribution of unresolved sources, including members of confirmed source classes and proposed gamma-ray emitters such as the radiation induced by dark matter annihilation and decay. Clues about the properties of the contributing sources are imprinted in the anisotropy characteristics of the gamma-ray background. We use 81 months of Pass 7 Reprocessed data from the Fermi Large Area Telescope to perform a measurement of the anisotropy angular power spectrum of the gamma-ray background. We analyze energies between 0.5 and 500 GeV, extending the range considered in the previous measurement based on 22 months of data. We also compute, for the first time, the cross-correlation angular power spectrum between different energy bins. We find that the derived angular spectra are compatible with being Poissonian, i.e. constant in multipole. Moreover, the energy dependence of the anisotropy suggests that the signal is due to two populations of sources, contributing, respectively, below and above 2 GeV. Finally, using data from state-of-the-art numerical simulations to model the dark matter distribution, we constrain the contribution from dark matter annihilation and decay in Galactic and extragalactic structures to the measured anisotropy. These constraints are competitive with those that can be derived from the average intensity of the isotropic gamma-ray background.
... The DGRB is thought to be predominantly of extragalactic origin: gamma-ray sources with a flux smaller than the sensitivity of Fermi LAT are not detected individually, producing instead a cumulative diffuse glow that contributes to the DGRB. Unresolved blazars [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25], misaligned Active Galactic Nuclei (MAGNs) [26,27,28,29], star-forming galaxies (SFGs) [30,31,32,33,34,35] and millisecond pulsars (MSPs) [36,37,38] are guaranteed components to the DGRB. More uncertain source classes, e.g. ...
... [13,105,52] also consider a luminosity-dependent density evolution for the X-ray LF but they assume a common power-law energy spectrum at gamma-ray frequencies, instead of the SED predicted by the blazar sequence. In Ref. [19] the formalism outlined before is fitted to both the blazar dN/dS computed from the 1FGL [106] and to the first Fermi LAT measurement of the DGRB in Ref. [8] (see also Ref. [107]). The result of the fit proves that it is possible to explain both observables with one single population of blazars. ...
... It is, thus, also independent on the complex issue of halo finding and on the uncertainties associated with it [339]. 19 This is, indeed, what the authors of Ref. [262] did, guided by the results of the Millennium-I and Millennium-II simulations [282,341]. Yet, as in the Halo Model approach, current N -body simulations do not reach the highest k values needed in Eq. (20) and, thus, extrapolations beyond the simulation resolution are again required. ...
We review the current understanding of the diffuse gamma-ray background
(DGRB). The DGRB is what remains of the total measured gamma-ray emission after
the subtraction of the resolved sources and of the diffuse Galactic
foregrounds. It is interpreted as the cumulative emission of sources that are
not bright enough to be detected individually. Yet, its exact composition
remains unveiled. Well-established astrophysical source populations (e.g.
blazars, misaligned AGNs, star-forming galaxies and millisecond pulsars) all
represent guaranteed contributors to the DGRB. More exotic scenarios, such as
dark matter annihilation or decay, may contribute as well. In this review, we
describe how these components have been modeled in the literature and how the
DGRB can be used to provide valuable information on each of them. We summarize
the observational information currently available on the DGRB, paying
particular attention to the most recent measurement of its intensity energy
spectrum by the Fermi LAT Collaboration. We also discuss the novel analyses of
the auto-correlation angular power spectrum of the DGRB and of its
cross-correlation with tracers of the large-scale structure of the Universe.
New data sets already (or soon) available are expected to provide further
insight on the nature of this emission. By summarizing where we stand on the
current knowledge of the DGRB, this review is intended both as a useful
reference for those interested in the topic and as a means to trigger new ideas
for further research.
... The information available from the anisotropy measured in [2] has been used, for example, in [30] together with the source count distribution of blazars to show that they contribute only by about 20-30% to the IGRB intensity, confirming with the anisotropy the result found via the source counts alone [31]. Anisotropy from blazars has been further studied in [32,33]. It has also been used to constrain the contribution of MSPs to the IGRB [12] showing that stronger constraints are obtained with respect to the case when intensity alone is used. ...
... We adopt the PLE 3 model from Tab. 2 of [6]. We note that other blazar models have been explored recently in [32] and [33], but they have been tuned taking into account the observed anisotropy so we will not consider them here. The model derived in [32] adopts an LDDE γ-ray luminosity function in order to fully explain only with blazars the anisotropy data [2]. ...
... We note that other blazar models have been explored recently in [32] and [33], but they have been tuned taking into account the observed anisotropy so we will not consider them here. The model derived in [32] adopts an LDDE γ-ray luminosity function in order to fully explain only with blazars the anisotropy data [2]. On the other hand [33] studied a new γ-ray emission model associated with plasma instabilities of the ultra-relativistic pair distribution which dissipate the kinetic energy of the TeV pairs and heat the intergalactic medium. ...
Radio-loud active galactic nuclei (AGN) are expected to contribute
substantially to both the intensity and anisotropy of the isotropic gamma-ray
background (IGRB). In turn, the measured properties of the IGRB can be used to
constrain the characteristics of proposed contributing source classes. We
consider individual subclasses of radio-loud AGN, including low-,
intermediate-, and high-synchrotron-peaked BL Lacertae objects, flat-spectrum
radio quasars, and misaligned AGN. Using updated models of the gamma-ray
luminosity functions of these populations, we evaluate the energy-dependent
contribution of each source class to the intensity and anisotropy of the IGRB.
We find that collectively radio-loud AGN can account for the entirety of the
IGRB intensity and anisotropy as measured by the Fermi Large Area Telescope
(LAT). Misaligned AGN provide the bulk of the measured intensity but a
negligible contribution to the anisotropy, while high-synchrotron-peaked BL
Lacertae objects provide the dominant contribution to the anisotropy. In
anticipation of upcoming measurements with the Fermi-LAT and the forthcoming
Cherenkov Telescope Array, we predict the anisotropy in the broader energy
range that will be accessible to future observations.
... We find that the SF source class remains dominant over the entire energy range, though we note that FSRQs are relatively important below a few GeV. Although the average contribution of BL Lacs (from previous studies) and that of mAGN (predicted by our model) is almost identical, anisotropy analyses of BL Lacs have placed significant constraints on their contributions to the IGRB and therefore these sources are not expected to contribute more than about 20% of the diffuse photons observed by Fermi [6,9,62,63]. ...
... However, for our models of electromagnetic cascades, the unresolved point-source contribution from BL Lacs and FSRQs is expected to outshine the diffuse cascade contribution from their on-axis jet emission. Thus, we do not expect that such an analysis would alter the BL Lac and FSRQ fluxes to the point that they contribute significantly to the IGRB [6,62,63]. ...
The total extragalactic γ -ray flux provides a powerful probe into the origin and evolution of the highest energy processes in our universe. An important component of this emission is the isotropic γ -ray background (IGRB), composed of sources that cannot be individually resolved by current experiments. Previous studies have determined that the IGRB can be dominated by either misaligned active galactic nuclei (mAGN) or star-forming galaxies (SFGs). However, these analyses are limited, because they have utilized binary source classifications and examined only one source class at a time. We perform the first combined joint-likelihood analysis that simultaneously correlates the γ -ray luminosity of extragalactic objects with both star-formation and mAGN activity. We find that SFGs produce 48 ⁺³³ -20 % of the total IGRB at 1 GeV and 56 ⁺⁴⁰ -23 % of the total IGRB at 10 GeV. The contribution of mAGN is more uncertain, but can also be significant. Future work to quantify the radio and infrared properties of nearby galaxies could significantly improve these constraints.
... Although the origin of the IGRB is still under debate, a significant fraction may originate from the emission from unresolved astrophysical sources, which are too faint to be detected on an individual basis. Possible candidates include blazars [6,7], star-forming galaxies [8,9], and radio galaxies [10,11]. ...
... To relate the X-ray luminosity to the γ-ray luminosity, we introduce the relation between the bolometric blazar luminosity P and disk X-ray luminosity as P = 10 q L X−ray [49]. The parameter q is set to be 4.21 as follows in Ref. [7]. For star-forming and radio galaxies, constructing the possible relation between M h and L γ is more uncertain since there exists limited data. ...
The extragalactic $\gamma$-ray background (EGB) arises from accumulation $\gamma$-ray emissions from resolved and unresolved extragalactic sources as well as diffuse processes. It is important to understand the origin of the EGB in the context of cosmological structure formation. Known astrophysical $\gamma$-ray sources such as blazars, star-forming galaxies, and radio galaxies are expected to trace the underlying cosmic matter density distribution. We explore the correlation of the EGB from Fermi-LAT data with the large-scale matter density distribution from the Subaru Hyper Suprime-Cam (HSC) SSP survey. We reconstruct an unbiased surface matter density distribution $\kappa$ at $z<1$ by applying weak-gravitational lensing analysis to the first-year HSC data. We then calculate the $\kappa - \gamma$ cross-correlation. Our measurement suggests overall weak correlation at angular scales of 30-60 arcmin. The result is essentially consistent with a null detection, and allows us to derive constraints on the statistical relationships between astrophysical $\gamma$-ray sources and dark matter halos. We make forecasts for the detectability of the cross-correlation in the final HSC data. Detection with a $\sim3\sigma$ confidence can be achieved with the full survey covering 1,400 squared degrees planned for HSC SSP.
... Although it has long been speculated that this emission is likely to originate from a large number of unresolved sources, the nature of those sources has only recently become more apparent. In particular, it is now known that this background receives non-negligible contributions from several classes of sources, including blazars (flat-spectrum radio quasars and BL Lac objects) [5][6][7][8], radio galaxies [9,10], and star-forming galaxies [11,12], along with smaller contributions from galaxy clusters [13], millisecond pulsars [14,15], and propagating ultra-high energy cosmic rays [16,17]. At this time, it appears plausible that a combination of these source classes could account for the observed intensity and spectrum of the diffuse gamma-ray background [18][19][20][21][22][23]. ...
... where L γ is the luminosity measured by Fermi/LAT between 0.1 and 100 GeV and L RC is the core luminosity measured for the radio galaxy at a frequency of 5 GHz, each in units of JCAP08(2016)019 erg/s. 6 We use the values of L RC as tabulated in ref. [33]. 7 The correlation between these two quantities is not perfect, however, and there exists galaxy-to-galaxy scatter in the observed ratios of the radio and gamma-ray emission, distinct from measurement uncertainties. ...
It has been suggested that unresolved radio galaxies and radio quasars (sometimes referred to as misaligned active galactic nuclei) could be responsible for a significant fraction of the observed diffuse gamma-ray background. In this study, we use the latest data from the Fermi Gamma-Ray Space Telescope to characterize the gamma-ray emission from a sample of 51 radio galaxies. In addition to those sources that had previously been detected using Fermi data, we report here the first statistically significant detection of gamma-ray emission from the radio galaxies 3C 212, 3C 411, and B3 0309+411B. Combining this information with the radio fluxes, radio luminosity function, and redshift distribution of this source class, we find that radio galaxies dominate the diffuse gamma-ray background, generating 83.3^{+27.4}_{-10.1}% of this emission at energies above ~1 GeV. We discuss the implications of this result and point out that it provides support for scenarios in which IceCube's high-energy astrophysical neutrinos also originate from the same population of radio galaxies.
... The Fermi LAT has detected many extragalactic sources: among the 1873 sources in the 2FGL catalog, there are 672 blazars (all classified according to the Roma BZ-CAT 15 ), 8 radio galaxies, 3 normal galaxies, 3 starburst galaxies and 2 Seyfert galaxies [2]. 16 The contribution to the IGRB from unresolved members of these extragalactic source classes has been studied over the years, e.g., [69][70][71][72][73][74][75][76][77][78][79][80]. In addition, some classes of Galactic sources, most notably millisecond pulsars, could contribute to the isotropic emission at large scale height in the Milky Way [81]. ...
... Overall, the origin and composition of the IGRB are still open questions. Because of the large number of blazars detected by the LAT, direct population studies are now feasible using gamma rays and there is arguably a guaranteed contribution from the blazar population [69][70][71][72][73]. The minimum contribution below 100 GeV from unresolved blazars has been estimated in ref. [70] to be close to 10%, the best estimate being 22 − 34% in the 0.1-100 GeV range (which agrees well with previous findings, e.g. ...
We search for evidence of dark matter (DM) annihilation in the isotropic
gamma-ray background (IGRB) measured with 50 months of Fermi Large Area
Telescope (LAT) observations. An improved theoretical description of the
cosmological DM annihilation signal, based on two complementary techniques and
assuming generic weakly interacting massive particle (WIMP) properties, renders
more precise predictions compared to previous work. More specifically, we
estimate the cosmologically-induced gamma-ray intensity to have an uncertainty
of a factor ~20 in canonical setups. We consistently include both the Galactic
and extragalactic signals under the same theoretical framework, and study the
impact of the former on the IGRB spectrum derivation. We find no evidence for a
DM signal and we set limits on the DM-induced isotropic gamma-ray signal. Our
limits are competitive for DM particle masses up to tens of TeV and, indeed,
are the strongest limits derived from Fermi LAT data at TeV energies. This is
possible thanks to the new Fermi LAT IGRB measurement, which now extends up to
an energy of 820 GeV. We quantify uncertainties in detail and show the
potential this type of search offers for testing the WIMP paradigm with a
complementary and truly cosmological probe of DM particle signals.
... The Fermi LAT has detected many extragalactic sources: among the 1873 sources in the 2FGL catalog, there are 672 blazars (all classified according to the Roma BZ-CAT 15 ), 8 radio galaxies, 3 normal galaxies, 3 starburst galaxies and 2 Seyfert galaxies [2]. 16 The contribution to the IGRB from unresolved members of these extragalactic source classes has been studied over the years, e.g., [69][70][71][72][73][74][75][76][77][78][79][80]. In addition, some classes of Galactic sources, most notably millisecond pulsars, could contribute to the isotropic emission at large scale height in the Milky Way [81]. ...
... Overall, the origin and composition of the IGRB are still open questions. Because of the large number of blazars detected by the LAT, direct population studies are now feasible using gamma rays and there is arguably a guaranteed contribution from the blazar population [69][70][71][72][73]. The minimum contribution below 100 GeV from unresolved blazars has been estimated in ref. [70] to be close to 10%, the best estimate being 22 − 34% in the 0.1-100 GeV range (which agrees well with previous findings, e.g. ...
We search for evidence of dark matter (DM) annihilation in the isotropic gamma-ray background (IGRB) measured with 50 months of Fermi Large Area Tele- scope (LAT) observations. An improved theoretical description of the cosmological DM annihilation signal, based on two complementary techniques and assuming generic weakly interacting massive particle (WIMP) properties, renders more precise predic- tions compared to previous work. More specifically, we estimate the cosmologically- induced gamma-ray intensity to have an uncertainty of a factor ∼ 20 in canonical setups. We consistently include both the Galactic and extragalactic signals under the same theoretical framework, and study the impact of the former on the IGRB spectrum derivation. We find no evidence for a DM signal and we set limits on the DM-induced isotropic gamma-ray signal. Our limits are competitive for DM particle masses up to tens of TeV and, indeed, are the strongest limits derived from Fermi LAT data at TeV energies. This is possible thanks to the new Fermi LAT IGRB measurement, which now extends up to an energy of 820 GeV. We quantify uncertainties in detail and show the potential this type of search offers for testing the WIMP paradigm with a complementary and truly cosmological probe of DM particle signals.
... Following Ref. [5], we consider flat-spectrum radio quasars and BL Lacertae cumulatively as one unique class of blazars (X = γ BLA ) 3 . The common spectral index α BLA is fixed to 2.2 and the γ-ray luminosity L is computed at 100 MeV, which implies A BLA (χ) = [1 + z(χ)] −α BLA . ...
... Considering one single population of blazars is well motivated when the shape of the energy spectrum is allowed to depend on the luminosity of the source. In that case, the flexibility of model well reproduces the differences between the two subclasses of blazars[5,7,10]. Our approach assumes one common spectral index αBLA and, therefore, is a more simplified description. ...
We recently proposed to cross-correlate the diffuse γ-ray emission with the gravita-tional lensing signal of cosmic shear. This represents a novel and promising strategy to search for annihilating or decaying dark matter (DM) candidates. In the present work, we demon-strate the potential of a tomographic-spectral approach: measuring the cross-correlation in separate bins of redshift and energy significantly improves the sensitivity to a DM signal. Indeed, the power of the proposed technique stems from the capability of simultaneously exploiting the different redshift scaling of astrophysical and DM components, their different energy spectra and their different angular shapes. The sensitivity to a particle DM signal is extremely promising even in the case the γ-ray emission induced by DM is a subdominant component in the isotropic γ-ray background. We quantify the prospects of detecting DM by cross-correlating the γ-ray emission from the Fermi large area telescope (LAT) with the cosmic shear measured by the Dark Energy Survey, using data sets that will be available in the near future. Under the hypothesis of a significant (but realistic) subhalo boost, such a measurement can deliver a 5σ detection of DM, if the DM particle has a mass lighter than 300 GeV and thermal annihilation rate. Data from the European Space Agency Euclid satellite (launch planned for 2020) will be even more informative: if used to reconstruct the properties of the DM particle, the cross-correlation of Euclid and Fermi-LAT will allow for a measurement of the DM mass within a factor of 1.5–2, even for moderate subhalo boosts, assuming the DM mass around 100 GeV and a thermal annihilation rate.
... However, the modeling of the sources' faint end distributions is non-trivial, and estimates of the contribution to the EGB from unresolved blazars range from ∼15 per cent to ∼100 per cent [e.g., [5][6][7]. On the other hand, the intrinsic spectral and flux properties of blazars constructed by Fermi LAT data, as well as the auto-correlation of EGB anisotropies [8], suggest that unresolved blazars can only contribute up to ∼20 per cent of EGB [e.g., [9][10][11][12]. Similarly, the contribution from SFGs and radio galaxies to the EGB can be significant but is subject to large uncertainties [13,14]. ...
... We note that our adopted model of blazars is different from one in the previous work of Ref. [21]. Our model reproduces the observed flux counts of resolved blazars, whereas the model in Ref. [21] is aimed at reproducing the flux counts as well as the anisotropy of the EGB [11]. The main difference lies in the faint slope of the gamma-ray luminosity function. ...
We present the first measurement of the cross-correlation of weak
gravitational lensing and the extragalactic gamma-ray background emission using
data from the Canada-France-Hawaii Lensing Survey and the Fermi Large Area
Telescope. The cross-correlation is a powerful probe of signatures of dark
matter annihilation, because both cosmic shear and gamma-ray emission originate
directly from the same DM distribution in the universe, and it can be used to
derive constraints on dark matter annihilation cross-section. We show that the
measured lensing-gamma correlation is consistent with a null signal. Comparing
the result to theoretical predictions, we exclude dark matter annihilation
cross sections of =10^{-24}-10^{-25} cm^3 s^-1 for a 100 GeV dark
matter. If dark matter halos exist down to the mass scale of 10^-6 M_sun, we
are able to place constraints on the thermal cross sections ~ 3 x
10^{-26} cm^3 s^-1 for a 10 GeV dark matter annihilation into tau^{+} tau^{-}.
Future gravitational lensing surveys will increase sensitivity to probe
annihilation cross sections of ~ 3 x 10^{-26} cm^3 s^-1 even for a
100 GeV dark matter. Detailed modeling of the contributions from astrophysical
sources to the cross correlation signal could further improve the constraints
by ~ 40-70 %.
... Among these, blazars are the most responsible candidates for such background fluctuations [202]. At high-latitude, blazars are the most numerous point sources and thus they are assumed to be the prime source of anisotropy in extragalactic gamma-ray sky [208][209][210][211][212][213]. A non-negligible amount of γ-ray emission can also arise from the star-forming and the radio galaxies. ...
With the growing interest in indirect detection for dark matter signature, the thesis aims to investigate the signal originating from the self-annihilation of dark matter candidates. The methods for targeting the dark matter signal are two-fold, on one hand, we explore the gamma rays resulting from dark matter particles. On the other hand, we focus on complementary radio properties.
... Distinguishing statistically isotropic sources -including essentially all extragalactic sources -E-mail: ericjbax@gmail.com is more challenging, but could be accomplished using statistics such as the power spectrum (e.g. Harding & Abazajian 2012 ) and the cross-correlation with some external catalog (e.g. Xia et al. 2011 ;Baxter et al. 2022 ). ...
Many sources contribute to the diffuse gamma-ray background (DGRB), including star forming galaxies, active galactic nuclei, and cosmic ray interactions in the Milky Way. Exotic sources, such as dark matter annihilation, may also make some contribution. The photon counts-in-pixels distribution is a powerful tool for analyzing the DGRB and determining the relative contributions of different sources. However, including photon energy information in a likelihood analysis of the counts-in-pixels distribution quickly becomes computationally intractable as the number of source types and energy bins increase. Here, we apply the likelihood-free method of Approximate Bayesian Computation (ABC) to the problem. We consider a mock analysis that includes contributions from dark matter annihilation in galactic subhalos as well as astrophysical backgrounds. We show that our results using ABC are consistent with the exact likelihood when energy information is discarded, and that significantly tighter parameter constraints can be obtained with ABC when energy information is included. ABC presents a powerful tool for analyzing the DGRB and understanding its varied origins.
... where ζ z is a cosmological evolution factor defined in equation F22 of Appendix F and L x the X-ray luminosity of the AGN sources. The prescribed photon fields in our framework are related to L x such that L bol /L x = 10 4.21 where the constant of proportionality is obtained by modeling the γ-ray luminosity function through the observed X-ray luminosity (see, e.g., Ueda et al. 2003;Inoue & Totani 2009;Harding & Abazajian 2012). This is broadly consistent with the more recent results on the blazar luminosity function (Ajello et al. 2012(Ajello et al. , 2014. ...
We present a bottom-up calculation of the flux of ultra-high energy cosmic rays (UHECRs) and high-energy neutrinos produced by powerful jets of active galactic nuclei (AGNs). By propagating test particles in 3D relativistic magnetohydrodynamic jet simulations, including a Monte Carlo treatment of sub-grid pitch-angle scattering and attenuation losses due to realistic photon fields, we study the spectrum and composition of the accelerated UHECRs and estimate the amount of neutrinos produced in such sources. We find that UHECRs may not be significantly affected by photodisintegration in AGN jets where the espresso mechanism efficiently accelerated particles, consistent with Auger's results that favor a heavy composition at the highest energies. Moreover, we present estimates and upper bounds for the flux of high-energy neutrinos expected from AGN jets. In particular, we find that: i) source neutrinos may account for a sizable fraction, or even dominate, the expected flux of cosmogenic neutrinos; ii) neutrinos from the beta-decay of secondary neutrons produced in nucleus photodisintegration could in principle contribute to the PeV neutrino flux observed by IceCube, but can hardly account for all of it; iii) UHECRs accelerated via the espresso mechanism lead to nearly isotropic neutrino emission, which suggests that nearby radio galaxies may be more promising as potential sources. We discuss our results in the light of multimessenger astronomy and current/future neutrino experiments.
... Previous works on the isotropic γ-ray emission (see, e.g., Harding & Abazajian 2012;Singal et al. 2012;Ajello et al. 2015) also found that blazars cannot account for all of the extragalactic isotropic γ-ray emission, although none of them used a method similar to the method adopted here. In Ackermann et al. (2015), the authors found that roughly half of the total extragalactic γ-ray emission at 100 GeV come from blazars which, in our work, would be equivalent to measuring the total number of photons in both ROIs (before masking) that are originated by blazars. ...
Astrophysical sources of very high energy (VHE; >100 GeV) γ- rays are rare, since GeV and TeV photons can be only emitted in extreme circumstances involving interactions of relativistic particles with local radiation and magnetic fields. In the context of the Fermi Large Area Telescope (LAT), only a few sources are known to be VHE emitters, where the largest fraction belongs to the rarest class of active galactic nuclei: the blazars. In this work, we explore Fermi-LAT data for energies >100 GeV and Galactic latitudes b > ∣50°∣ in order to probe the origin of the extragalactic isotropic γ -ray emission. Since the production of such VHE photons requires very specific astrophysical conditions, we would expect that the majority of the VHE photons from the isotropic γ -ray emission originate from blazars or other extreme objects like star-forming galaxies, γ -ray bursts, and radio galaxies, and that the detection of a single VHE photon at the adopted Galactic latitudes would be enough to unambiguously trace the presence of such a counterpart. Our results suggest that blazars are, by far, the dominant class of sources above 100 GeV, although they account for only 22.8 − 4.1 + 4.5 % of the extragalactic VHE photons. The remaining 77 − 4.5 + 4.1 % of the VHE photons still have an unknown origin.
... For instance, a significant part of the anisotropic gamma-ray flux comes from regions near the galactic plane. Distinguishing statistically isotropic sources -including essentially all extragalactic sourcesis more challenging, but could be accomplished using statistics such as the power spectrum (e.g Harding & Abazajian 2012) and the cross-correlation with some external catalog (e.g. Xia et al. 2011). ...
Many sources contribute to the diffuse gamma-ray background (DGRB), including star forming galaxies, active galactic nuclei, and cosmic ray interactions in the Milky Way. Exotic sources, such as dark matter annihilation, may also make some contribution. The photon counts-in-pixels distribution is a powerful tool for analyzing the DGRB and determining the relative contributions of different sources. However, including photon energy information in a likelihood analysis of the counts-in-pixels distribution quickly becomes computationally intractable as the number of source types and energy bins increase. Here, we apply the likelihood-free method of Approximate Bayesian Computation (ABC) to the problem. We consider a mock analysis that includes contributions from dark matter annihilation in galactic subhalos as well as astrophysical backgrounds. We show that our results using ABC are consistent with the exact likelihood when energy information is discarded, and that significantly tighter parameter constraints can be obtained with ABC when energy information is included. ABC presents a powerful tool for analyzing the DGRB and understanding its varied origins.
... Star-forming and radio galaxies can also provide a non-negligible contribution in γ-ray sky. But at the highlatitude gamma-ray sky, blazars are the most numerous point sources and they are thought to be the main source of anisotropy in the extragalactic gamma-ray background [87][88][89][90][91][92]. ...
Tucana-II (Tuc-II), a recently discovered and confirmed Ultra Faint Dwarf Spheroidal galaxy, has a high mass to light ratio as well as a large line-of-sight stellar velocity dispersion, thus making it an ideal candidate for an indirect dark matter (DM) search. In this paper, we have analyzed nine years of γ-ray data obtained from the Fermi-LAT instrument from the direction of Tuc-II. The fact that a very weak significant γ-ray excess (2.2σ) over the background of Tuc-II have been detected from the location of this galaxy. We have observed that this excess of γ-ray emission from the of location Tuc-II rises with longer periods of data. If WIMP pair annihilation is assumed for this faint emission, for b ¯ b annihilation channel the test statistics (TS) value peaks at DM mass ∼ 14 GeV and for τ + τ − annihilation channel it peaks at DM mass 4 GeV. It is then called for an estimation of the 95% confidence level upper limit of the possible velocity weighted self-annihilation cross-section of the DM particles (WIMPs) within Tuc-II by fitting the observed γ-ray flux with spectra expected for DM annihilation. The estimated upper limits of the cross-sections from Tuc-II are then compared with two other dwarf galaxies that are considered to be good DM candidates in several studies. We have also compared our results with the cross-sections obtained in various popular theoretical models of the WIMPs to find that our results impose reasonable tight constraints on the parameter spaces of those DM models. In the concluding section, we compared our results with the similar results obtained from a combined dSph analysis by the Fermi-LAT collaboration as well as the results obtained from the studies of DM in the dwarf galaxies by the major ground-based Cherenkov experiments.
... More recent studies have shown that the combination of these source classes dominates the observed IGRB [21,22]. In contrast, a smaller fraction of this emission originates from blazars [53][54][55][56][57], 1 along with contributions from other source classes, such as merging galaxy clusters [58][59][60]. Truly diffuse processes, such as cascades generated in the propagation of ultra-high energy cosmic rays [61,62] or cosmic-ray interactions with circum-galactic gas [63] are also thought to contribute at a modest level. ...
If the dark matter is unstable, the decay of these particles throughout the universe and in the halo of the Milky Way could contribute significantly to the isotropic gamma-ray background (IGRB) as measured by Fermi. In this article, we calculate the high-latitude gamma-ray flux resulting from dark matter decay for a wide range of channels and masses, including all contributions from inverse Compton scattering and accounting for the production and full evolution of cosmological electromagnetic cascades. We also make use of recent multi-wavelength analyses that constrain the astrophysical contributions to the IGRB, enabling us to more strongly restrict the presence any component arising from decaying dark matter. Over a wide range of decay channels and masses (from GeV to EeV and above), we derive stringent lower limits on the dark matter's lifetime, generally in the range of τ ∼ (1-5)× 10 ²⁸ s.}.
... It has been shown that Fermi's isotropic gamma-ray background (IGRB) [62] is dominated at high-energies by emission from unresolved, non-blazar AGN [63] (see also refs. [64,65]), along with smaller but non-negligible contributions from star-forming galaxies [66][67][68] and blazars [69][70][71][72][73] (possibly among other sources, including galaxy clusters [74], millisecond pulsars [75,76], and propagating ultra-high energy cosmic rays [77,78]). 3 More quantitatively, ref. [63] concludes that non-blazar AGN account for 83.3 +27.4 −10.1 % of the photons above 1 GeV that make up the IGRB (see also refs. ...
The excess of neutrino candidate events detected by IceCube from the direction of TXS 0506+056 has generated a great deal of interest in blazars as sources of high-energy neutrinos. In this study, we analyze the publicly available portion of the IceCube dataset, performing searches for neutrino point sources in spatial coincidence with the blazars and other active galactic nuclei contained in the Fermi 3LAC and the Roma BZCAT catalogs, as well as in spatial and temporal coincidence with flaring sources identified in the Fermi Collaboration's All-Sky Variability Analysis (FAVA). We find no evidence that blazars generate a significant flux of high-energy neutrinos, and conclude that no more than 5-15% of the diffuse flux measured by IceCube can originate from this class of objects. While we cannot rule out the possibility that TXS 0506+056 has at times generated significant neutrino emission, we find that such behavior cannot be common among blazars, requiring TXS 0506+056 to be a rather extreme outlier and not representative of the overall blazar population. The bulk of the diffuse high-energy neutrino flux must instead be generated by a significantly larger population of less-luminous sources, such as non-blazar active galactic nuclei.
... More recent studies have shown that the combination of these source classes dominates the observed IGRB [21,22]. In contrast, a smaller fraction of this emission originates from blazars [53][54][55][56][57], 1 along with contributions from other source classes, such as merging galaxy clusters [58][59][60]. Truly diffuse processes, such as cascades generated in the propagation of ultra-high energy cosmic rays [61,62] or cosmic-ray interactions with circum-galactic gas [63] are also thought to contribute at a modest level. ...
If the dark matter is unstable, the decay of these particles throughout the universe and in the halo of the Milky Way could contribute significantly to the isotropic gamma-ray background (IGRB) as measured by Fermi. In this article, we calculate the high-latitude gamma-ray flux resulting from dark matter decay for a wide range of channels and masses, including all contributions from inverse Compton scattering and accounting for the production and full evolution of cosmological electromagnetic cascades. We also make use of recent multi-wavelength analyses that constrain the astrophysical contributions to the IGRB, enabling us to more strongly restrict the presence any component arising from decaying dark matter. Over a wide range of decay channels and masses (from GeV to EeV and above), we derive stringent lower limits on the dark matter's lifetime, generally in the range of $\tau \sim (1-5)\times 10^{28}$ s.
... It has been shown that Fermi's isotropic gamma-ray background (IGRB) [57] is dominated at high-energies by emission from unresolved, non-blazar AGN [58] (see also Refs. [59,60]), along with smaller but non-negligible contributions from star-forming galaxies [61][62][63] and blazars [64][65][66][67][68] (possibly among other sources, including galaxy clusters [69], millisecond Figure 10. As in Fig. 4, but for non-blazar AGN. ...
The excess of neutrino candidate events detected by IceCube from the direction of TXS 0506+056 has generated a great deal of interest in blazars as sources of high-energy neutrinos. In this study, we analyze the publicly available portion of the IceCube dataset, performing searches for neutrino point sources in spatial coincidence with the blazars and other active galactic nuclei contained in the Fermi 3LAC and the Roma BZCAT catalogs, as well as in spatial and temporal coincidence with flaring sources identified in the Fermi Collaboration's All-Sky Variability Analysis (FAVA). We find no evidence that blazars generate a significant flux of high-energy neutrinos, and conclude that no more than 5-15% of the diffuse flux measured by IceCube can originate from this class of objects. While we cannot rule out the possibility that TXS 0506+056 has at times generated significant neutrino emission, we find that such behavior cannot be common among blazars, requiring TXS 0506+056 to be a rather extreme outlier and not representative of the overall blazar population. The bulk of the diffuse high-energy neutrino flux must instead be generated by a significantly larger population of less-luminous sources, such as non-blazar active galactic nuclei.
... Although the detailed origin of this emission is still being debated, there is considerable empirical support for a scenario in which both non-blazar active galaxies [63] and star-forming galaxies [64] (see also Refs. [65][66][67][68]) provide the largest contributions, along with smaller but potentially non-negligible contributions from blazars [67,[69][70][71][72], galaxy clusters [73], propagating ultra-high energy cosmic rays [74,75], and perhaps even annihilating dark matter particles [76][77][78][79]. More quantitatively, Ref. [63] concludes that unresolved non-blazar active galaxies account for no less than 59% of the IGRB photons above 1 GeV, while Ref. [64] finds that starforming galaxies are responsible for at least 24% of the IGRB intensity above 1 GeV (each at the 2σ confidence level). ...
Due to shielding, direct detection experiments are in some cases insensitive to dark matter candidates with very large scattering cross sections with nucleons. In this paper, we revisit this class of models, and derive a simple analytic criterion for conservative but robust direct detection limits. While large spin-independent cross sections seem to be ruled out, we identify potentially viable parameter space for dark matter with a spin-dependent cross section with nucleons in the range of $10^{-27} {\rm cm}^2 < \sigma_{{\rm DM}-p} < 10^{-24} \, {\rm cm}^{2}$. With these parameters, cosmic-ray scattering with dark matter in the extended halo of the Milky Way could generate a novel and distinctive gamma-ray signal at high galactic latitudes. Such a signal could be observable by Fermi or future space-based gamma-ray telescopes.
... In particular, it has been shown in recent studies that radio galaxies [39] and star-forming galaxies [40] (see also refs. [41][42][43][44]) together provide the dominant contributions to the IGRB (smaller but potentially non-negligible contributions also come from blazars [43,[45][46][47][48], as well as from galaxy clusters [49], propagating ultra-high energy cosmic rays [50,51], and perhaps even annihilating dark matter particles [52][53][54][55]). In quantitative terms, ref. [39] concludes that unresolved radio galaxies account for no less than 59% of the IGRB photons above 1 GeV, while ref. ...
Multi-messenger data suggest that radio galaxies (i.e. non-blazar active galaxies) are perhaps the most likely class of sources for the diffuse flux of high-energy neutrinos reported by the IceCube Collaboration. In this study, we consider the gamma-ray spectrum observed from four nearby radio galaxies (Centaurus A, PKS 0625-35, NGC 1275 and IC 310) and constrain the intensity and spectral shape of the emission injected from these sources, accounting for the effects of attenuation and contributions from electromagnetic cascades (initiated both within the radio galaxy itself and during extragalactic propagation). Assuming that this gamma-ray emission is generated primarily through the interactions of cosmic-ray protons with gas, we calculate the neutrino flux predicted from each of these sources. Although this scenario is consistent with the constraints published by the IceCube and ANTARES Collaborations, the predicted fluxes consistently fall within an order of magnitude of the current point source sensitivity. The prospects appear very encouraging for the future detection of neutrino emission from the nearest radio galaxies.
... (2.1), (2.2), (2.3) and (2.4), we need its accretion disk luminosity L AD , bolometric radiation luminosity L rad , and specific luminosity at 1 keV EL E (keV). For this purpose we follow the blazar emission model in ref. [16], which adopted the accretion-disk model of [29,30] and the blazar sequence model of [31,32]. Specifically, in order to derive the neutrino luminosity of Fermi-LAT detected blazars, we need L AD , L rad , and EL E (keV) as functions of L γ , the gamma-ray luminosity integrated over the spectrum above 100 MeV. ...
IceCube has detected a cumulative flux of PeV neutrinos, which origin is unknown. Blazars, active galactic nuclei with relativistic jets pointing to us, are long and widely expected to be one of the strong candidates of high energy neutrino sources. The neutrino production depends strongly on the cosmic ray power of blazar jets, which is largely unknown. The recent null results in stacking searches of neutrinos for several blazar samples by IceCube put upper limits on the neutrino fluxes from these blazars. Here we compute the cosmic ray power and PeV neutrino flux of Fermi-LAT blazars, and find that the upper limits for known blazar sources give stringent constraint on the cosmic ray loading factor of blazar jets (i.e., the ratio of the cosmic ray to bolometric radiation luminosity of blazar jets), ξcr (2–10)ζ⁻¹ (with ζ 1 the remained fraction of cosmic ray energy when propagate into the blazar broad line region) for flat cosmic ray spectrum, and that the cumulative PeV neutrino flux contributed by all-sky blazars is a fraction (10–50)% of the IceCube detected flux.
... In a recent study [28], it was demonstrated that the isotropic gamma-ray background (IGRB) measured by the Fermi Gamma-Ray Space Telescope [29] is dominated by emission from unresolved radio galaxies, along with a smaller but non-negligible contribution from blazars [30][31][32][33][34] (possibly among other sources, including star-forming galaxies [35,36], galaxy clusters [37], millisecond pulsars [38,39], propagating ultra-high energy cosmic rays [40,41], and/or annihilating dark matter particles [42][43][44][45]). 1 This result was made possible by utilizing an empirical correlation that had been previously identified between the radio and JCAP09(2016)002 gamma-ray luminosities of this class of sources [47,48]. More quantitatively, ref. [28] concluded that unresolved radio galaxies account for 77.2 +25.4 −9.4 % of the E γ > 1 GeV photons that make up Fermi's IGRB. ...
We present an argument that radio galaxies (active galaxies with mis-aligned jets) are likely to be the primary sources of the high-energy astrophysical neutrinos observed by IceCube. In particular, if the gamma-ray emission observed from radio galaxies is generated through the interactions of cosmic-ray protons with gas, these interactions can also produce a population of neutrinos with a flux and spectral shape similar to that measured by IceCube. We present a simple physical model in which high-energy cosmic rays are confined within the volumes of radio galaxies, where they interact with gas to generate the observed diffuse fluxes of neutrinos and gamma rays. In addition to simultaneously accounting for the observations of Fermi and IceCube, radio galaxies in this model also represent an attractive class of sources for the highest energy cosmic rays.
... More information can be recovered and exploited by investigating the anisotropy properties of the measured IGRB. Three measures of anisotropy currently are being studied: i) the auto-correlation or angular power spectrum (APS) of the IGRB [59,117,[117][118][119][120][121][122][123][124][125][126][127][128][129][130][131], ii) the cross-correlation with Large Scale Structure (LSS) tracers (galaxy catalogs and lensing surveys) [60,[106][107][108][109][110][111][112][113][114][115][116], and iii) pixel fluctuations (also referred to as one-point statistics) [152,153,[224][225][226][227][228]. ...
The nature of dark matter is a longstanding enigma of physics; it may consist of particles beyond the Standard Model that are still elusive to experiments. Among indirect search techniques, which look for stable products from the annihilation or decay of dark matter particles, or from axions coupling to high-energy photons, observations of the $\gamma$-ray sky have come to prominence over the last few years, because of the excellent sensitivity of the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope mission. The LAT energy range from 20 MeV to above 300 GeV is particularly well suited for searching for products of the interactions of dark matter particles. In this report we describe methods used to search for evidence of dark matter with the LAT, and review the status of searches performed with up to six years of LAT data. We also discuss the factors that determine the sensitivities of these searches, including the magnitudes of the signals and the relevant backgrounds, considering both statistical and systematic uncertainties. We project the expected sensitivities of each search method for 10 and 15 years of LAT data taking. In particular, we find that the sensitivity of searches targeting dwarf galaxies, which provide the best limits currently, will improve faster than the square root of observing time. Current LAT limits for dwarf galaxies using six years of data reach the thermal relic level for masses up to 120 GeV for the $b\bar{b}$ annihilation channel for reasonable dark matter density profiles. With projected discoveries of additional dwarfs, these limits could extend to about 250 GeV. With as much as 15 years of LAT data these searches would be sensitive to dark matter annihilations at the thermal relic cross section for masses to greater than 400 GeV (200 GeV) in the $b\bar{b}$ ($\tau^+ \tau^-$) annihilation channels.
... In 2012 the Fermi LAT collaboration measured the anisotropy of the IGRB for the first time [222], enabling new constraints to be placed on the contribution of dark matter [222][223][224] and other source populations [222,[225][226][227] to the IGRB based on requiring that the predicted anisotropy not exceed that measured in the data. As blazars are expected to contribute the majority of the IGRB anisotropy, robust predictions for the amplitude of their anisotropy can enhance sensitivity to dark matter signals by limiting the anisotropy available to be attributed to dark matter. ...
The indirect detection of dark matter annihilation and decay using observations of photons, charged cosmic rays, and neutrinos offers a promising means of identifying the particle nature of this elusive component of the universe. The last decade has seen substantial advances in observational data sets, complemented by new insights from numerical simulations, which together have enabled for the first time strong constraints on dark matter particle models, and have revealed several intriguing hints of possible signals. This review provides an introduction to indirect detection methods and an overview of recent results in the field.
We present a bottom-up calculation of the flux of ultrahigh-energy cosmic rays (UHECRs) and high-energy neutrinos produced by powerful jets of active galactic nuclei (AGNs). By propagating test particles in 3D relativistic magnetohydrodynamic jet simulations, including a Monte Carlo treatment of sub-grid pitch-angle scattering and attenuation losses due to realistic photon fields, we study the spectrum and composition of the accelerated UHECRs and estimate the amount of neutrinos produced in such sources. We find that UHECRs may not be significantly affected by photodisintegration in AGN jets where the espresso mechanism efficiently accelerates particles, consistent with Auger’s results that favor a heavy composition at the highest energies. Moreover, we present estimates and upper bounds for the flux of high-energy neutrinos expected from AGN jets. In particular, we find that (i) source neutrinos may account for a sizable fraction, or even dominate, the expected flux of cosmogenic neutrinos; (ii) neutrinos from the β -decay of secondary neutrons produced in nucleus photodisintegration end up in the teraelectronvolt to petaelectronvolt band observed by IceCube, but can hardly account for the observed flux; (iii) UHECRs accelerated via the espresso mechanism lead to nearly isotropic neutrino emission, which suggests that nearby radio galaxies may be more promising as potential sources. We discuss our results in light of multimessenger astronomy and current/future neutrino experiments.
Recent observations have shown that pulsars are surrounded by extended regions which emit TeV-scale gamma rays through the inverse Compton scattering of very high energy electrons and positrons. Such TeV halos are responsible for a large fraction of the Milky Way’s TeV-scale gamma-ray emission. In this paper, we calculate the gamma-ray spectrum from the population of TeV halos located within the Andromeda Galaxy, predicting a signal that is expected to be detectable by the Cherenkov Telescope Array. We also calculate the contribution from TeV halos to the isotropic gamma-ray background, finding that these sources should contribute significantly to this flux at the highest measured energies, constituting up to ∼20% of the signal observed above ∼0.1 TeV. We also comment on the implications of our results for the origin of the diffuse neutrino flux detected by IceCube.
The Milky Way's dark matter halo is expected to host numerous low-mass subhalos with no detectable associated stellar component. Such subhalos are invisible unless their dark matter annihilates to visible states such as photons. One of the established methods for identifying candidate subhalos is to search for individual unassociated gammaray sources with properties consistent with the dark matter expectation. However, robustly ruling out an astrophysical origin for any such candidate is challenging. In this work, we present a complementary approach that harnesses information about the entire population of subhalos-such as their spatial and mass distribution in the Galaxy-to search for a signal of annihilating dark matter. Using simulated data, we show that the collective emission from subhalos can imprint itself in a unique way on the statistics of observed photons, even when individual subhalos may be too dim to be resolved on their own. Additionally, we demonstrate that, for the models we consider, the signal can be identified even in the face of unresolved astrophysical point-source emission of extragalactic and Galactic origin. This establishes a new search technique for subhalos that is complementary to established methods, and that could have important ramifications for gamma-ray dark matter searches using observatories such as the Fermi Large Area Telescope and the Cerenkov Telescope Array.
In this thesis I investigate the evolution of high-energy neutrinos, and their ability to diagnose the environment of their origin. AGN are identified as candidates for high-energy neutrino production, and the radio-loud population is particularly attractive due to observed energetic regions in the jet features. I derive source populations at different redshifts and brightness using data obtained from AGN surveys detecting X-ray and γ-ray bright sources. These trace the accretion disc and beamed jet luminosities, respectively, and reflect AGN populations varying in observable properties. Two hadronic interaction models are considered to determine neutrino production efficiencies in AGN. The region of high-energy particle production is located at the base of the AGN jet, where relevant interactions are expected to occur. The diffuse neutrino emission on Earth is calculated as the sum of the contributions from various cosmological epochs, and X-ray luminosities. I find that the bulk of AGN sources would produce a neutrino flux far exceeding the current upper limits set on the received neutrino emission. The only neutrino energy spectra consistent with these limits are due to neutrino production in blazars. Additionally, the importance of a reliable luminosity scaling model is demonstrated, and hence the need for an improved understanding of the radiative processes in jets. To further investigate the physical processes relevant to neutrino production, I construct a neutrino luminosity function for blazar sources. The expected neutrino energy distribution is produced by assuming typical parameters, such as luminosity and Lorentz factor, for each class along the blazar sequence. This will establish constraints on possible AGN sources, the relative neutrino duty cycle, and production efficiencies across the sequence.
With a decade of γ-ray data from the Fermi-LAT telescope, we can now hope to answer how well we understand the local Universe at γ-ray frequencies. On the other hand, with γ-ray data alone it is not possible to directly access the distance of the emission and to point out the origin of unresolved sources. This obstacle can be overcome by cross-correlating the γ-ray data with catalogs of objects with well-determined redshifts and positions. In this work, we cross-correlate Fermi-LAT skymaps with the 2MPZ catalog to study the local z<0.2γ-ray Universe, where about 10% of the total unresolved γ-ray background is produced. We find the signal to be dominated by AGN emissions, while star forming galaxies provide a subdominant contribution. Possible hints for a particle DM signal are discussed.
Cette thèse vise à étudier le phénomène dit de « cascades électromagnétiques cosmologiques ». Ces cascadessont typiquement générées dans le milieu intergalactique par l’absorption de rayons gamma sur les photons du fond optique / UV et par la production de paires électron / positron associés. Ces leptons eux-mêmes interagissent avec les photons du fond diffus cosmologique via diffusion inverse Compton pour produire de nouveaux rayons gamma qui eux même peuvent s’annihiler, générant à partir d’un unique photon primaire toute une gerbe de photons et de particules secondaires. D’un point de vue observationnel, le développement de cette cascade introduit trois effets : une déformation du spectre à haute énergie, un retard temporel dans l’arrivée des rayons gamma et une extension de la taille apparente de la source.Les cascades électromagnétiques cosmologiques ont commencé à être étudiées dans les années soixante. Mais ce n’est qu’à partir des années 2010 avec l’arrivée du satellite Fermi (entre autres) et des observations dans la bande au GeV et au TeV que la discipline a explosé. Le phénomène est particulièrement important. D’une part il altère le spectre observé des sources rendant difficile la compréhension de la physique de ces dernières. D’autre part les cascades se développant dans le milieu extragalactique, elles sont très sensibles à la composition de ce dernier (fond diffus de photons, champ magnétique). Or ce milieu étant très ténu, il est difficile à étudier. Les cascades deviennent alors une formidable sonde pour accéder à sa compréhension et pouvoir en comprendre l’origine qui remonte au commencement de l’Univers.Pourtant les cascades cosmologiques sont un phénomène complexe faisant intervenir des interactions difficiles à modéliser (sections efficaces complexes) et le transport de particules dans un Univers en expansion (cosmologie). Face à cette complexité les expressions analytiques sont vite limitées et le passage au numérique devient inévitable. Dans le cadre de cette thèse un code de simulation Monte Carlo a donc été développé visant à reproduire aussi précisément que possible le phénomène des cascades. Ce code a été testé et validé en le confrontant aux expressions analytiques.Grâce à ce code, le rôle des différents paramètres physiques impactant le développement de la cascade a été étudié de manière systématique. Cette étude a permis de mieux comprendre la physique du phénomène. En particulier, l’impact des propriétés du milieu extragalactique (fond diffus extragalactique, champ magnétique extragalactique) sur les observables a été mis en évidence.Finalement, une seconde étude a été menée pour mesurer la contribution des cascades au fond gamma extragalactique. Des travaux récents montrent qu’une grande partie de l’émission diffuse à très haute énergie provient de sources ponctuelles non résolues (blazars en particulier). Ces sources gamma (résolues et non résolues) doivent en principe initier des cascades qui peuvent contribuer au fond diffus. En partant d’une modélisation de l’émission des blazars à différents redshifts, l’absorption et la contribution des cascades ont alors été calculées à l’aide du code Monte Carlo. Les résultats montrent que la contribution des cascades au fond gamma extragalactique pourrait violer les limites Fermi mais l’excès doit encore être confirmé.
We investigate the nature of the extragalactic unresolved γ-ray background (UGRB) by cross-correlating several galaxy catalogs with sky maps of the UGRB built from 78 months of Pass 8 Fermi-Large Area Telescope data. This study updates and improves similar previous analyses in several aspects. First, the use of a larger γ-ray data set allows us to investigate the energy dependence of the cross-correlation in more detail, using up to eight energy bins over a wide energy range of [0.25,500] GeV. Second, we consider larger and deeper catalogs (2MASS Photometric Redshift catalog, 2MPZ; WISE × SuperCOSMOS, WI×SC; and SDSS DR12 photometric redshift data set) in addition to the ones employed in the previous studies (NVSS and SDSS QSOs). Third, we exploit the redshift information available for the above catalogs to divide them into redshift bins and perform the cross-correlation separately in each of them. Our results confirm, with higher statistical significance, the detection of cross-correlation signals between the UGRB maps and all the catalogs considered, on angular scales smaller than 1°. Significances range from for NVSS, for SDSS DR12 and WI×SC, to for 2MPZ and for SDSS QSOs. Furthermore, including redshift tomography, the significance of the SDSS DR12 signal strikingly rises up to and that of WI×SC to . We offer a simple interpretation of the signal in the framework of the halo model. The precise redshift and energy information allows us to clearly detect a change over redshift in the spectral and clustering behavior of the γ-ray sources contributing to the UGRB.
For constraints on dark matter, we have studied the cross-correlation of gravitational lensing and another possible probe of dark matter distribution, the extragalactic gamma-ray background (EGB). The origin of EGB is among the most interesting problems in astrophysics. Among the potential new contributors to the EGB is the emission due to dark matter annihilation. The dark matter distribution that causes gravitational lensing would also be a gamma-ray source, if dark matter particles annihilate into standard model particles. Thus, the cross-correlation of gravitational lensing and the EGB can be a powerful probe of signature of dark matter annihilation. We performed the first measurement of the cross-correlation using the real data set obtained from Canada-France-Hawaii Telescope Lensing survey and the Fermi Large Area Telescope. We found that the measured cross-correlation is consistent with a null signal. Comparing the result to theoretical predictions based on structure formation, we put a cosmological constraint on dark matter annihilation cross section.
The source-count distribution as a function of their flux, dN/dS, is one of
the main quantities characterizing gamma-ray source populations. We employ
statistical properties of the Fermi-LAT photon counts map to measure the
composition of the extragalactic gamma-ray sky at high latitudes (|b|>30 deg)
between 1 GeV and 10 GeV. We present a new method, generalizing the use of
standard pixel-count statistics, to decompose the total observed gamma-ray
emission into: (a) point-source contributions, (b) the Galactic foreground
contribution, and (c) a truly diffuse isotropic background contribution. Using
the 6-year Fermi-LAT data set (P7REP), we show that the dN/dS distribution in
the regime of so far undetected point sources can be consistently described
with a power-law of index between 1.9 and 2.0. We measure dN/dS down to an
integral flux of ~2x10^{-11} cm^{-2}s^{-1}, improving beyond the 3FGL catalog
detection limit by about one order of magnitude. The overall dN/dS distribution
is consistent with a broken power law, with a break at
2.1^{+1.0}_{-1.3}x10^{-8} cm^{-2}s^{-1}. The power-law index
n_1=3.1^{+0.7}_{-0.5} for bright sources above the break hardens to
n_2=1.97+-0.03 for fainter sources below the break. A possible second break of
the dN/dS distribution is constrained to be at fluxes below 6x10^{-11}
cm^{-2}s^{-1} at 95% confidence level. The high-latitude gamma-ray sky between
1 GeV and 10 GeV is shown to be composed of ~25% point-sources, ~69% diffuse
Galactic foreground emission, and ~6% isotropic diffuse background.
We present the cross-correlation angular power spectrum of cosmic shear and gamma-rays produced by the annihilation/decay of Weakly Interacting Massive Particle (WIMP) dark matter (DM), and by astrophysical sources. We show that this observable can provide novel information on the composition of the Extra-galactic Gamma-ray Background (EGB), since the amplitude and shape of the cross-correlation signal depend on which class of sources is responsible for the gamma-ray emission. If the DM contribution to the EGB is significant (at least in a definite energy range), although compatible with current observational bounds, its strong correlation with the cosmic shear (since both signals peak at large halo masses) makes such signature potentially detectable by combining Fermi-LAT data with forthcoming galaxy surveys, like Dark Energy Survey and Euclid.
Similarly to gravitational lensing effects like cosmic shear, cosmological γ-ray emission too is to some extent a tracer of the distribution of dark matter (DM) in the Universe. Intervening DM structures source gravitational lensing distortions of distant galaxy images, and those same galaxies can emit γ rays, either because they host astrophysical sources, or directly by particle DM annihilations or decays occurring in the galactic halo. If such γ rays exhibit correlation with the cosmic shear signal, this will provide novel information on the composition of the extragalactic γ-ray background.
We search for evidence of dark matter (DM) annihilation in the isotropic gamma-ray background (IGRB) measured with 50 months of Fermi Large Area Telescope (LAT) observations. An improved theoretical description of the cosmological DM annihilation signal, based on two complementary techniques and assuming generic weakly interacting massive particle (WIMP) properties, renders more precise predictions compared to previous work. More specifically, we estimate the cosmologically-induced gamma-ray intensity to have an uncertainty of a factor ~ 20 in canonical setups. We consistently include both the Galactic and extragalactic signals under the same theoretical framework, and study the impact of the former on the IGRB spectrum derivation. We find no evidence for a DM signal and we set limits on the DM-induced isotropic gamma-ray signal. Our limits are competitive for DM particle masses up to tens of TeV and, indeed, are the strongest limits derived from Fermi LAT data at TeV energies. This is possible thanks to the new Fermi LAT IGRB measurement, which now extends up to an energy of 820 GeV. We quantify uncertainties in detail and show the potential this type of search offers for testing the WIMP paradigm with a complementary and truly cosmological probe of DM particle signals.
We present a determination of the distributions of gamma-ray photon flux --
the so called LogN-LogS relation -- and photon spectral index for blazars,
based on the third extragalactic source catalog of the Fermi Gamma-ray Space
Telescope's Large Area Telescope, and considering the photon energy range from
100 MeV to 100 GeV. The dataset consists of the 774 blazars in the so-called
"Clean" sample detected with a greater than approximately seven sigma detection
threshold and located above $\pm$20 deg Galactic latitude. We use
non-parametric methods verified in previous works to reconstruct the intrinsic
distributions from the observed ones which account for the data truncations
introduced by observational bias and includes the effects of the possible
correlation between the flux and photon index. The intrinsic flux distribution
can be represented by a broken power law with a high flux power-law index of
-2.43$\pm$0.08 and a low flux power-law index of -1.87$\pm$0.10. The intrinsic
photon index distribution can be represented by a Gaussian with mean of
2.62$\pm$0.05 and width of 0.17$\pm$0.02. We also report the intrinsic
distributions for the sub-populations of BL Lac and FSRQ type blazars
separately and these differ substantially. We then estimate the contribution of
FSRQs and BL Lacs to the diffuse extragalactic gamma-ray background radiation.
Under the simplistic assumption that the flux distributions probed in this
analysis continue to arbitrary low flux, we calculate that the best fit
contribution of FSRQs is 35% and BL Lacs 17% of the total gamma-ray output of
the Universe in this energy range.
The recent discovery of ten new dwarf galaxy candidates by the Dark Energy
Survey (DES) and the Panoramic Survey Telescope and Rapid Response System
(Pan-STARRS) could increase the Fermi Gamma-Ray Space Telescope's sensitivity
to annihilating dark matter particles, potentially enabling a definitive test
of the dark matter interpretation of the long-standing Galactic Center
gamma-ray excess. In this paper, we compare the previous analyses of Fermi data
from the directions of the new dwarf candidates (including the relatively
nearby Reticulum II) and perform our own analysis, with the goal of
establishing the statistical significance of any gamma-ray signal from these
sources. We confirm the presence of an excess from Reticulum II, with a
spectral shape that is compatible with the Galactic Center signal. The
significance of this emission is greater than that observed from 99.84% of
randomly chosen high-latitude blank-sky locations, corresponding to a local
detection significance of 3.2 sigma. We improve upon the standard blank-sky
calibration approach through the use of multi-wavelength catalogs, which allow
us to avoid regions that are likely to contain unresolved gamma-ray sources.
The origin of the extragalactic $\gamma$-ray background (EGB) has been
debated for some time. { The EGB comprises the $\gamma$-ray emission from
resolved and unresolved extragalactic sources, such as blazars, star-forming
galaxies and radio galaxies, as well as radiation from truly diffuse
processes.} This letter focuses on the blazar source class, the most numerous
detected population, and presents an updated luminosity function and spectral
energy distribution model consistent with the blazar observations performed by
the {\it Fermi} Large Area Telescope (LAT). We show that blazars account for
50$^{+12}_{-11}$\,\% of the EGB photons ($>$0.1\,GeV), and that {\it Fermi}-LAT
has already resolved $\sim$70\,\% of this contribution. Blazars, and in
particular low-luminosity hard-spectrum nearby sources like BL Lacs, are
responsible for most of the EGB emission above 100\,GeV. We find that the
extragalactic background light, which attenuates blazars' high-energy emission,
is responsible for the high-energy cut-off observed in the EGB spectrum.
Finally, we show that blazars, star-forming galaxies and radio galaxies can
naturally account for the amplitude and spectral shape of the background in the
0.1--820\,GeV range, leaving only modest room for other contributions. This
allows us to set competitive constraints on the dark-matter annihilation cross
section.
Angular anisotropy techniques for cosmic diffuse radiation maps are powerful probes, even for quite small data sets. A popular
observable is the angular power spectrum; we present a detailed study applicable to any unbinned source skymap $S(\boldsymbol {n})$ from which N random, independent events are observed. Its exact variance, which is due to the finite statistics, depends only on $S(\boldsymbol {n})$ and N; we also derive an unbiased estimator of the variance from the data. First-order effects agree with previous analytic estimates.
Importantly, heretofore unidentified higher order effects are found to contribute to the variance and may cause the uncertainty
to be significantly larger than previous analytic estimates – potentially orders of magnitude larger. Neglect of these higher
order terms, when significant, may result in a spurious detection of the power spectrum. On the other hand, this would indicate
the presence of higher order spatial correlations, such as a large bispectrum, providing new clues about the sources. Numerical
simulations are shown to support these conclusions. Applying the formalism to an ensemble of Gaussian-distributed skymaps,
the noise-dominated part of the power-spectrum uncertainty is significantly increased at high multipoles by the new, higher
order effects. This work is important for harmonic analyses of the distributions of diffuse high-energy γ-rays, neutrinos,
and charged cosmic rays, as well as for populations of sparse point sources such as active galactic nuclei.
The Fermi-LAT collaboration has studied the gamma-ray emission from a stacked
population of dwarf spheroidal galaxies and used this information to set
constraints on the dark matter annihilation cross section. Interestingly, their
analysis uncovered an excess with a test statistic (TS) of 8.7. If interpreted
naively, this constitutes a 2.95 sigma local excess (p-value=0.003), relative
to the expectations of their background model. In order to further test this
interpretation, the Fermi-LAT team studied a large number of blank sky
locations and found TS>8.7 excesses to be more common than predicted by their
background model, decreasing the significance of their dwarf excess to 2.2
sigma (p-value=0.027). We argue that these TS>8.7 blank sky locations are
largely the result of unresolved blazars, radio galaxies, and starforming
galaxies, and show that multi-wavelength information can be used to reduce the
degree to which such sources contaminate the otherwise blank sky. In
particular, we show that masking regions of the sky that lie within 1 degree of
sources contained in the BZCAT or CRATES catalogs reduces the fraction of blank
sky locations with TS>8.7 by more than a factor of two. Taking such
multi-wavelength information into account can enable experiments such as Fermi
to better characterize their backgrounds and increase their sensitivity to dark
matter in dwarf galaxies, the most important of which remain largely
uncontaminated by unresolved point sources. We also note that for the range of
dark matter masses and annihilation cross sections currently being tested by
studies of dwarf spheroidal galaxies, simulations predict that Fermi should be
able to detect a significant number of dark matter subhalos. These subhalos
constitute a population of sub-threshold gamma-ray point sources and represent
an irreducible background for searches for dark matter annihilation in dwarf
galaxies.
We study high-energy neutrino production in the inner jets of radio-loud
active galactic nuclei (AGN), taking into account effects of external photon
fields and the blazar sequence. We show that the resulting diffuse neutrino
intensity is dominated by quasar-hosted blazars, in particular, flat spectrum
radio quasars, and that PeV neutrino production due to photohadronic
interactions with broadline radiation is unavoidable if the inner jets of
blazars are cosmic-ray sources. The resulting neutrino spectrum has a cutoff
feature around PeV energies since the main target photons are Ly$\alpha$
emission. Because of infrared photons provided by the dust torus, neutrino
spectra above PeV energies are too hard to be consistent with the IceCube data
unless the proton spectral index is steeper than 2.5, or the maximum proton
energy is $\lesssim100$ PeV. Thus, although the cumulative background can be as
high as $E_\nu^2\Phi_\nu\sim{10}^{-8}~{\rm GeV}~{\rm cm}^{-2}~{\rm s}^{-1}~{\rm
sr}^{-1}$ for the cosmic-ray loading factor of $\xi_{\rm cr}\sim10$-$100$, it
is difficult to explain the IceCube signal in the inner jet model unless more
complicated situations are invoked. Even so, interestingly, future detectors
such as the Askaryan Radio Array can detect $\sim0.1$-$1$ EeV neutrinos. We
find that the diffuse neutrino intensity from radio-loud AGN is dominated by
blazars with gamma-ray luminosity of $\gtrsim10^{48}~{\rm erg}~{\rm s}^{-1}$,
and the arrival directions of $\sim1$-$100$ PeV neutrinos correlate with the
luminous blazars detected by Fermi.
The annihilation of dark matter particles in the halo of galaxies may end up into γ-rays, which travel almost unperturbed
till to their detection at the Earth. This annihilation signal can exhibit an anisotropic behaviour quantified by the angular
power spectrum, whose properties strongly depend on the dark matter distribution and its clumpiness. We use high resolution
pure dark matter N-body simulations to quantify the contribution of different components (main halo and satellites) to the global signal as
a function of the analytical profile adopted to describe the numerical results. We find that the smooth main halo dominates
the angular power spectrum of the γ-ray signal up to quite large multipoles, where the subhaloes anisotropy signal starts
to emerge, but the transition multipole strongly depends on the assumed radial profile. The extrapolation down to radii not
resolved by current numerical simulations can affect both the normalization and the shape of the γ-ray angular power spectrum. For the subhaloes described by an asymptotically cored dark matter distribution,
the angular power spectrum shows an overall smaller normalization and a flattening at high multipoles. Our results show the
criticality of the dark matter density profile shape in γ-ray anisotropy searches, and evaluate quantitatively the intrinsic
errors occurring when extrapolating the dark matter radial profiles down to spatial scales not yet explored by numerical simulations.
Several classes of astrophysical sources contribute to the approximately
isotropic gamma-ray background measured by the Fermi Gamma-Ray Space Telescope.
In this paper, we use Fermi's catalog of gamma-ray sources (along with
corresponding source catalogs at infrared and radio wavelengths) to build and
constrain a model for the contributions to the extragalactic gamma-ray
background from astrophysical sources, including radio galaxies, star-forming
galaxies, and blazars. We then combine our model with Fermi's measurement of
the gamma-ray background to derive constraints on the dark matter annihilation
cross section, including contributions from both extragalactic and galactic
halos and subhalos. The resulting constraints are competitive with the
strongest current constraints from the Galactic Center and dwarf spheroidal
galaxies. As Fermi continues to measure the gamma-ray emission from a greater
number of astrophysical sources, it will become possible to more tightly
constrain the astrophysical contributions to the extragalactic gamma-ray
background. We project that with 10 years of data, Fermi's measurement of this
background combined with the improved constraints on the astrophysical source
contributions will yield a sensitivity to dark matter annihilations that
exceeds the strongest current constraints by a factor of ~ 5 - 10.
Both cosmic shear and cosmological gamma-ray emission stem from the presence of dark matter (DM) in the universe: DM structures are responsible for the bending of light in the weak-lensing regime and those same objects can emit gamma rays, either because they host astrophysical sources (active galactic nuclei or star-forming galaxies) or directly by DM annihilations (or decays, depending on the properties of the DM particle). Such gamma rays should therefore exhibit strong correlation with the cosmic shear signal. In this Letter, we compute the cross-correlation angular power spectrum of cosmic shear and gamma rays produced by the annihilation/decay of weakly interacting massive particle DM, as well as by astrophysical sources. We show that this observable provides novel information on the composition of the extragalactic gamma-ray background (EGB), since the amplitude and shape of the cross-correlation signal strongly depend on which class of sources is responsible for the gamma-ray emission. If the DM contribution to the EGB is significant (at least in a definite energy range), although compatible with current observational bounds, its strong correlation with the cosmic shear makes such signal potentially detectable by combining Fermi Large Area Telescope data with forthcoming galaxy surveys, like the Dark Energy Survey and Euclid. At the same time, the same signal would demonstrate that the weak-lensing observables are indeed due to particle DM matter and not to possible modifications of general relativity.
Very high-energy gamma-ray measurements of distant blazars do not clearly
show the signature of the attenuation due to the pair production with the
extragalactic background light. Recent studies showed that these objects can be
well explained by secondary gamma rays emitted by cascades induced by
ultra-high-energy cosmic rays. The secondary gamma rays will enable one to
detect a large number of blazars with future ground based gamma- ray telescopes
such as Cherenkov Telescope Array (CTA). We show that the secondary emission
process will allow CTA to detect 100, 130, 150, 87, and 8 blazars above 30 GeV,
100 GeV, 300 GeV, 1 TeV, and 10 TeV, respectively, up to z~8 assuming the
intergalactic magnetic field (IGMF) strength B = 10^-17 G and an unbiased all
sky survey with 0.5 hr exposure at each Field of View, where total observing
time is ~540 hr. These numbers will be 79, 96, 110, 63, and 6 up to z~5 in the
case of B = 10^-15 G. This large statistics of sources will be a clear evidence
of the secondary gamma-ray scenarios and a new key to studying the IGMF
statistically. We also find that a wider and shallower survey is favored to
detect more and higher redshift sources even if we take into account secondary
gamma rays.
Indirect searches for products of dark matter annihilation and decay face the
challenge of identifying an uncertain and subdominant signal in the presence of
uncertain backgrounds. Two valuable approaches to this problem are (1) using
analysis methods which take advantage of different features in the energy
spectrum and angular distribution of the signal and backgrounds, and (2) more
accurate characterization of backgrounds, which allows for more robust
identification of possible signals. I review the status of indirect searches
with gamma rays using two promising targets, the Inner Galaxy and the Isotropic
Gamma-Ray Background. For both targets, uncertainties in the properties of
backgrounds is a major limitation to the sensitivity of indirect searches. I
then highlight approaches which can enhance the sensitivity of indirect
searches using these targets.
We present the first catalog of active galactic nuclei (AGNs) detected by the Large Area Telescope (LAT), corresponding to 11 months of data collected in scientific operation mode. The First LAT AGN Catalog (1LAC) includes 671 ?-ray sources located at high Galactic latitudes (|b|>10°) that are detected with a test statistic greater than 25 and associated statistically with AGNs. Some LAT sources are associated with multiple AGNs, and consequently, the catalog includes 709 AGNs, comprising 300 BL Lacertae objects, 296 flat-spectrum radio quasars, 41 AGNs of other types, and 72 AGNs of unknown type. We also classify the blazars based on their spectral energy distributions as archival radio, optical, and X-ray data permit. In addition to the formal 1LAC sample, we provide AGN associations for 51 low-latitude LAT sources and AGN "affiliations" (unquantified counterpart candidates) for 104 high-latitude LAT sources without AGN associations. The overlap of the 1LAC with existing ?-ray AGN catalogs (LBAS, EGRET, AGILE, Swift, INTEGRAL, TeVCat) is briefly discussed. Various properties—such as ?-ray fluxes and photon power-law spectral indices, redshifts, ?-ray luminosities, variability, and archival radio luminosities—and their correlations are presented and discussed for the different blazar classes. We compare the 1LAC results with predictions regarding the ?-ray AGN populations, and we comment on the power of the sample to address the question of the blazar sequence
We have conducted a detailed investigation of the broadband spectral properties of the γ-ray selected blazars of the Fermi LAT Bright AGN Sample (LBAS). By combining our accurately estimated Fermi γ-ray spectra with Swift, radio, infra-red, optical, and other hard X-ray/γ-ray data, collected within 3 months of the LBAS data taking period, we were able to assemble high-quality and quasi-simultaneous spectral energy distributions (SED) for 48 LBAS blazars. The SED of these γ-ray sources is similar to that of blazars discovered at other wavelengths, clearly showing, in the usual log ν-log ν F ν representation, the typical broadband spectral signatures normally attributed to a combination of low-energy synchrotron radiation followed by inverse Compton emission of one or more components. We have used these SED to characterize the peak intensity of both the low- and the high-energy components. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broadband colors (i.e., the radio to optical, αro, and optical to X-ray, αox, spectral slopes) and from the γ-ray spectral index. Our data show that the synchrotron peak frequency (νS peak) is positioned between 1012.5 and 1014.5 Hz in broad-lined flat spectrum radio quasars (FSRQs) and between 1013 and 1017 Hz in featureless BL Lacertae objects. We find that the γ-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron-inverse Compton scenarios. However, simple homogeneous, one-zone, synchrotron self-Compton (SSC) models cannot explain most of our SED, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving external Compton radiation or multiple SSC components are required to reproduce the overall SED and the observed spectral variability. While more than 50% of known radio bright high energy peaked (HBL) BL Lacs are detected in the LBAS sample, only less than 13% of known bright FSRQs and LBL BL Lacs are included. This suggests that the latter sources, as a class, may be much fainter γ-ray emitters than LBAS blazars, and could in fact radiate close to the expectations of simple SSC models. We categorized all our sources according to a new physical classification scheme based on the generally accepted paradigm for Active Galactic Nuclei and on the results of this SED study. Since the LAT detector is more sensitive to flat spectrum γ-ray sources, the correlation between νS peak and γ-ray spectral index strongly favors the detection of high energy peaked blazars, thus explaining the Fermi overabundance of this type of sources compared to radio and EGRET samples. This selection effect is similar to that experienced in the soft X-ray band where HBL BL Lacs are the dominant type of blazars.
The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view (FoV), high-energy γ-ray telescope, covering the energy range from below 20 MeV to more than 300 GeV. The LAT was built by an international collaboration with contributions from space agencies, high-energy particle physics institutes, and universities in France, Italy, Japan, Sweden, and the United States. This paper describes the LAT, its preflight expected performance, and summarizes the key science objectives that will be addressed. On-orbit performance will be presented in detail in a subsequent paper. The LAT is a pair-conversion telescope with a precision tracker and calorimeter, each consisting of a 4 × 4 array of 16 modules, a segmented anticoincidence detector that covers the tracker array, and a programmable trigger and data acquisition system. Each tracker module has a vertical stack of 18 (x, y) tracking planes, including two layers (x and y) of single-sided silicon strip detectors and high-Z converter material (tungsten) per tray. Every calorimeter module has 96 CsI(Tl) crystals, arranged in an eight-layer hodoscopic configuration with a total depth of 8.6 radiation lengths, giving both longitudinal and transverse information about the energy deposition pattern. The calorimeter's depth and segmentation enable the high-energy reach of the LAT and contribute significantly to background rejection. The aspect ratio of the tracker (height/width) is 0.4, allowing a large FoV (2.4 sr) and ensuring that most pair-conversion showers initiated in the tracker will pass into the calorimeter for energy measurement. Data obtained with the LAT are intended to (1) permit rapid notification of high-energy γ-ray bursts and transients and facilitate monitoring of variable sources, (2) yield an extensive catalog of several thousand high-energy sources obtained from an all-sky survey, (3) measure spectra from 20 MeV to more than 50 GeV for several hundred sources, (4) localize point sources to 0.3-2 arcmin, (5) map and obtain spectra of extended sources such as SNRs, molecular clouds, and nearby galaxies, (6) measure the diffuse isotropic γ-ray background up to TeV energies, and (7) explore the discovery space for dark matter.
Recent detections of the starburst galaxies M82 and NGC 253 by gamma-ray telescopes suggest that galaxies rapidly forming massive stars are more luminous at gamma-ray energies compared to their quiescent relatives. Building upon those results, we examine a sample of 69 dwarf, spiral, and luminous and ultraluminous infrared galaxies at photon energies 0.1-100 GeV using 3 years of data collected by the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope (Fermi). Measured fluxes from significantly detected sources and flux upper limits for the remaining galaxies are used to explore the physics of cosmic rays in galaxies. We find further evidence for quasi-linear scaling relations between gamma-ray luminosity and both radio continuum luminosity and total infrared luminosity which apply both to quiescent galaxies of the Local Group and low-redshift starburst galaxies (conservative P-values 0.05 accounting for statistical and systematic uncertainties). The normalizations of these scaling relations correspond to luminosity ratios of log (L 0.1-100 GeV/L 1.4 GHz) = 1.7 ± 0.1(statistical) ± 0.2(dispersion) and log (L 0.1-100 GeV/L 8-1000 μm) = –4.3 ± 0.1(statistical) ± 0.2(dispersion) for a galaxy with a star formation rate of 1 M ☉ yr–1, assuming a Chabrier initial mass function. Using the relationship between infrared luminosity and gamma-ray luminosity, the collective intensity of unresolved star-forming galaxies at redshifts 0 < z < 2.5 above 0.1 GeV is estimated to be 0.4-2.4 × 10–6 ph cm–2 s–1 sr–1 (4%-23% of the intensity of the isotropic diffuse component measured with the LAT). We anticipate that ~10 galaxies could be detected by their cosmic-ray-induced gamma-ray emission during a 10 year Fermi mission.
During the period from the launch of the Compton Observatory in 1991
April through 1993 September, the Energetic Gamma-Ray Experiment
Telescope (EGRET) has detected, with high significance (>5σ),
33 blazar-type QSOs which are sources of high-energy gamma-rays and
another eight which show strong evidence for high-energy gamma-ray
emission. Many of the gamma-ray loud blazars have redshifts z ˜ 1
with the most distant source having a redshift of z = 2.2. The
V/VMAX test has been applied to the sample of gamma-ray loud
AGNs seen during the phase I and II observing periods, and evidence for
the evolution of these sources consistent with pure luminosity evolution
is presented. The effects of observational biases due to gamma-ray
flux-detection limits and optical identifications are considered. Using
the Ve/Va test, we find the parameters describing
the evolution, and by de-evolving the sample, we make an estimate of the
luminosity function of these objects. Finally, using this luminosity
function, we make an estimate of the gamma-ray loud AGN contribution to
the diffuse extragalactic emission.
We estimate the contribution of radio-loud active galactic nuclei to the
gamma-ray background using the recent detections by GRO of BL Lacs and
flat-spectrum radio quasars at energies about 100 MeV. From the recently
derived radio luminosity functions of these objects, together with the
observed mean values of the gamma-ray to radio luminosity, we derive
contributions of about 30 and 35 per cent, respectively, at 100 MeV. The
total contribution of radio-loud sources is 66+31 or -24 per cent,
leaving little room for a substantial contribution from radio-quiet
active galactic nuclei, none of which has been detected so far. Our
results assume that the detected sources are typical of the radio-loud
population and not strongly variable; further gamma-ray detections and
variability studies should allow a refinement of our estimate.
This is the first of a series of papers aimed at characterizing the populations detected in the high-latitude sky of the Fermi-LAT survey. In this work, we focus on the intrinsic spectral and flux properties of the source sample. We show that when selection effects are properly taken into account, Fermi sources are on average steeper than previously found (e.g., in the bright source list) with an average photon index of 2.40 ± 0.02 over the entire 0.1-100 GeV energy band. We confirm that flat spectrum radio quasars have steeper spectra than BL Lacertae objects with an average index of 2.48 ± 0.02 versus 2.18 ± 0.02. Using several methods, we build the deepest source count distribution at GeV energies, deriving that the intrinsic source (i.e., blazar) surface density at F 100 ≥ 10–9 ph cm–2 s–1 is 0.12+0.03 –0.02 deg–2. The integration of the source count distribution yields that point sources contribute 16(±1.8)% (±7% systematic uncertainty) of the GeV isotropic diffuse background. At the fluxes currently reached by LAT, we can rule out the hypothesis that pointlike sources (i.e., blazars) produce a larger fraction of the diffuse emission.
The combination of seven-year data from WMAP and improved astrophysical data rigorously tests the standard cosmological model and places new constraints on its basic parameters and extensions. By combining the WMAP data with the latest distance measurements from the baryon acoustic oscillations (BAO) in the distribution of galaxies and the Hubble constant (H 0) measurement, we determine the parameters of the simplest six-parameter ΛCDM model. The power-law index of the primordial power spectrum is ns = 0.968 ± 0.012 (68% CL) for this data combination, a measurement that excludes the Harrison-Zel'dovich-Peebles spectrum by 99.5% CL. The other parameters, including those beyond the minimal set, are also consistent with, and improved from, the five-year results. We find no convincing deviations from the minimal model. The seven-year temperature power spectrum gives a better determination of the third acoustic peak, which results in a better determination of the redshift of the matter-radiation equality epoch. Notable examples of improved parameters are the total mass of neutrinos, ∑m ν < 0.58 eV(95%CL), and the effective number of neutrino species, N eff = 4.34+0.86 –0.88 (68% CL), which benefit from better determinations of the third peak and H 0. The limit on a constant dark energy equation of state parameter from WMAP+BAO+H 0, without high-redshift Type Ia supernovae, is w = –1.10 ± 0.14 (68% CL). We detect the effect of primordial helium on the temperature power spectrum and provide a new test of big bang nucleosynthesis by measuring Yp = 0.326 ± 0.075 (68% CL). We detect, and show on the map for the first time, the tangential and radial polarization patterns around hot and cold spots of temperature fluctuations, an important test of physical processes at z = 1090 and the dominance of adiabatic scalar fluctuations. The seven-year polarization data have significantly improved: we now detect the temperature-E-mode polarization cross power spectrum at 21σ, compared with 13σ from the five-year data. With the seven-year temperature-B-mode cross power spectrum, the limit on a rotation of the polarization plane due to potential parity-violating effects has improved by 38% to (68% CL). We report significant detections of the Sunyaev-Zel'dovich (SZ) effect at the locations of known clusters of galaxies. The measured SZ signal agrees well with the expected signal from the X-ray data on a cluster-by-cluster basis. However, it is a factor of 0.5-0.7 times the predictions from "universal profile" of Arnaud et al., analytical models, and hydrodynamical simulations. We find, for the first time in the SZ effect, a significant difference between the cooling-flow and non-cooling-flow clusters (or relaxed and non-relaxed clusters), which can explain some of the discrepancy. This lower amplitude is consistent with the lower-than-theoretically expected SZ power spectrum recently measured by the South Pole Telescope Collaboration.
We summarize a study where we test the hypothesis that local black holes (BH) are relics of AGN activity. We compare the mass
function of BHs in the local universe with that expected from AGN relics, which are BHs grown entirely with mass accretion
during AGN phases. The local BH mass function (BHMF) is estimated by applying the well-known correlations between BH mass,
bulge luminosity and stellar velocity dispersion to galaxy luminosity and velocity functions. The density of BHs in the local
universe is
rBH = 4.6-1.4+1.9h0.72 105M\odotMpc-3\ensuremath{\rho_\mathrm{BH}} = 4.6_{-1.4}^{+1.9}\, h_{0.7}^2 \ensuremath{\,\times 10^5\ensuremath{~\mathrm{M}_\odot}\mathrm{\,Mpc}^{-3}}
. The relic BHMF is derived from the continuity equation with the only assumption that AGN activity is due to accretion onto
massive BHs and that merging is not important. We find that the relic BHMF at z=0 is generated mainly at z<3. Moreover, the BH growth is anti-hierarchical in the sense that smaller BHs (
MBH < 107M\odot\ensuremath{M_\mathrm{BH}}
We show that the current extragalactic gamma-ray background (EGB) measurement
below 100 GeV sets an upper limit on EGB itself at very high energy (VHE) above
100 GeV. The limit is conservative for the electromagnetic cascade emission
from VHE EGB interacting with the cosmic microwave-to-optical background
radiation not to exceed the current EGB measurement. The cascade component fits
the measured VHE EGB spectrum rather well. However, once we add the
contribution from known source classes, the Fermi VHE EGB observation exceeds
or even violates the limit, which is approximated as E^2dN/dE < 4.5x10^-5
(E/100 GeV)^-0.7 MeV/cm^2/s/sr. The upper limit above 100 GeV is useful in the
future to probe the EGB origin and the new physics like axion-like particles
and Lorentz-invariance violation.
Recent detections of the starburst galaxies M82 and NGC 253 by gamma-ray
telescopes suggest that galaxies rapidly forming massive stars are more
luminous at gamma-ray energies compared to their quiescent relatives. Building
upon those results, we examine a sample of 69 dwarf, spiral, and luminous and
ultraluminous infrared galaxies at photon energies 0.1-100 GeV using 3 years of
data collected by the Large Area Telescope (LAT) on the \textit{Fermi Gamma-ray
Space Telescope} (\textit{Fermi}). Measured fluxes from significantly detected
sources and flux upper limits for the remaining galaxies are used to explore
the physics of cosmic rays in galaxies. We find further evidence for
quasi-linear scaling relations between gamma-ray luminosity and both radio
continuum luminosity and total infrared luminosity which apply both to
quiescent galaxies of the Local Group and low-redshift starburst galaxies
(conservative $P$-values $\lesssim0.05$ accounting for statistical and
systematic uncertainties). The normalizations of these scaling relations
correspond to luminosity ratios of $\log(L_{0.1-100 \rm{GeV}}/L_{1.4 \rm{GHz}})
= 1.7 \pm 0.1_{\rm (statistical)} \pm 0.2_{\rm (dispersion)}$ and
$\log(L_{0.1-100 \rm{GeV}}/L_{8-1000 \mu\rm{m}}) = -4.3 \pm 0.1_{\rm
(statistical)} \pm 0.2_{\rm (dispersion)}$ for a galaxy with a star formation
rate of 1 $M_{\odot}$ yr$^{-1}$, assuming a Chabrier initial mass function.
Using the relationship between infrared luminosity and gamma-ray luminosity,
the collective intensity of unresolved star-forming galaxies at redshifts
$0<z<2.5$ above 0.1 GeV is estimated to be 0.4-2.4 $\times 10^{-6}$ ph
cm$^{-2}$ s$^{-1}$ sr$^{-1}$ (4-23% of the intensity of the isotropic diffuse
component measured with the LAT). We anticipate that $\sim10$ galaxies could be
detected by their cosmic-ray induced gamma-ray emission during a 10-year
\textit{Fermi} mission.
Recent observations of isotropic diffuse backgrounds by Fermi and IceCube allow us to get more insight into distant very-high-energy (VHE) and ultra-high-energy (UHE) gamma-ray/neutrino emitters, including cosmic-ray accelerators/sources.
First, we investigate the contribution of intergalactic cascades induced by gamma-rays and/or cosmic rays (CRs) to the diffuse gamma-ray background (DGB) in view of the latest Fermi data. We identify a possible ``VHE Excess" from the fact that the Fermi data are well above expectations for an attenuated power law, and show that cascades induced by VHE gamma rays (above ~ 10 TeV) and/or VHECRs (below ~ 1019 eV) may significantly contribute to the DGB above ~ 100 GeV.
The relevance of the cascades is also motivated by the intergalactic cascade interpretations of extreme TeV blazars such as 1ES 0229+200, which suggest very hard intrinsic spectra.
This strengthens the importance of future detailed VHE DGB measurements.
Then, more conservatively, we derive general constraints on the cosmic energy budget of high-energy gamma rays and neutrinos based on recent Fermi and IceCube observations of extragalactic background radiation. We demonstrate that these multi-messenger constraints are useful and the neutrino limit is very powerful for VHE/UHE hadronic sources.
Furthermore, we show the importance of constraints from individual source surveys by future imaging atmospheric Cherenkov telescopes such as Cherenkov Telescope Array, and demonstrate that the cascade hypothesis for the VHE DGB can be tested by searching for distant emitters of cascaded gamma rays.
Interactions of cosmic rays with the interstellar gas and radiation fields of
the Milky Way provide the majority of the gamma rays observed by the Fermi
Gamma Ray Space Telescope. In addition to the gas which is densely concentrated
along the Galactic Disk, hydrodynamical simulations and observational evidence
favor the presence of a halo of hot (T~10^6 K) ionized hydrogen (H_II),
extending with non-negligible densities out to the virial radius of the Milky
Way. We show that cosmic ray collisions with this circum-galactic gas should be
expected to provide a significant flux of gamma rays, on the order of 10% of
the observed isotopic gamma ray background at energies above 1 GeV. In
addition, gamma rays originating from the extended H_II halos of other galaxies
along a given line-of-sight should contribute to this background at a similar
level.
We present the first catalog of active galactic nuclei (AGNs) detected by the Large Area Telescope (LAT), corresponding to 11 months of data collected in scientific operation mode. The First LAT AGN Catalog (1LAC) includes 671 gamma-ray sources located at high Galactic latitudes (|b| > 10 degrees) that are detected with a test statistic greater than 25 and associated statistically with AGNs. Some LAT sources are associated with multiple AGNs, and consequently, the catalog includes 709 AGNs, comprising 300 BL Lacertae objects, 296 flat-spectrum radio quasars, 41 AGNs of other types, and 72 AGNs of unknown type. We also classify the blazars based on their spectral energy distributions as archival radio, optical, and X-ray data permit. In addition to the formal 1LAC sample, we provide AGN associations for 51 low-latitude LAT sources and AGN "affiliations" (unquantified counterpart candidates) for 104 high-latitude LAT sources without AGN associations. The overlap of the 1LAC with existing gamma-ray AGN catalogs (LBAS, EGRET, AGILE, Swift, INTEGRAL, TeVCat) is briefly discussed. Various properties-such as gamma-ray fluxes and photon power-law spectral indices, redshifts, gamma-ray luminosities, variability, and archival radio luminosities-and their correlations are presented and discussed for the different blazar classes. We compare the 1LAC results with predictions regarding the gamma-ray AGN populations, and we comment on the power of the sample to address the question of the blazar sequence.
We place new constraints on the contribution of blazars to the large-scale
isotropic gamma-ray background (IGRB) by jointly analyzing the measured source
count distribution (logN-logS) of blazars and the measured intensity and
anisotropy of the IGRB. We find that these measurements point to a consistent
scenario in which unresolved blazars make less than 20% of the IGRB intensity
at 1-10 GeV while accounting for the majority of the measured anisotropy in
that energy band. These results indicate that the remaining fraction of the
IGRB intensity is made by a component with a low level of intrinsic anisotropy.
We determine upper limits on the anisotropy from non-blazar sources, adopting
the best-fit parameters of the measured source count distribution to calculate
the unresolved blazar anisotropy. In addition, we show that the anisotropy
measurement excludes some recently proposed models of the unresolved blazar
population.
The contribution of unresolved sources to the diffuse gamma-ray background
could induce anisotropies in this emission on small angular scales. We analyze
the angular power spectrum of the diffuse emission measured by the Fermi LAT at
Galactic latitudes |b| > 30 deg in four energy bins spanning 1 to 50 GeV. At
multipoles \ell \ge 155, corresponding to angular scales \lesssim 2 deg,
angular power above the photon noise level is detected at >99.99% CL in the 1-2
GeV, 2-5 GeV, and 5-10 GeV energy bins, and at >99% CL at 10-50 GeV. Within
each energy bin the measured angular power takes approximately the same value
at all multipoles \ell \ge 155, suggesting that it originates from the
contribution of one or more unclustered source populations. The amplitude of
the angular power normalized to the mean intensity in each energy bin is
consistent with a constant value at all energies, C_P/ ^2 = 9.05 +/- 0.84 x
10^{-6} sr, while the energy dependence of C_P is consistent with the
anisotropy arising from one or more source populations with power-law photon
spectra with spectral index \Gamma_s = 2.40 +/- 0.07. We discuss the
implications of the measured angular power for gamma-ray source populations
that may provide a contribution to the diffuse gamma-ray background.
The intergalactic magnetic field (IGMF) may leave an imprint on the angular
anisotropy of the extragalactic gamma-ray background through its effect on
electromagnetic cascades triggered by interactions between very high energy
photons and the extragalactic background light. A strong IGMF will deflect
secondary particles produced in these cascades and will thus tend to isotropize
lower energy cascade photons, thereby inducing a modulation in the anisotropy
energy spectrum of the gamma-ray background. Here we present a simple,
proof-of-concept calculation of the magnitude of this effect and demonstrate
that current Fermi data already seem to prefer non-negligible IGMF values. The
anisotropy energy spectrum of the Fermi gamma-ray background could thus be used
as a probe of the IGMF strength.
The gamma-ray energy spectra of bright blazars of the LAT Bright AGN Sample LBAS) are investigated using Fermi-LAT data. Spectral properties hardness, curvature, and variability) established using a data set accumulated over 6 months of operation are presented and discussed for different blazar classes and subclasses: flat spectrum radio quasars (FSRQs), low-synchrotron peaked BLLacs (LSP-BLLacs), intermediate-synchrotron peaked BLLacs (ISP-BLLacs), and high-synchrotron peaked BLLacs (HSP-BLLacs). The distribution of photon index G, obtained from a power-law fit above 100 MeV) is found to correlate strongly with blazar subclass. The change in spectral index from that averaged over the 6 months observing period is < 0.2-0.3 when the flux varies by about an order of magnitude, with a tendency toward harder spectra when the flux is brighter for FSRQs and LSP-BLLacs. A strong departure from a single power-law spectrum appears to be a common feature for FSRQs. This feature is also present for some high-luminosity LSP-BLLacs, and a small number of ISP-BLLacs. It is absent in all LBAS HSP-BLLacs. For 3C 454.3 and AO 0235+164, the two brightest FSRQ source and LSP-BLLac source, respectively, a broken power law (BPL) gives the most acceptable of power law, BPL, and curved forms. The consequences of these findings are discussed.
The extragalactic background light (EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rays with energy above ~10 GeV during propagation from sources at cosmological distances. This results in a redshift- and energy-dependent attenuation of the γ-ray flux of extragalactic sources such as blazars and gamma-ray bursts (GRBs). The Large Area Telescope on board Fermi detects a sample of γ-ray blazars with redshift up to z ~ 3, and GRBs with redshift up to z ~ 4.3. Using photons above 10 GeV collected by Fermi over more than one year of observations for these sources, we investigate the effect of γ-ray flux attenuation by the EBL. We place upper limits on the γ-ray opacity of the universe at various energies and redshifts and compare this with predictions from well-known EBL models. We find that an EBL intensity in the optical-ultraviolet wavelengths as great as predicted by the "baseline" model of Stecker et al. can be ruled out with high confidence.
Fermi has provided the largest sample of {\gamma}-ray selected blazars to
date. In this work we use a complete sample of FSRQs detected during the first
year of operation to determine the luminosity function (LF) and its evolution
with cosmic time. The number density of FSRQs grows dramatically up to redshift
\sim0.5-2.0 and declines thereafter. The redshift of the peak in the density is
luminosity dependent, with more luminous sources peaking at earlier times; thus
the LF of {\gamma}-ray FSRQs follows a luminosity-dependent density evolution
similarly to that of radio-quiet AGN. Also using data from the Swift Burst
Alert Telescope we derive the average spectral energy distribution of FSRQs in
the 10 keV-100 GeV band and show that there is no correlation of the peak
{\gamma}-ray luminosity with {\gamma}-ray peak frequency. The coupling of the
SED and LF allows us to predict that the contribution of FSRQs to the Fermi
isotropic {\gamma}-ray background is 9.3(+1.6/-1.0) (\pm3% systematic
uncertainty) in the 0.1-100GeV band. Finally we determine the LF of unbeamed
FSRQs, finding that FSRQs have an average Lorentz factor of {\gamma} =
11.7(+3.3/-2.2), that most are seen within 5\circ of the jet axis, and that
they represent only ~0.1 % of the parent population.
We present a determination of the distributions of the photon spectral index and gamma-ray flux—the so-called log N-log S relation—for the 352 blazars detected with a greater than approximately 7σ detection threshold and located above ±20° Galactic latitude by the Large Area Telescope of the Fermi
Gamma-ray Space Telescope in its first year catalog. Because the flux detection threshold depends on the photon index, the observed raw distributions do not provide the true log N-log S counts or the true distribution of the photon index. We use the non-parametric methods developed by Efron and Petrosian to reconstruct the intrinsic distributions from the observed ones which account for the data truncations introduced by observational bias and includes the effects of the possible correlation between the two variables. We demonstrate the robustness of our procedures using a simulated data set of blazars and then apply these to the real data and find that for the population as a whole the intrinsic flux distribution can be represented by a broken power law with high and low indices of –2.37 ± 0.13 and –1.70 ± 0.26, respectively, and the intrinsic photon index distribution can be represented by a Gaussian with mean of 2.41 ± 0.13 and width of 0.25 ± 0.03. We also find the intrinsic distributions for the sub-populations of BL Lac and flat spectrum radio quasar type blazars separately. We then calculate the contribution of Fermi blazars to the diffuse extragalactic gamma-ray background radiation. Under the assumption that the flux distribution of blazars continues to arbitrarily low fluxes, we calculate the best-fit contribution of all blazars to the total extragalactic gamma-ray output to be 60%, with a large uncertainty.
The Fermi gamma-ray satellite has recently detected gamma-ray emissions from
radio galaxy cores. From these samples, we first examine the correlation
between the luminosities at 5 GHz, L_{5GHz}, and at 0.1-10 GeV, L_{gamma}, of
these gamma-ray loud radio galaxies. We find that the correlation is
significant with L_{gamma} \propto L_{5GHz}^{1.16} based on a partial
correlation analysis. Using this correlation and the radio luminosity function
(RLF) of radio galaxies, we further explore the contribution of gamma-ray loud
radio galaxies to the unresolved extragalactic gamma-ray background (EGRB). The
gamma-ray luminosity function is obtained by normalizing the RLF to reproduce
the source count distribution of the Fermi gamma-ray loud radio galaxies. We
find that gamma-ray loud radio galaxies will explain ~25% of the unresolved
Fermi EGRB flux above 100 MeV and will also make a significant contribution to
the EGRB in the 1-30 MeV energy band. Since blazars explain 22% of the EGRB
above 100 MeV, radio loud active galactic nuclei (AGNs) population explains
~47% of the unresolved EGRB. We further make an interpretation on the origin of
the EGRB. The observed EGRB spectrum at 0.2-100 GeV does not show an absorption
signature by the extragalactic background light. Thus, the dominant population
of the origin of EGRB at very high energy (>30 GeV) might be nearby gamma-ray
emitting sources or sources with very hard gamma-ray spectrum.
We present new theoretical estimates of the relative contributions of
unresolved blazars and star-forming galaxies to the extragalactic gamma-ray
background (EGB) and discuss constraints on the contributions from alternative
mechanisms such as dark matter annihilation and truly diffuse gamma-ray
production. We find that the Fermi source count data do not rule out a scenario
in which the EGB is dominated by emission from unresolved blazars, though
unresolved star-forming galaxies may also contribute significantly to the
background, within order-of-magnitude uncertainties. In addition, we find that
the spectrum of the unresolved star-forming galaxy contribution cannot explain
the EGB spectrum found by EGRET at energies between 50 and 200 MeV, whereas the
spectrum of unresolved FSRQs, when accounting for the energy-dependent effects
of source confusion, could be consistent with the combined spectrum of the
low-energy EGRET EGB measurements and the Fermi-LAT EGB measurements.
We examine the constraints on the luminosity-dependent density evolution
model for the evolution of blazars given the observed spectrum of the diffuse
gamma-ray background (DGRB), blazar source-count distribution, and the blazar
spectral energy distribution sequence model, which relates the observed the
blazar spectrum to its luminosity. We show that the DGRB observed by the Large
Area Telescope (LAT) aboard the Fermi Gamma Ray Space Telescope can be produced
entirely by gamma-ray emission from blazars and nonblazar active galactic
nuclei, and that our blazar evolution model is consistent with and constrained
by the spectrum of the DGRB and flux source-count distribution function of
blazars observed by Fermi-LAT. Our results are consistent with previous work
that used EGRET spectral data to forecast the Fermi-LAT DGRB. The model
includes only three free parameters, and forecasts that >~ 95% of the flux from
blazars will be resolved into point sources by Fermi-LAT with 5 years of
observation, with a corresponding reduction of the flux in the DGRB by a factor
of ~2 to 3 (95% confidence level), which has implications for the Fermi-LAT's
sensitivity to dark matter annihilation photons.
We investigate the cosmological evolution of the hard X-ray luminosity function (HXLF) of Active Galactic Nuclei (AGN) in the 2-10 keV luminosity range of 10(41.5) - 10(46.5) erg s(-1) as a function of redshift up to 3, utilizing a highly complete sample consisting of 247 hard X-ray selected AGNs. We find that (i) the fraction of X-ray absorbed AGNs decreases with the intrinsic luminosity and (ii) the evolution of the HXLF of the whole AGNs is best described with a luminosity dependent density evolution where the cutoff redshift increases with the luminosity. Our results directly constrain the evolution of AGNs that produce a major part of the hard X-ray background (XRB), thus solving its origin quantitatively. Based on these results, we discuss the growth history of supermassive black holes (SMBHs) in galactic centers.
As very high energy (VHE) photons propagate through the extragalactic
background light (EBL), they interact with the soft photons of the EBL
and initiate electromagnetic cascades of photons and electrons. The
collective intensity of a cosmological population emitting at VHEs (such
as blazars) will be attenuated at the highest energies through
interactions with the EBL and enhanced at lower energies by the
resulting cascade. As such, depending on the space density and spectra
of the sources and the model of the EBL, cascade radiation can provide a
significant contribution to the extragalactic gamma-ray background
(EGB). Through deflections of the charged particles of the cascade, an
intergalactic magnetic field (IGMF) may leave an imprint on the
anisotropy properties of the EGB. The impact of a strong IGMF is to
isotropize lower energy cascade photons, inducing a modulation in the
anisotropy energy spectrum of the EGB. We discuss the implications of
cascade radiation for the origins of the EGB and the nature of the IGMF,
as well as insight that will be provided by data from the Fermi Large
Area Telescope in the upcoming years.
The origin of the extragalactic gamma-ray background is a pressing cosmological mystery. The Fermi Gamma-Ray Space Telescope has recently measured the intensity and spectrum of this background; both are substantially different from previous measurements. We present a novel calculation of the gamma-ray background from normal star-forming galaxies. Contrary to longstanding expectations, we find that numerous but individually faint normal galaxies may comprise the bulk of the Fermi signal, rather than rare but intrinsically bright active galaxies. This result has wide-ranging implications, including: the possibility to probe the cosmic star-formation history with gamma rays; the ability to infer the cosmological evolution of cosmic rays and galactic magnetic fields; and an increased likelihood to identify subdominant components from rare sources (e.g., dark matter annihilation).
During the period from the launch of the Compton Observatory in 1991 April through 1993 September, the Energetic Gamma-Ray Experiment Telescope (EGRET) has detected, with high significance (>5σ), 33 blazar-type QSOs which are sources of high-energy gamma-rays and another eight which show strong evidence for high-energy gamma-ray emission. Many of the gamma-ray loud blazars have redshifts z ∼ 1 with the most distant source having a redshift of z = 2.2. The K/Kmax test has been applied to the sample of gamma-ray loud AGNs seen during the phase I and II observing periods, and evidence for the evolution of these sources consistent with pure luminosity evolution is presented. The effects of observational biases due to gamma-ray flux-detection limits and optical identifications are considered. Using the Ve/Va test, we find the parameters describing the evolution, and by de-evolving the sample, we make an estimate of the luminosity function of these objects. Finally, using this luminosity function, we make an estimate of the gamma-ray loud AGN contribution to the diffuse extragalactic emission.
We present a new theoretical estimate of the contribution of unresolved blazars to the extragalactic gamma-ray background (EGB). We find that the Fermi source count data do not rule out a scenario in which the EGB is dominated by emission from unresolved blazars. The spectrum of unresolved FSRQs, when accounting for the energy-dependent effects of source confusion, could be consistent with the combined spectrum of both the low-energy EGRET EGB measurements between 50 and 200 MeV and the Fermi-LAT EGB measurements above 200 MeV.
The Wilkinson Microwave Anisotropy Probe (WMAP) is currently mapping temperature and polarization anisotropies of the cosmic microwave background radiation on the full sky in 5 microwave bands. We summarize major scientific results obtained from the first year data released on February 11, 2003: (1) precision determinations of the cosmological parameters, (2) discovery of early reionization of the universe, and (3) implications for the Inflationary paradigm.
We interpret recent gamma-ray observations of active galactic nuclei (AGNs) made with the Whipple Observatory, Granat and especially the Compton Gamma Ray Observatory. The gamma-ray data show that there are two distinct classes of AGNs defined by their redshift and luminosity distributions and high-energy spectral properties. Sources in the first class, which are generally associated with AGNs classified in other wavelength ranges as Seyferts, have redshifts z ≲ 0.06 and 50-150 keV luminosities in the range 1041-1044 ergs s-1. These sources display spectral softenings at ˜100 keV energies, with no measured emission at photon energies E > several MeV. This class includes radio-quiet AGNs in addition to radio galaxies apparently viewed at large angles with respect to the radio jet axis. The redshifts of objects in the second class, which are associated with AGNs classified as blazars, are as large as z ≅ 2.3, and the range of 100 MeV-5 GeV luminosities, assuming isotropic emission, extends to 1049 ergs s-1. The ≈20 MeV-30 GeV gamma-ray luminosity often dominates the bolometric luminosity in objects of this class. These sources probably represent AGNs that are observed nearly along the axis of a radio jet. Some AGNs show evidence from the high-energy data for transitional behavior between the two classes. We consider whether the qualitatively different properties of the two gamma-ray classes provide evidence for quasi-isotropic emission from the Seyferts and beamed emission from the blazars. Comparison of the observed redshift and luminosity distributions with model distributions derived from a treatment of the cosmological statistics of isotropic and beamed sources gives, however, inconclusive results. We treat gamma- ray transparency arguments for beaming, avoiding earlier unproven assumptions that X-rays and E > 100 MeV gamma rays originate from the same site. The pair-production optical depth of E > 100 MeV gamma rays interacting with other gamma rays is much less than 1 and does not require beaming, but data from OSSE give evidence for beaming in a few blazars. We generalize to the gamma-ray regime the Elliot-Shapiro relation, which is based on the assumption that AGN radiation is isotropically emitted and that the luminosity is generated by Eddington-limited accretion. Available gamma-ray data do not yet demonstrate a strong conflict with this limit. The generalization of the Eddington-luminosity limit to the Klein-Nishina limit suggests, however, a new type of object that can accrete at luminosities much greater than 1046 M8 ergs s-1 by radiating photons at gamma-ray energies. Here M8 is the black hole mass in units of 108 Msun. Beaming arguments from gamma-ray observations require more observations of blazars but superluminal observations probably still provide the most compelling evidence for bulk relativistic motion in blazars.
DOI:https://doi.org/10.1103/PhysRevD.85.109901
It is suggested that the variable radio emission from active galactic
nuclei and quasars originates within a collimated relativistic jet of
the type that has been inferred to supply extended radio sources with
mass, momentum, and energy and is directly observed in several cases. An
idealized description of the steady radio emission from a relativistic
jet is presented, the variable component of the emission is associated
with shock waves traveling in the jet, and the dynamical and radiative
properties of accelerated clouds and of velocity disturbances that
steepen to form propagating shocks are examined. It is shown that
several observed features of compact radio sources can be interpreted on
the assumption that these sources are relativistic jets viewed at small
angles to their axes. Some general observational tests are proposed.
We revisit the concept of a blazar sequence that relates the synchrotron peak frequency (νpeak) in blazars with synchrotron peak luminosity (L peak, in νL ν) using a large sample of radio-loud active galactic nuclei. We present observational evidence that the blazar sequence is formed from two populations in the synchrotron νpeak-L peak plane, each forming an upper edge to an envelope of progressively misaligned blazars, and connecting to an adjacent group of radio galaxies having jets viewed at much larger angles to the line of sight. When binned by jet kinetic power (L kin; as measured through a scaling relationship with extended radio power), we find that radio core dominance decreases with decreasing synchrotron L peak, revealing that sources in the envelope are generally more misaligned. We find population-based evidence of velocity gradients in jets at low kinetic powers (~1042-1044.5 erg s–1), corresponding to Fanaroff-Riley (FR) I radio galaxies and most BL Lac objects. These low jet power "weak-jet" sources, thought to exhibit radiatively inefficient accretion, are distinguished from the population of non-decelerating, low synchrotron-peaking (LSP) blazars and FR II radio galaxies ("strong" jets) which are thought to exhibit radiatively efficient accretion. The two-population interpretation explains the apparent contradiction of the existence of highly core-dominated, low-power blazars at both low and high synchrotron peak frequencies, and further implies that most intermediate synchrotron peak sources are not intermediate in intrinsic jet power between LSP and high synchrotron-peaking (HSP) sources, but are more misaligned versions of HSP sources with similar jet powers.
We examine the properties of the gamma-ray active galactic nuclei (AGNs) detected by EGRET. We calculate the evolution and luminosity function of the gamma-ray-loud AGNs, and we estimate the contribution this source class makes to the diffuse extragalactic gamma-ray background. These calculations are based on a sample of EGRET observations complete through the Compton Gamma-Ray Observatory observing Cycle 4. We use the Ve/Va test to measure the luminosity evolution and the nonparametric c--method to estimate the de-evolved luminosity function. We also consider inclusion in the 1 Jy radio catalog of Kühr et al. (1981) as an additional criterion to help guard against selection effects resulting from incomplete redshift determinations for AGNs in our sample. Our results include evidence for a low-luminosity cutoff in the gamma-ray AGN luminosity distribution, a more precise estimate of the contribution of these objects to the diffuse extragalactic gamma-ray background, and constraints on the gamma-ray production mechanism based upon the statistics implied by relativistic beaming of the emission. We discuss the implications of these findings for models of AGN unification and the relationship of gamma-ray to radio emission.
A study of the statistics of cosmological black hole jet sources is applied to EGRET blazar data and predictions are made for GLAST. Black hole jet sources are modeled as collimated relativistic plasma outflows with radiation beamed along the jet axis due to strong Doppler boosting. The comoving rate density of blazar flares is assumed to follow a blazar formation rate (BFR), modeled by analytic functions based on astronomical observations and fits to EGRET data. The redshift and size distributions of γ-ray blazars observed with EGRET, separated into BL Lac objects (BLs) and flat spectrum radio quasar (FSRQ) distributions, are fit with monoparametric functions for the distributions of the jet Lorentz factor Γ, comoving directional power l, and spectral slope. A BFR factor ≈10× greater at z 1 than at present is found to fit the FSRQ data. A smaller comoving rate density and greater luminosity of BL flares at early times compared to the present epoch fits the BL data. Based on the EGRET observations, ≈1000 blazars consisting of ≈800 FSRQs and FR 2 radio galaxies and ≈200 BL Lac objects and FR 1 radio galaxies will be detected with GLAST during the first year of the mission. Additional AGN classes, such as hard-spectrum BL Lac objects that were mostly missed with EGRET, could add more GLAST sources. The FSRQ and BL contributions to the EGRET γ-ray background at 1 GeV are estimated at the level of ≈10%-15% and ≈2%-4%, respectively. EGRET and GLAST sensitivities to blazar flares are considered in the optimal case, and a GLAST analysis method for blazar detection is outlined.
We investigate the cosmological evolution of the hard X-ray luminosity function (HXLF) of active galactic nuclei (AGNs) in the 2-10 keV luminosity range of 1041.5-1046.5 ergs s-1 as a function of redshift up to 3. From a combination of surveys conducted at photon energies above 2 keV with HEAO 1, ASCA, and Chandra, we construct a highly complete (>96%) sample consisting of 247 AGNs over the wide flux range of 10-10 to 3.8 × 10-15 ergs cm-2 s-1 (2-10 keV). For our purpose, we develop an extensive method of calculating the intrinsic (before absorption) HXLF and the absorption (NH) function. This utilizes the maximum likelihood method, fully correcting for observational biases with consideration of the X-ray spectrum of each source. We find that (1) the fraction of X-ray absorbed AGNs decreases with the intrinsic luminosity and (2) the evolution of the HXLF of all AGNs (including both type I and type II AGNs) is best described with a luminosity-dependent density evolution (LDDE) where the cutoff redshift increases with the luminosity. Our results directly constrain the evolution of AGNs that produce a major part of the hard X-ray background, thus solving its origin quantitatively. A combination of the HXLF and the NH function enables us to construct a purely "observation-based" population synthesis model. We present basic consequences of this model and discuss the contribution of Compton-thick AGNs to the rest of the hard X-ray background.
We present a calculation of the blazar contribution to the extragalactic diffuse γ-ray background (EGRB) in the EGRET energy range. Our model is based on inverse-Compton scattering as the dominant γ-ray production process in the jets of flat spectrum radio quasars (FSRQs) and BL Lac objects, and on the unification scheme of radio-loud AGN. According to this picture, blazars represent the beamed fraction of the Fanaroff—Riley radio galaxies (FR galaxies).
The observed log N-log S distribution and redshift distribution of both FSRQs and BL Lacs constrain our model. Depending slightly on the evolutionary behaviour of blazars, we find that unresolved AGN underproduce the intensity of the extragalactic background radiation. With our model only 20–40 per cent of the extragalactic background emission can be explained by unresolved blazars if we integrate to a maximum redshift of Zmax=3. For Zmax=5, blazars could account for 40–80 per cent of the EGRB. Roughly 70–90 per cent of the AGN contribution to the EGRB would result from BL Lacs. While the systematic uncertainties in our estimate for the FSRQ contribution appear small, in the case of BL Lacs our model parameters are not consistent with the results from studies in other wavelength regimes, and therefore may have larger systematic uncertainties. Thus we end up with two possibilities, depending on whether we underpredict or overpredict the BL Lac contribution: either unresolved AGN cannot account for the entire EGRB, or unresolved BL Lacs produce the observed background.
We predict a significant flattening of the γ-ray log N-log S function in the next two decades of flux below the EGRET threshold.
We quantify the importance of mass accretion during active galactic nuclei (AGN) phases in the growth of supermassive black holes (BHs) by comparing the mass function of black holes in the Local Universe with that expected from AGN relics, which are black holes grown entirely with mass accretion during AGN phases. The local BH mass function (BHMF) is estimated by applying the well-known correlations between BH mass, bulge luminosity and stellar velocity dispersion to galaxy luminosity and velocity functions. We find that different correlations provide the same BHMF only if they have the same intrinsic dispersion. The density of supermassive black holes in the Local Universe that we estimate is ρBH= 4.6+1.9−1.4h20.7× 105 M⊙ Mpc−3. The relic BHMF is derived from the continuity equation with the only assumption that AGN activity is due to accretion on to massive BHs and that merging is not important. We find that the relic BHMF at z= 0 is generated mainly at z < 3 where the major part of the growth of a BH takes place. Moreover, BH growth is antihierarchical in the sense that smaller BHs (MBH < 107 M⊙) grow at lower redshifts (z < 1) with respect to more massive ones (z∼ 1–3). Unlike previous work, we find that the BHMF of AGN relics is perfectly consistent with the local BHMF, indicating that local BHs were mainly grown during AGN activity. This agreement is obtained while satisfying, at the same time, the constraints imposed from the X-ray background (XRB). The comparison between the local and relic BHMFs also suggests that the merging process is not important in shaping the relic BHMF, at least at low redshifts (z < 3), and allows us to estimate the average radiative efficiency (ɛ), the ratio between emitted and Eddington luminosity (λ) and the average lifetime of active BHs. Our analysis thus suggests the following scenario: local BHs grew during AGN phases in which accreting matter was converted into radiation with efficiencies ɛ= 0.04–0.16 and emitted at a fraction λ= 0.1–1.7 of the Eddington luminosity. The average total lifetime of these active phases ranges from ≃ 4.5 × 108 yr for MBH < 108 M⊙ to ≃ 1.5 × 108 yr for MBH > 109 M⊙, but can become as large as ∼109 yr for the lowest acceptable ɛ and λ values.
We examine the disc–jet connection in stellar mass and supermassive black holes by investigating the properties of their compact emission in the X-ray and radio bands. We compile a sample of ∼100 active galactic nuclei with measured masses, 5-GHz core emission, and 2–10 keV luminosities, together with eight galactic black holes with a total of ∼50 simultaneous observations in the radio and X-ray bands. Using this sample, we study the correlations between the radio (LR) and the X-ray (LX) luminosity and the black hole mass (M). We find that the radio luminosity is correlated with bothM and LX, at a highly significant level. In particular, we show that the sources define a ‘Fundamental Plane’ in the three-dimensional (log LR, log LX, log M) space, given by log LR= (0.60+0.11−0.11) log LX+ (0.78+0.11−0.09) log M+ 7.33+4.05−4.07, with a substantial scatter of σR= 0.88. We compare our results to the theoretical relations between radio flux, black hole mass, and accretion rate derived by Heinz & Sunyaev. Such relations depend only on the assumed accretion model and on the observed radio spectral index. Therefore, we are able to show that the X-ray emission from black holes accreting at less than a few per cent of the Eddington rate is unlikely to be produced by radiatively efficient accretion, and is marginally consistent with optically thin synchrotron emission from the jet. On the other hand, models for radiatively inefficient accretion flows seem to agree well with the data.
[Abridged] While star-forming galaxies could be major contributors to the
cosmic GeV gamma-ray background, they are expected to be MeV-dim because of the
"pion bump" falling off below ~100 MeV. We investigate the MeV background from
star-forming galaxies by running one-zone models of cosmic ray populations,
taking into account the leptonic emission, including Inverse Compton (IC) and
bremsstrahlung, as well as nuclear lines, emission from core collapse
supernovae, and positron annihilation emission, besides the pionic emission. We
use the Milky Way and the GeV-TeV detected starbursts M82 and NGC 253 as
templates of normal and starburst galaxies, and compare our models to radio and
GeV-TeV gamma-ray data. We find that (1) IC losses off the CMB flatten out the
pion bump at high z for normal galaxies, (2) we cannot rule out that starbursts
have significant MeV emission if their magnetic field strengths are low, and
(3) cascades can contribute to the MeV emission of starbursts if they emit
mainly hadronic gamma rays. The star-forming galaxy contribution to the GeV
background is uncertain by an order of magnitude, depending on how much of the
cosmic star-formation is in starbursts. Our fiducial model predicts that ~1/3
of the unresolved GeV background is from star-forming galaxies, with comparable
contributions from normal and starburst galaxies. About ~2% of the claimed 1
MeV background is diffuse emission from star-forming galaxies; we place a firm
upper limit of ~10% contribution based on the requirement that star-forming
galaxies do not overpower the observed gamma-ray background at any energy. The
low star-forming galaxy contribution arises because the observed gamma-ray
background spectrum steeply falls with energy, while the star-forming
contribution slowly increases with energy in the MeV range. A different source
class must emit the observed MeV background, if it is real.
Fermi has resolved several star-forming galaxies, but the vast majority of the star-forming universe is unresolved, and thus contributes to the extragalactic gamma-ray background (EGB). Here, we calculate the contribution of star-forming galaxies to the EGB in the Fermi range from 100 MeV to 100 GeV due to inverse-Compton (IC) scattering of the interstellar photon field by cosmic-ray electrons. We first construct one-zone models for individual star-forming galaxies assuming that supernovae power the acceleration of cosmic rays. We develop templates for both normal and starburst galaxies, accounting for differences in the cosmic-ray electron propagation and in the interstellar radiation fields. For both types of star-forming galaxies, the same IC interactions leading to gamma rays also substantially contribute to the energy loss of the high-energy cosmic-ray electrons. Consequently, a galaxy's IC emission is determined by the relative importance of IC losses in the cosmic-ray electron energy budget ("partial calorimetry"). We calculate the cosmological contribution of star-forming galaxies to the EGB using our templates and the cosmic star formation rate distribution. For all of our models, we find that the IC EGB contribution is almost an order of magnitude less than the peak of the emission due to cosmic-ray ion interactions (mostly pionic p
crp
ism → π0 → γγ); even at the highest Fermi energies, IC is subdominant. The flatter IC spectrum increases the high-energy signal of the pionic+IC sum, bringing it closer to the EGB spectral index observed by Fermi. Partial calorimetry ensures that the overall IC signal is relatively well constrained, with only uncertainties in the amplitude and spectral shape for plausible model choices. We conclude with a brief discussion on how the pionic spectral feature and other methods can be used to measure the star-forming component of the EGB.
We show the conclusions claimed in the manuscript arXiv:1202.5309v1 by Cuoco,
Komatsu and Siegal-Gaskins (CKS) are not generally valid. The results in CKS
are based on a number of simplifying assumptions regarding the source
population below the detection threshold and the threshold flux itself, and do
not apply to many physical models of the blazar population. Physical blazar
population models that match the measured source counts above the observational
threshold can account for 60% of the diffuse gamma-ray background intensity
between 1-10 GeV, while the assumptions in CKS limit the intensity to <30%. The
shortcomings of the model considered in CKS arise from an over-simplified
blazar source model. A number of the simplifying assumptions are unjustified,
including: first, the adoption of an assumed power-law source-count
distribution, dN/dS, to arbitrary low source fluxes, which is not exhibited in
physical models of the blazar population; and, second, the lack of blazar
spectral information in calculating the anisotropy of unresolved gamma-ray
blazar emission. We also show that the calculation of the unresolved blazars'
anisotropy is very sensitive to the spectral distribution of the unresolved
blazars through the adopted source resolution threshold value, and must be
taken into account in an accurate anisotropy calculation.
We report on the first Fermi Large Area Telescope (LAT) measurements of the so-called "extra-galactic" diffuse gamma-ray emission (EGB). This component of the diffuse gamma-ray emission is generally considered to have an isotropic or nearly isotropic distribution on the sky with diverse contributions discussed in the literature. The derivation of the EGB is based on detailed modeling of the bright foreground diffuse Galactic gamma-ray emission, the detected LAT sources, and the solar gamma-ray emission. We find the spectrum of the EGB is consistent with a power law with a differential spectral index gamma = 2.41 +/- 0.05 and intensity I(>100 MeV) = (1.03 +/- 0.17) x 10(-5) cm(-2) s(-1) sr(-1), where the error is systematics dominated. Our EGB spectrum is featureless, less intense, and softer than that derived from EGRET data.
The second catalog of active galactic nuclei (AGNs) detected by the Fermi
Large Area Telescope (LAT) in two years of scientific operation is presented.
The Second LAT AGN Catalog (2LAC) includes 1017 gamma-ray sources located at
high Galactic latitudes (|b|>10{\deg}) that are detected with a test statistic
greater than 25 and associated statistically with AGNs. However some of these
are affected by analysis issues and some are associated with multiple AGNs.
Consequently we define a clean sample which includes 886 AGNs, comprising 395
BL Lacertae objects (BL Lacs), 310 flat-spectrum radio quasars (FSRQs), 157
candidate blazars of unknown type (i.e., with broad-band blazar characteristics
but with no optical spectral measurement yet), eight misaligned AGNs, four
narrow-line Seyfert 1 (NLS1s), 10 AGNs of other types and two starburst
galaxies. Where possible, the blazars have been further classified based on
their spectral energy distributions (SEDs) as archival radio, optical, and
X-ray data permit. While almost all FSRQs have a synchrotron-peak frequency
10^14 Hz, about half of the BL Lacs have a synchrotron-peak frequency >10^15
Hz. The 2LAC represents a significant improvement relative to the First LAT AGN
Catalog (1LAC), with 52% more associated sources. The full characterization of
the newly detected sources will require more broad-band data. Various
properties, such as gamma-ray fluxes and photon power law spectral indices,
redshifts, gamma-ray luminosities, variability, and archival radio
luminosities---and their correlations are presented and discussed for the
different blazar classes. The general trends observed in 1LAC are confirmed.
An analytic relation between the statistics of photons in pixels and the
number counts of multi-photon point sources is used to constrain the
distribution of gamma-ray point sources below the Fermi detection limit at
energies above 1 GeV and at latitudes below and above 30 degrees. The derived
source-count distribution is consistent with the distribution found by the
Fermi collaboration based on the first Fermi point source catalogue. In
particular, we find that the contribution of resolved and unresolved active
galactic nuclei (AGN) to the total gamma-ray flux is below 20% - 25%. In the
best fit model, the AGN-like point source fraction is 17% +- 2%. Using the fact
that the Galactic emission varies across the sky while the extra-galactic
diffuse emission is isotropic, we put a lower limit of 51% on Galactic diffuse
emission and an upper limit of 32% on the contribution from extra-galactic weak
sources, such as star-forming galaxies. Possible systematic uncertainties are
discussed.
In this work, starting from 21 months of data from the Fermi-Large Area
Telescope, we derive maps of the residual isotropic gamma-ray emission, a
relevant fraction of which is expected to be contributed by the extragalactic
diffuse gamma-ray background. We compute the angular two-point auto-correlation
function of the residual Fermi-LAT maps at energies E>1GeV, E>3GeV and E>30GeV
well above the Galactic plane and find no significant correlation signal. This
is, indeed, what is expected if the EGB were contributed by BL Lacertae, Flat
Spectrum Radio Quasars or star-forming galaxies, since, in this case, the
predicted signal is very weak. Then, we search for the Integrated Sachs-Wolfe
signature by cross-correlating the Fermi-LAT maps with the WMAP7-Cosmic
Microwave Background map. We find a cross-correlation consistent with zero,
even though the expected signal is larger than that of the EGB
auto-correlation. Finally, in an attempt to constrain the nature of the
gamma-ray background we cross-correlate the Fermi-LAT maps with the angular
distributions of objects that may contribute to the EGB: QSOs in the SDSS-DR6
catalog, NVSS galaxies, 2MASS galaxies and LRG in the SDSS catalog. The
cross-correlation is always consistent with zero, in agreement with theoretical
expectations, but we find (with low statistical significance) some interesting
features that may indicate that some specific classes of objects contribute to
the EGB. A chi2 analysis confirms that the correlation properties of the
21-month data do not provide strong constraints of the EGB origin. However, the
results suggest that the situation will significantly improve with the 5- and
10-year Fermi-LAT data. The future EGB analysis will then allow placing
significant constraints on the nature of the EGB and might provide in addition
a detection of the ISW signal. (Abridged)