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Spectral Properties of Bright Fermi-detected Blazars in the Gamma-ray Band
Abstract
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 (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 six month 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 gives the most acceptable of power law, broken power law, and curved forms. The consequences of these findings are discussed. Comment: 15 figures, 2 tables, accepted by The Astrophysical Journal
... A shift in these peaks with blazar luminosity indicates that the gamma-ray spectrum becomes harder for lower luminosity blazars. This has been observed by noting that FSRQs have high luminosity and soft spectra, while BL Lacs have harder spectra with lower luminosities [38] [39] [40] [41]. The importance of this spectral hardening at low luminosities cannot be overstated: the gamma-ray spectra of unresolved, lower-luminosity blazars should be harder than the spectra of resolved blazars. ...
... Because the luminosity integral diverges if γ 1 > 1, we place a lower bound of L γ,min = 10 42 erg s −1 on this integral. This is an order of magnitude lower luminosity than any Fermi-LAT observed blazar [39] [40]. That is, we impose a step-function cutoff of blazar GLF at L γ,min . ...
... It was, however, shown in Refs. [39] [40] that a single power-law spectrum does a very poor job of fitting individual blazars, which display curvature and breaks in their spectra that are correlated with their total luminosity. Ref. [2] [3] [4] show that blazar gamma-ray spectra are soft for high-luminosity blazars and get much harder for lowluminosity blazars, and the wide variation of blazar spectra has been confirmed by Fermi- LAT [40]. ...
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 energy of the break varied by no more than a factor of ∼ 3, while the flux varied by as much as a factor of 10 [7]. This spectral curvature has been found in other blazars as well, although a broken power-law is not always preferred over a log-parabola fit, which has one less free parameter [4]. The spectral break (curvature) is not always present in flaring states of 3C 454.3 and other blazars [116]. ...
Blazars are active galactic nuclei with relativistic jets pointed at the Earth, making them extremely bright at essentially all wavelengths, from radio to gamma rays. I review the modeling of this broadband spectral energy distributions of these objects, and what we have learned, with a focus on gamma rays.
... They assume this extrapolation to be an upper limit to the intrinsic TeV spectrum thus giving them a strict upper limit to the EBL absorption. Another technique is to examine the nature of the connection between the HE spectrum of a source to its VHE spectrum [6, 31, 32], namely, whether or not a single power law is justified or if a broken power law model should be invoked. Other attempts, including work by [33], use a spline method to adjust the shape of the EBL such that when they deabsorb a TeV blazar, the resulting intrinsic spectrum is physically feasible. ...
Gamma-rays propagating distances on the scale of clusters of galaxies are
likely to be extinguished via electron-positron pair production off of the
ambient extragalactic background light (EBL). The spectrum of the EBL is
produced by starlight (and starlight reprocessed by dust) from these clusters.
The attenuation of 40 - 400 GeV gamma-rays has been observed by \textit{Fermi}
and used to measure the EBL spectrum over energies 1 eV -10 eV out to redshift
$z\sim 1$. Measurements of several TeV blazers are consistent with attenuation,
attributed to the EBL at redshift $z\sim 0.1$. Here we simultaneously analyze a
set of TeV blazers at $z\sim 0.1$ to measure the optical depth for 100 GeV - 10
TeV gamma-rays, which interact with EBL of energies 0.05 eV - 5 eV. Using a
suite of models for the EBL, we show that the optical depth indicated by TeV
blazar attenuation is in good agreement with the optical depths measured by
\textit{Fermi} at lower gamma-ray energies and higher redshifts.
... The high photon statistics from the brightest sources allows also to study their spectra with unprecedented accuracy. These studies have shown that the spectra of high-luminosity sources, flat-spectra radio quasars (FSRQs) and low-energy synchrotron peaked BL Lac objects, are much better described by a broken power law than by a simple powerlaw or any smoothly curved models [1] [3] (see Fig. 1a). The works dedicated to 3C 454.3 [5] [4] found the spectral hardness–flux correlation at a nearly constant break energy. ...
The brightest blazars detected by the Fermi Gamma-ray Space Telescope Large
Area Telescope (Fermi/LAT) show significant breaks in their spectra at a few
GeV. The sharpness and the position of the breaks can be well reproduced by
absorption of $\gamma$-rays via photon--photon pair production on He ii and H i
Lyman recombination continua (LyC) produced in the broad-line region (BLR).
Using 138 weeks of LAT observations of the brightest GeV blazar 3C 454.3 we
find a power-law dependence of the peak energy on flux and discover
anti-correlation between flux and the column density of the He ii LyC which is
responsible for absorption of the >2.5 GeV photons in this object. The strength
and the variability of the absorption implies the location of the gamma-ray
emitting zone close to the boundary of the high-ionization part of the BLR and
moving away from the black hole when the flux increases. A combination of the
GeV breaks with the detection of a few powerful blazars in the TeV range puts
strong constraints on the BLR size. Additional spectral breaks at ~100 and ~400
GeV due to absorption by the Balmer and Paschen lines could be detected by the
Cherenkov Telescope Array.
The past decade has seen a dramatic improvement in the quality of data
available at both high (HE: 100 MeV to 100 GeV) and very high (VHE: 100 GeV to
100 TeV) gamma-ray energies. With three years of data from the Fermi Large Area
Telescope (LAT) and deep pointed observations with arrays of Cherenkov
telescope, continuous spectral coverage from 100 MeV to $\sim10$ TeV exists for
the first time for the brightest gamma-ray sources. The Fermi-LAT is likely to
continue for several years, resulting in significant improvements in high
energy sensitivity. On the same timescale, the Cherenkov Telescope Array (CTA)
will be constructed providing unprecedented VHE capabilities. The optimisation
of CTA must take into account competition and complementarity with Fermi, in
particularly in the overlapping energy range 10$-$100 GeV. Here we compare the
performance of Fermi-LAT and the current baseline CTA design for steady and
transient, point-like and extended sources.
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