June 2015
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1,629 Reads
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2 Citations
The Astrophysical Journal
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June 2015
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1,629 Reads
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2 Citations
The Astrophysical Journal
January 2014
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141 Reads
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3 Citations
Astronomy and Astrophysics
Previously unremarkable, the extragalactic radio source GB 1310+487 showed a gamma-ray flare on 2009 November 18, reaching a daily flux of ~10^-6 photons/cm^2/s at energies E>100 MeV and becoming one of the brightest GeV sources for about two weeks. Its optical spectrum is not typical for a blazar, instead, it resembles those of narrow emission-line galaxies. We investigate changes of the object's radio-to-GeV spectral energy distribution (SED) during and after the prominent GeV flare with the aim to determine the nature of the object and constrain the origin of the variable high-energy emission. The data collected by the Fermi and AGILE satellites at gamma-ray energies, Swift at X-ray and ultraviolet, Kanata, NOT, and Keck telescopes at optical, OAGH and WISE at infrared, and IRAM 30m, OVRO 40m, Effelsberg 100m, RATAN-600, and VLBA at radio, are analysed together to trace the SED evolution on timescales of months. The gamma-ray/radio-loud narrow-line active galactic nucleus (AGN) is located at redshift z=0.638. It is shining through an unrelated foreground galaxy at z=0.500. The AGN light is likely amplified by a factor of a few because of gravitational lensing. The AGN SED shows a two-humped structure typical of blazars and gamma-ray-loud NLSy1 galaxies, with the high-energy (inverse-Compton) emission dominating by more than an order of magnitude over the low-energy (synchrotron) emission during gamma-ray flares. The difference between the two SED humps is smaller during the low-activity state. Fermi observations reveal a strong correlation between the gamma-ray flux and spectral index, with the hardest spectrum observed during the brightest gamma-ray state. If the gamma-ray flux is a mixture of synchrotron self-Compton (SSC) and external Compton (EC) emission, the observed GeV spectral variability may result from varying relative contributions of these two emission components.
September 2013
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109 Reads
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274 Citations
The Astrophysical Journal
Fermi has provided the largest sample of gamma-ray selected blazars to date. In this work we use a uniformly selected set of 211 BL Lacertae (BL Lac) objects detected by it Fermi during its first year of operation. We have obtained redshift constraints for 206 out of the 211 BL Lacs in our sample making it the largest and most complete sample of BL Lacs available in the literature. We use this sample to determine the luminosity function of BL Lacs and its evolution with cosmic time. We find that for most BL Lac classes, the evolution is positive with a space density peaking at modest redshift (z~1.2). The low-luminosity, high-synchrotron peaked (HSP) BL Lacs are an exception, showing strong negative evolution, with number density increasing for z$\lesssim$0.5. Since this rise corresponds to a drop-off in the density of flat-spectrum radio quasars (FSRQs), a possible interpretation is that these HSPs represent an accretion-starved end-state of an earlier merger-driven gas-rich phase. We additionally find that the known BL Lac correlation between luminosity and photon spectral index persists after correction for the substantial observational selection effects with implications for the so called `blazar sequence'. Finally, estimating the beaming corrections to the luminosity function, we find that BL Lacs have an average Lorentz factor of $\gamma=6.1^{+1.1}_{-0.8}$, and that most are seen within 10$^{\circ}$ of the jet axis.
January 2013
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48 Reads
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226 Citations
The Astrophysical Journal
We report on spectroscopic observations covering most of the 475 BL Lacs in the 2nd Fermi LAT catalog of AGN. Including archival measurements (correcting several erroneous literature values) we now have spectroscopic redshifts for 44% of the BL Lacs. We establish firm lower redshift limits via intervening absorption systems and statistical lower limits via searches for host galaxies for an additional 51% of the sample leaving only 5% of the BL Lacs unconstrained. The new redshifts raise the median spectroscopic z from 0.23 to 0.33 and include redshifts as large as z=2.471. Spectroscopic redshift minima from intervening absorbers have ~ z= 0.70, showing a substantial fraction at large z and arguing against strong negative evolution. We find that detected BL Lac hosts are bright ellipticals with black hole masses M_\bullet ~ 10^{8.5-9}, substantially larger than the mean of optical AGN and LAT Flat Spectrum Radio Quasar samples. A slow increase in M_\bullet with z may be due to selection bias. We find that the power-law dominance of the optical spectrum extends to extreme values, but this does not strongly correlate with the gamma-ray properties, suggesting that strong beaming is the primary cause of the range in continuum dominance.
June 2012
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460 Reads
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124 Citations
The Astrophysical Journal
The Fermi Large Area Telescope (LAT) First Source Catalog (1FGL) provided spatial, spectral, and temporal properties for a large number of γ-ray sources using a uniform analysis method. After correlating with the most-complete catalogs of source types known to emit γ rays, 630 of these sources are "unassociated" (i.e., have no obvious counterparts at other wavelengths). Here, we employ two statistical analyses of the primary γ-ray characteristics for these unassociated sources in an effort to correlate their γ-ray properties with the active galactic nucleus (AGN) and pulsar populations in 1FGL. Based on the correlation results, we classify 221 AGN-like and 134 pulsar-like sources in the 1FGL unassociated sources. The results of these source "classifications" appear to match the expected source distributions, especially at high Galactic latitudes. While useful for planning future multiwavelength follow-up observations, these analyses use limited inputs, and their predictions should not be considered equivalent to "probable source classes" for these sources. We discuss multiwavelength results and catalog cross-correlations to date, and provide new source associations for 229 Fermi-LAT sources that had no association listed in the 1FGL catalog. By validating the source classifications against these new associations, we find that the new association matches the predicted source class in ~80% of the sources.
April 2012
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106 Reads
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40 Citations
The Astrophysical Journal
Whether a correlation exists between the radio and gamma-ray flux densities of blazars is a long-standing question, and one that is difficult to answer confidently because of various observational biases which may either dilute or apparently enhance any intrinsic correlation between radio and gamma-ray luminosities. We introduce a novel method of data randomization to evaluate quantitatively the effect of these biases and to assess the intrinsic significance of an apparent correlation between radio and gamma-ray flux densities of blazars. The novelty of the method lies in a combination of data randomization in luminosity space (to ensure that the randomized data are intrinsically, and not just apparently, uncorrelated) and significance assessment in flux space (to explicitly avoid Malmquist bias and automatically account for the limited dynamical range in both frequencies). The method is applicable even to small samples that are not selected with strict statistical criteria. For larger samples we describe a variation of the method in which the sample is split in redshift bins, and the randomization is applied in each bin individually; this variation is designed to yield the equivalent to luminosity-function sampling of the underlying population in the limit of very large, statistically complete samples. We show that for a smaller number of redshift bins, the method yields a worse significance, and in this way it is conservative in that it does not assign a stronger, artificially enhanced significance. We demonstrate how our test performs as a function of number of sources, strength of correlation, and number of redshift bins used, and we show that while our test is robust against common-distance biases and associated false positives for uncorrelated data, it retains the power of other methods in rejecting the null hypothesis of no correlation for correlated data.
March 2012
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79 Reads
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217 Citations
The Astrophysical Journal
We report on optical spectroscopy of 165 Flat Spectrum Radio Quasars (FSRQs) in the Fermi 1LAC sample, which have helped allow a nearly complete study of this population. Fermi FSRQ show significant evidence for non-thermal emission even in the optical; the degree depends on the gamma-ray hardness. They also have smaller virial estimates of hole mass than the optical quasar sample. This appears to be largely due to a preferred (axial) view of the gamma-ray FSRQ and non-isotropic (H/R ~ 0.4) distribution of broad-line velocities. Even after correction for this bias, the Fermi FSRQ show higher mean Eddington ratios than the optical population. A comparison of optical spectral properties with Owens Valley Radio Observatory radio flare activity shows no strong correlation.
August 2011
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245 Reads
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117 Citations
The Astrophysical Journal
We present a detailed statistical analysis of the correlation between radio and gamma-ray emission of the Active Galactic Nuclei (AGN) detected by Fermi during its first year of operation, with the largest datasets ever used for this purpose. We use both archival interferometric 8.4 GHz data (from the VLA and ATCA, for the full sample of 599 sources) and concurrent single-dish 15 GHz measurements from the Owens Valley Radio Observatory (OVRO, for a sub sample of 199 objects). Our unprecedentedly large sample permits us to assess with high accuracy the statistical significance of the correlation, using a surrogate-data method designed to simultaneously account for common-distance bias and the effect of a limited dynamical range in the observed quantities. We find that the statistical significance of a positive correlation between the cm radio and the broad band (E>100 MeV) gamma-ray energy flux is very high for the whole AGN sample, with a probability <1e-7 for the correlation appearing by chance. Using the OVRO data, we find that concurrent data improve the significance of the correlation from 1.6e-6 to 9.0e-8. Our large sample size allows us to study the dependence of correlation strength and significance on specific source types and gamma-ray energy band. We find that the correlation is very significant (chance probability <1e-7) for both FSRQs and BL Lacs separately; a dependence of the correlation strength on the considered gamma-ray energy band is also present, but additional data will be necessary to constrain its significance.
May 2011
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521 Reads
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518 Citations
The Astrophysical Journal Supplement Series
The Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope provides an unprecedented opportunity to study gamma-ray blazars. To capitalize on this opportunity, beginning in late 2007, about a year before the start of LAT science operations, we began a large-scale, fast-cadence 15 GHz radio monitoring program with the 40 m telescope at the Owens Valley Radio Observatory. This program began with the 1158 northern (δ > –20°) sources from the Candidate Gamma-ray Blazar Survey and now encompasses over 1500 sources, each observed twice per week with about 4 mJy (minimum) and 3% (typical) uncertainty. Here, we describe this monitoring program and our methods, and present radio light curves from the first two years (2008 and 2009). As a first application, we combine these data with a novel measure of light curve variability amplitude, the intrinsic modulation index, through a likelihood analysis to examine the variability properties of subpopulations of our sample. We demonstrate that, with high significance (6σ), gamma-ray-loud blazars detected by the LAT during its first 11 months of operation vary with almost a factor of two greater amplitude than do the gamma-ray-quiet blazars in our sample. We also find a significant (3σ) difference between variability amplitude in BL Lacertae objects and flat-spectrum radio quasars (FSRQs), with the former exhibiting larger variability amplitudes. Finally, low-redshift (z < 1) FSRQs are found to vary more strongly than high-redshift FSRQs, with 3σ significance. These findings represent an important step toward understanding why some blazars emit gamma-rays while others, with apparently similar properties, remain silent.
January 2011
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12 Reads
Since 2007, the Owens Valley Radio Observatory (OVRO) 40 meter telescope has been engaged in an intensive fast-cadence gamma-ray blazar monitoring program, observing about 1500 objects twice per week. Using our intrinsic modulation index method and careful likelihood analyses, we find that gamma-ray loud objects associated with Fermi 1LAC sources in our sample demonstrate radio variability amplitudes significantly larger than do gamma-ray quiet objects. We also find significant differences in variability amplitude between flat spectrum radio quasars and BL Lacertae objects within our sample as well as possible evidence for cosmological evolution of variability amplitude.
... A detailed analysis of ongoing X-ray follow-up observations of the region will be presented in A.B.P. et al. (manuscript in preparation). Similarly, no sources are reported at the location of M81 in any of the Fermi-LAT catalogues 21 . The nearest catalogued source (at a separation of 52′) is 4FGL J0955.7 + 6940, known to be associated with M82 22 . ...
May 2010
The Astrophysical Journal
... It was listed as a gamma-ray emitter in the first Fermi-LAT bright catalog in 2009, where it was classified as a potential TeV emitter. It was first detected at TeV energies by the MAGIC experiment in 2011 after triggering an optical high flux state [119,120] and was detected later by the VERITAS experiment as well [121]. ...
November 2009
... We excluded sources that had viable active galactic nucleus (AGN) counterparts (Abdo et al. 2010c) and sources with statistically significant variability as defined in 1FGL, as pulsars had long been recognized as a nonvariable γ-ray population (e.g., McLaughlin et al. 1996; however, see Roberts et al. 2002b;Mayer et al. 2013;Stappers et al. 2014 for discussions of γ-ray variability from pulsar wind and accretion flows; and Allafort et al. 2013 for the discovery of a rare example of γ-ray pulsar variability). We also excluded sources in the Fermi-LAT Bright Source List (Abdo et al. 2009c) since those had largely been surveyed with the GBT at 820 MHz by Ransom et al. (2011). There is no clear relation between γ-ray and radio pulsed flux (3PC), which likely depends to a large degree on beaming geometry, and so targeting a fainter γ-ray population would not be expected to yield a smaller (or larger) fraction of radio pulsar counterparts. ...
July 2009
The Astrophysical Journal
... Generally speaking, the low-energy hump is caused by synchrotron radiation of relativistic electrons moving in the magnetic field (Marscher and Gear 1985). Based on the peak frequency of the low-energy hump, blazars are divided into low-synchrotron-peaked (LSP; i.e., ν S p < 10 14 Hz), intermediate-synchrotron-peaked (ISP; i.e., 10 14 < ν S p < 10 15 Hz), high-synchrotron-peaked (HSP; i.e., ν S p > 10 15 Hz), and extreme high-synchrotron-peaked (EHSP; i.e., ν S p > 10 17 Hz) blazars (Padovani and Giommi 1995;Costamante et al. 2001;Abdo, Ackermann, Agudo, et al. 2010a). In the leptonic model, the high-energy hump is attributed to the inverse Compton scattering (IC) from the same population of relativistic electrons that emit the synchrotron emission. ...
May 2010
The Astrophysical Journal
... The Fermi-LAT covers an energy range from ∼ 100 MeV to >300 GeV. It is a pair-conversion telescope with a large field of view and an angular resolution of ∼ 0.6 • at 1 GeV (Abdo et al. 2009;Atwood et al. 2009). The publicly available data was accessed via the new Fermi LAT Light Curve Repository (LCR) database (Abdollahi et al. 2023a). ...
May 2009
The Astrophysical Journal
... Utilizing various methods Shaw et al. 2013;Ajello et al. 2014;Shaw et al. 2014;Kaur et al. 2017), the redshifts of ∼200 brightest BL Lac objects detected by the Fermi Large Area Telescope (LAT; The Fermi-LAT collaboration 2019) have been obtained. Sixteen BL Lac objects were found to be at high redshifts z > 1 Kaur et al. 2017), which indicates that their luminosity is comparable to FSRQs (e.g., bolometric luminosity, L bol > 10 46 erg s −1 (Ghisellini et al. 2017)). ...
September 2013
The Astrophysical Journal
... Though there is a paucity of such data, we found a few noteworthy observations that support the blazar nature of these objects. NVSS J064632+445116 was predicted as a candidate γ-ray emitter by [18], whereas NVSS J135406−020603 is included in the ROMA-BZCAT [19]. The quasar NVSS J212912−153841 is a hard X-ray spectrum luminous blazar and included in the 70 month Swift-Burst Alert Telescope catalog [20]. ...
December 2004
... We find no 4FGL-DR2 sources are located within 0.5 • of the targets. In the BZCAT [55], CRATES [56], CGraBS [57] and WISE blazar candidate catalogs [58], no potential γ-ray emitters are found to be within 0.5 • of these two dSphs. ...
April 2009
... Only in the Fourth Fermi Catalog [4] was it classified as a BL Lac object with a photon index 2. The redshift was also unknown until recently when the observations of [5,6] determined it to be z=0.1285. The MAGIC telescopes observed TXS 1515-273 during February 2019 as a part of a MWL campaign organized after flaring activity in the high-energy gamma-ray band (HE, 0.5 MeV ≤ ≤ 100 GeV) was reported by the Fermi-LAT [7]. ...
... Finally, applying visibility constraints and introducing the optical magnitude cut R ≤ 17.5, we obtained the unbiased γ-loud monitoring sample of 62 sources. The control sample of γ-quiet sources was selected from Candidate Gamma-Ray Blazar Survey (CGRaBS) sources [37] that had not been detected by Fermi at that time (not included in 2FGL). We introduced constraints on the 15 GHz mean flux density ≥ 0.06 Jy and intrinsic modulation index as defined by Richards et al. [38] m ≥ 0.05. ...
June 2009