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Register a new userA Determination of the Gamma-ray Flux and Photon Spectral Index Distributions of Blazars from the Fermi-LAT 3LAC
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J. Singal
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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 Telescopes 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.
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(Abridged) In this paper, multi-wavelength data are compiled for a sample of 1425 Fermi blazars to calculate their spectral energy distributions (SEDs). A parabolic function, $log( u F_{ u}) = P_1(log u - P_2)^2 + P_3,$ is used for SED fitting. Synchrotron peak frequency ($log u_p$), spectral curvature ($P_1$), peak flux ($ u_{rm p}F_{rm u_p}$), and integrated flux ($ u F_{ u}$) are successfully obtained for 1392 blazars (461 flat spectrum radio quasars-FSRQs, 620 BL Lacs-BLs and 311 blazars of uncertain type-BCUs, 999 sources have known redshifts). Monochromatic luminosity at radio 1.4 GHz, optical R band, X-ray at 1 keV and $gamma$-ray at 1 GeV, peak luminosity, integrated luminosity and effective spectral indexes of radio to optical ($alpha_{rm RO}$), and optical to X-ray ($alpha_{rm OX}$) are calculated. The Bayesian classification is employed to log$ u_{rm p}$ in the rest frame for 999 blazars with available redshift and the results show that 3 components are enough to fit the $log u_{rm p}$ distribution, there is no ultra high peaked subclass. Based on the 3 components, the subclasses of blazars using the acronyms of Abdo et al. (2010a) are classified, and some mutual correlations are also studied. Conclusions are finally drawn as follows: (1) SEDs are successfully obtained for 1392 blazars. The fitted peak frequencies are compared with common sources from samples available (Sambruna et al. 1996, Nieppola et al. 2006, 2008, Abdo et al. 2010a). (2) Blazars are classified as low synchrotron peak sources (LSPs) if $log u_{rm p}$(Hz) $leq 14.0$, intermediate synchrotron peak sources (ISPs) if $14.0 < log u_{rm p}$(Hz) $leq 15.3$, and high synchrotron peak sources (HSPs) if $log u_{rm p}$(Hz) $> 15.3$. (3) $gamma$-ray emissions are strongly correlated with radio emissions. (...)
We present multiwavelength spectral analyses of two Fermi-LAT blazars, OJ 287 and 3C 279, that are part of the Boston University multiwaveband polarization program. The data have been compiled from observations with Fermi, RXTE, the VLBA, and various ground-based optical and radio telescopes. We simulate the dynamic spectral energy distributions (SEDs) within the framework of a multi-slice, time-dependent leptonic jet model for blazars, with radiation feedback, in the internal shock scenario. We use the physical jet parameters obtained from the VLBA monitoring to guide our modeling efforts. We discuss the role of intrinsic parameters and the interplay between synchrotron and inverse Compton radiation processes responsible for producing the resultant SEDs.
Blazars are a subclass of active galactic nuclei (AGNs) with extreme observation properties, which is caused by the beaming effect, expressed by a Doppler factor, in a relativistic jet. Doppler factor is an important parameter in the blazars paradigm to indicate all of the observation properties, and many methods were proposed to estimate its value. In this paper, we present a method following Mattox et al. to calculate the lower limit on gamma-ray Doppler factor for 809 selected Fermi/LAT-detected gamma-ray blazars by adopting the available gamma-ray and X-ray data. Our sample included 342 flat-spectrum radio quasars (FSRQs) and 467 BL Lac objects (BL Lacs), out of which 507 sources are compiled with available radio core-dominance parameter (R) from our previous study. Our calculation shows that the average values of the lower limit on gamma-ray Doppler factor for FSRQs and BL Lacs are 6.87 and 4.31, respectively. We compare and discuss our results with those from the literature. We found that the derived lower limit on gamma-ray Doppler factor for some sources are higher than that from the radio estimation, which could be possibly explained by the jet bending within those blazars. Our results also suggest that the gamma-ray and radio regions perhaps share the same relativistic effects. The gamma-ray Doppler factor has been found to be correlated with both the gamma-ray luminosity and core-dominance parameter, implying that the jet is possibly continuous in the gamma-ray bands, and R is perhaps an indicator for a beaming effect.
We present the redshift evolutions and distributions of the gamma-ray luminosity and photon spectral index of flat spectrum radio quasar (FSRQ) type blazars, using non-parametric methods to obtain the evolutions and distributions directly from the data. The sample we use for analysis consists of almost all FSRQs observed with a greater than approximately 7 sigma detection threshold in the first year catalog of the Fermi Gamma-ray Space Telescopes Large Area Telescope, with redshfits as determined from optical spectroscopy by Shaw et al. We find that FSQRs undergo rapid gamma-ray luminosity evolution, but negligible photon index evolution, with redshift. With these evolutions accounted for we determine the density evolution and luminosity function of FSRQs, and calculate their total contribution to the extragalactic gamma-ray background radiation, resolved and unresolved, which is found to be 16(+10/-4)%, in agreement with previous studies.
The detection of periodicities in light curves of active galacticnuclei (AGN) could have profound consequences for our understanding of the nature and radiation physics of these objects. At high energies (HE; E>100 MeV) 5 blazars (PG 1553+113,PKS 2155-304, 0426-380, 0537-441, 0301-243) have been reported to show year-like quasi-periodic variations (QPVs) with significance >3 sig. As these findings are based on few cycles only, care needs to be taken to properly account for random variations which can produce intervals of seemingly periodic behaviour. We present results of an updated timing analysis for 6 blazars (adding PKS 0447-439), utilizing suitable methods to evaluate their long term variability properties and to search for QPVs in their light curves. We generate gamma-ray light curves covering almost 10 years, study their timing properties and search for QPVs using the Lomb-Scargle Periodogram and the Wavelet Z-transform. Extended Monte Carlo simulations are used to evaluate the statistical significance. Comparing their probability density functions (PDFs), all sources (except PG 1553+113) exhibit a clear deviation from a Gaussian distribution, but are consistent with being log-normal, suggesting that the underlying variability is of a non-linear, multiplicative nature. Apart from PKS 0301-243 the power spectral density for all investigated blazars is close to flicker noise (PL slope -1). Possible QPVs with a local significance ~ 3 sig. are found in all light curves (apart from PKS 0426-380 and 0537-441), with observed periods between (1.7-2.8) yr. The evidence is strongly reduced, however, if evaluated in terms of a global significance. Our results advise caution as to the significance of reported year-like HE QPVs in blazars. Somewhat surprisingly, the putative, redshift-corrected periods are all clustering around 1.6 yr. We speculate on possible implications for QPV generation.
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