We present the results of the γ-ray flux distribution study on the brightest blazars which are observed by the Fermi-LAT. We selected 50 brightest blazars based on the maximum number of detection reported in the LAT third AGN catalog. We performed standard unbinned maximum likelihood analysis on the LAT data during the period between August 2008 and December 2016, in order to obtain the average monthly flux. After quality cuts, blazars for which at least 90% of the total flux was survived were selected for the further study, and this includes 19 FSRQs and 19 BL Lacs. The Anderson-Darling and χ 2 tests suggest that the integrated monthly flux follow a log-normal distribution for all sources, except for three FSRQs for which neither a normal nor a log-normal distribution was preferred. A double log-normal flux distribution tendency were observed in these sources, though it has to be confirmed with improved statistics. We also found that, the standard deviation of the lognormal flux distribution increases with the mean spectral index of the blazar, and can be fitted with a line of slope 0.24 ± 0.04. We repeat our study on three additional brightest unclassified blazars to identify their flux distribution properties. Based on the features of their log-normal flux distribution, we infer these unclassified blazars may be closely associated with FSRQs.We also highlight that considering the log-normal behavior of the flux distribution of blazars, averaging their long term flux in linear scale can largely under estimate the nominal flux and this discrepancy can propagate down to the estimation of source parameters through spectral modeling.

We perform a detailed spectral study of a recent flaring activity from the Flat Spectrum Radio Quasar (FSRQ), 3C 454.3, observed simultaneously in optical, UV, X-ray and γ-ray energies during 16 to 28 August, 2015. The source reached its peak γ-ray flux of (1.9 ± 0.2) × 10 −05 ph cm −2 s −1 on 22 August. The time averaged broadband spectral energy distribution (SED) is obtained for three time periods, namely "flaring state"; covering the peak γ-ray flux, "post flaring state"; immediately following the peak flare and "quiescent state"; separated from the flaring event and following the post flaring state. The SED corresponding to the flaring state is investigated using different emission models involving synchrotron, synchrotron self Compton (SSC) and external Compton (EC) mechanisms. Our study suggests that the X-ray and γ-ray emission from 3C 454.3 cannot be attributed to a single emission mechanism and instead, one needs to consider both SSC and EC mechanisms.Moreover, the target photon energy responsible for the EC process corresponds to an equivalent temperature of 564 K, suggesting that the flare location lies beyond the broad line emitting region of the FSRQ. SED fitting of the other two flux states further supports these inferences.

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We present a detailed study of flux and index distributions of three blazars [one flat-spectrum radio quasar (FSRQ) and two BL Lacertae objects (BL Lacs)] by using 16 yr of Rossi X-ray Timing Explorer (RXTE) archival data. The three blazars were chosen such that their flux and index distributions have sufficient number of data points (≥90) with relatively less uncertainty $\left(\overline{\sigma _{\rm err}^{2}}/\sigma ^{2} < 0.2\right)$ in light curves. Anderson–Darling (AD) test and histogram fitting show that flux distribution of FSRQ 3C 273 is lognormal, while its photon index distribution is Gaussian. This result is consistent with linear Gaussian perturbation in the particle acceleration time-scale, which produces lognormal distribution in flux. However, for two BL Lacs, viz. Mrk 501 and Mrk 421, AD test shows that their flux distributions are neither Gaussian nor lognormal, and their index distributions are non-normal. The histogram fitting of Mrk 501 and Mrk 421 suggests that their flux distributions are more likely to be a bimodal, and their index distributions are double Gaussian. Since, Sinha et al. had shown that Gaussian distribution of index produces a lognormal distribution in flux, double Gaussian distribution of index in Mrk 501 and Mrk 421 indicates that their flux distributions are probably double lognormal. Observation of double lognormal flux distribution with double Gaussian distribution in index reaffirms two flux states hypothesis. Further, the difference observed in the flux distribution of FSRQ (3C 273) and BL Lacs (Mrk 501 and Mrk 421) at X-rays suggests that the low-energy emitting electrons have a single lognormal flux distribution, while the high-energy ones have a double lognormal flux distribution.