Log-normal flux distribution of bright Fermi blazars

Abstract

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.

Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501

“…Thanks to its constant monitoring, a great deal of data is available to study the statistical properties of AGN light curves. Studies on the brightest blazars seen by the LAT show the same trend when the light curves are binned on a monthly timescale [11]. However, the Fermi-LAT, having a collection area of ∼1 m 2 , lacks the necessary sensitivity to perform a detailed study also on fainter objects, where it can become hard to gain enough statistics on short timescales to perform these kind of studies, as shown in Section 2.…”

Evaluating quasi-periodic variations in the γ-ray light curves of Fermi-LAT blazars

“…The almost continuous detection of the investigated sources over time bins of seven days provides a good opportunity to investigate whether the probability density function (PDF) for the long-term γ-ray emission reveals any preference for a gaussian (normal) or a log-normal flux distribution. It is expected that such a feature offers an important clue for understanding the central engine of a blazer (e.g., Uttley et al 2005;Shah et al 2018). A log-normal distribution of γ-ray fluxes, for example, would suggest that the mechanism driving the variability is multiplicative in nature rather than additive.…”

“…A log-normal distribution of γ-ray fluxes, for example, would suggest that the mechanism driving the variability is multiplicative in nature rather than additive. Such multiplicative behaviour might possibly be related to accretion disk fluctuations (Lyubarskii 1997;Arévalo & Uttley 2006) or the particle acceleration process itself (Shah et al 2018). As a possible caveat we note that PDF of stochastic processes with power spectral density (PSD) steeper than index −1 can show deviations from normality owing to divergence of power at low frequencies (e.g., Vaughan et al 2003).…”

Science Case for a Wide Field-of-View Very-High-Energy Gamma-ray Observatory in the Southern Hemisphere

“…While the flux distribution of blazars follows a normal distributions at GeV energies, the fluxes measured by current IACTs have a clear preference for log-normal distributions, suggesting multiplicative rather than additive processes at their origin [109][110][111]. However, the observations of most IACTs are biased, as the observations are usually not carried out in a monitoring mode but often based on triggers by other instruments.…”