The Nature of γ-Ray Variability in Blazars

Abstract

We present an in-depth and systematic analysis of a sample of 20 powerful blazars, including 12 BL Lacs and 8 flat spectrum radio quasars, utilizing Fermi/LAT observations from the period 2008-2018 using various analysis tools such as flux distribution, symmetry analysis, and time series analysis. Results show that blazars with steeper γ-ray spectral indexes are found to be more variable; and the γ-ray flux distribution closely resemble both normal and lognormal probability distribution functions. The statistical variability properties of the sources as studied by power spectral density analysis are consistent with flicker noise (P (ν) ∝ 1/ν) -an indication of long-memory processes at work. Statistical analysis of the distribution of flux rise and decay rates in the light curves of the sources, aimed at distinguishing between particle acceleration and energy dissipation timescales, counter-intuitively suggests that both kinds of rates follow a similar distribution and the derived mean variability timescales are in the order of a few weeks. The corresponding emission region size is used to constrain location of γ-ray production sites in the sources to be a few parsecs. Additionally, using Lomb-Scargle periodogram aided with extensive Monte Carlo simulations, we detected year timescale quasi-periodic oscillations in the sources S5 0716+714, Mrk 421, ON +325, PKS 1424-418 and PKS 2155-304; and the detection significance was computed taking proper account of the red-noise and other artifacts inherent in the observations. We explain our results in the light of current blazar models with relativistic shocks propagating down the jet viewed close to the line of sight.

Exploring the variability properties of gamma-ray emission from blazars

“…Blazars are usually variable in practically all accessible bands, on diverse timescales and amplitudes. The general nature of the variability appears to be aperiodic, with the exception of a few sources such as Mrk 501, Mrk 421, S5 0716+714, PKS 1424 which are reported to show quasiperiodic oscillations [3,5]. The source in general are far away in the sense that the the telescopes with current capability hardly can resolve the central engine.…”

“…In this proceeding, I present some of highlight of the works on -ray variability study of a sample of 20 blazar using the decade-long observation from Fermi/LAT telescope (see [3,4]. Section 2, the analysis methods are discussed and in Section 3 result and discussion are presented.…”

“…To generate the light curves the data were binned in weekly bins and the task gtlike was run to carry out maximum-likelihood analysis, and to extract the source flux the events above test statistics 10 (equivalently 3 ) [33] were considered . For details on the data processing refer to [3] https://fermi.gsfc.nasa.gov/ssc/data/analysis/software/ https://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/likelihood_tutorial.html…”

Building a robust sample of Fermi-LAT blazars that exhibit periodic gamma-ray emission

“…The typical approach to search for periodicity at -ray energies in blazars is to analyze a limited sample of a few blazars and apply two or three time-series algorithms [e.g., 3,21]. In [10], we developed a pipeline to perform a search for periodicities in hundreds of blazars.…”

Periodicity Analysis of Mrk 501 and Mrk 421 in Gamma Rays

“…Using the spectral analysis RobPer, Benitez et al [8] also found a periodicity of 310 d in optical, gamma, and X-ray data, also supporting the BBH hypothesis. [9] found a 280 d period in FERMI data, and Nilsson et al [11] found a period of 477 d in optical data. Contrary to these findings, Sandrinelli et al [7] analyzed gamma ray and optical light curves and found no significant periodicity in either.…”