Short-Term X-ray Variability during Different Activity Phases of Blazars S5 0716+714 and PKS 2155-304

Abstract

We explored the statistical properties of short-term X-ray variability using long-exposure XMM-Newton data during high X-ray variability phases of blazars S5 0716+714 and PKS 2155-304. In general, the hardness ratio shows correlated variations with the source flux state (count rate), but in a few cases, mainly the bright phases, the trend is complex with both correlation and anti-correlation, indicating spectral evolution. Stationarity tests suggest the time series are non-stationarity or have trend stationarity. Except for one, none of the histogram fits resulted in a reduced-χ2∼1 for a normal and log-normal profile but a normal profile is favored in general. On the contrary, the Anderson–Darling test favors log-normal with a test-statistic value lower for log-normal over normal for all the observations, even if out of significance limits. None of the IDs show linear RMS-flux relation. The contrary inferences from different widely used statistical methods indicate that a careful analysis is needed while the complex behavior of count rate with hardness ratio suggests spectral evolution over a few 10 s of kilo-seconds during bright phases of the sources. In these cases, the spectrum extracted from whole observation may not be meaningful for spectral studies and certainly not a true representation of the spectral state of the source.

An XMM–Newton catalogue of BL Lacs

“…show a compact, one-sided core jet (Bach et al 2006) (Kushwaha & Pal 2020). During these observations the average flux was of the order of 1.0 • 10 −11 erg cm −2 s −1 in the 0.2 -10 keV range.…”

Study of Intra-Day Flux Distributions of Blazars Using XMM-Newton Satellite

“…BL Lac is studied by Giebels and Degrange (2009) [19] using the RXTE-PCA observations, and it is the first blazar in which lognormal X-ray variability is detected, and a linear correlation between the excess rms and the average flux was also found. The flux distribution of blazar S5 0716+714 in X-ray with XMM-Newton observations was studied on a short-term timescale by Kushwaha and Pal (2020) [25] and Mohorian et al (2022) [38]. 3C 273 is studied in X-ray using 16 years of Rossi X-ray Timing Explorer (RXTE) archival data by Khatoon et al 2020 [39].…”

“…Such small Gaussian perturbations propagating in blazar jets could produce non-linear flux distributions and can explain the lognormal behaviour. Kushwaha and Pal (2020) [25] studied blazars during high variability phases on IDV timescales and found the flux distributions show a normal profile compared to lognormal ones.…”

“…The lognormality of the flux distribution in blazars was detected using Rossi X-ray Timing Explorer (RXTE) observations [19]. The lognormal behaviour of flux distribution is observed in other blazars at various timescales and wavelengths ( [20][21][22][23][24][25][26] and references therein). However, the dominance of non-thermal emission hinders our understanding of the accretion disc-jet connection in jet-dominated sources.…”

Optical variability power spectrum analysis of blazar sources on intranight timescales

“…In addition to assuming the power spectral shape, the method requires supplying a value of mean and standard deviation (σ) of the flux values to reproduce the flux distribution and match the variance (Meyer et al 2019). We have assumed single power-law PSDs with a given β (to reproduce the PSD shape) and supplied mean and σ of the logarithmically transformed flux values which is found to be an adequate representation of flux distribution on shorter ( days) timescales for a few cases (e.g., H. E. S. S. Collaboration et al 2010;Kushwaha & Pal 2020). For this purpose, the mean and the σ are computed by fitting a Gaussian function to the flux distribution.…”