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

TL;DR

This study analyzes short-term X-ray variability in blazars S5 0716+714 and PKS 2155-304, revealing complex spectral evolution and challenging assumptions about statistical distributions and linear RMS-flux relations.

Contribution

It provides a detailed statistical analysis of X-ray variability during high activity phases, highlighting the need for careful interpretation of spectral and variability data.

Findings

Hardness ratio correlates with flux but shows complex behavior during bright phases.

Histograms favor a normal distribution overall, but Anderson-Darling test suggests a lognormal distribution.

No linear RMS-flux relation was observed in the data.

Abstract

We explored the statistical properties of short-term X-ray variability using long-exposure {\it XMM-Newton} data during high X-ray variability phases of blazars S5 0716+714 and PKS 2155-304. In general, 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 correlation and anti-correlation both, indicating spectral evolution. Stationarity tests suggest the time series as non-stationarity or have trend stationarity. Except for one, none of the histograms fit resulted in a reduced-\(\chi^2 \sim 1\) for a normal and log-normal profile but a normal profile is favored in general. On the contrary, the Anderson-Darling test favors lognormal 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 widely used different 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 10s 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.