Long-term hard X-ray variability properties of Swift-BAT blazars

TL;DR

This study analyzes over 13 years of Swift-BAT data to understand long-term hard X-ray variability in blazars, revealing that many are less variable than previously thought and that spectral variability is rare.

Contribution

First long-term analysis of blazar hard X-ray variability using Swift-BAT data, highlighting differences from prior short-term variability studies.

Findings

Approximately 37% of blazars show no significant monthly variability.

FSRQs and BL Lacs with variability exhibit similar flux changes.

Only five blazars show significant spectral variability over long timescales.

Abstract

We present results from the first dedicated study in the time domain of the hard X-ray variability behavior of blazars on long timescales based on $\sim$13 years of continuous hard X-ray data in the 14-195 keV band. We use monthly-binned data from the recent 157-month Swift-BAT catalog to characterize the hard X-ray variability of 127 blazars and search for potential differences between the variability of BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs). A significant portion of the blazars in the sample ($\sim$37%) do not show statistically significant hard X-ray variability on monthly timescales, which is deeply at odds with previous studies that show that blazars are highly variable in the X-rays and other energy bands on a wide range of timescales. We also find that, on average, the FSRQs and BL Lacs for which we do detect variability exhibit similar flux variability; this suggests that the variability in these FSRQs is not necessarily driven by variations in the source function of scattered external radiation arriving from extended regions, and that it is instead possibly driven by processes that lead to variations in particle injection. In addition, only five blazars in our sample show significant spectral variability in the long-term light curves. For 3 blazars, we find that a power law that changes slope on monthly timescales is sufficient to characterize the variable hard X-ray spectrum, suggesting that, at least for some bright blazars, the long-term spectra in the hard X-rays may be described in a relatively simple fashion.