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

We present results from the first dedicated study in the time domain of the hard X-ray variability behaviour of blazars on long time-scales based on similar to 13 yr of continuous hard X-ray data in the 14-195 keV band. We use monthly binned data from the recent 157-month Swift-BAT (Burst Alert Telescope) catalogue 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 (similar to 37 per cent) do not show statistically significant hard X-ray variability on monthly time-scales, 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 time-scales. 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 three blazars, we find that a power law that changes slope on monthly time-scales 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.