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We present results from the first dedicated study in the time domain of thehard X-ray variability behavior of blazars on long timescales based on $\sim$13years of continuous hard X-ray data in the 14-195 keV band. We usemonthly-binned data from the recent 157-month Swift-BAT catalog to characterizethe hard X-ray variability of 127 blazars and search for potential differencesbetween the variability of BL Lacertae objects (BL Lacs) and flat-spectrumradio quasars (FSRQs). A significant portion of the blazars in the sample($\sim$37%) do not show statistically significant hard X-ray variability onmonthly timescales, which is deeply at odds with previous studies that showthat blazars are highly variable in the X-rays and other energy bands on a widerange of timescales. We also find that, on average, the FSRQs and BL Lacs forwhich we do detect variability exhibit similar flux variability; this suggeststhat the variability in these FSRQs is not necessarily driven by variations inthe source function of scattered external radiation arriving from extendedregions, and that it is instead possibly driven by processes that lead tovariations in particle injection. In addition, only five blazars in our sampleshow significant spectral variability in the long-term light curves. For 3blazars, we find that a power law that changes slope on monthly timescales issufficient 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 maybe described in a relatively simple fashion.

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