Investigating the variability of Swift-BAT blazars with NICER

Abstract

ABSTRACT We present results of X-ray spectral and time-domain variability analyses of four faint, ‘quiescent’ blazars from the Swift-BAT 105-month catalogue. We use observations from a recent, 5-month long NICER campaign, as well as archival BAT data. Variations in the 0.3–2 keV flux are detected on minute, ∼weekly, and monthly time-scales, but we find that the fractional variability Fvar on these time-scales is <25 per cent and decreases on longer time-scales, implying generally low-amplitude variability across all sources and showing very low variability on monthly time-scales ($F_{\rm var}\lesssim 13{{\ \rm per\ cent}}$), which is at odds with previous studies that show that blazars are highly variable in the X-rays on a wide range of time-scales. Moreover, we find that the flux variability on very short time-scales appears to be characterized by long periods of relative quiescence accompanied by occasional short bursts, against the relatively time-stationary nature of the variability of most other AGN light curves. Our analysis also shows that the broadband X-ray spectra (0.3–195 keV) of our sources can be described with different power-law models. As is the case with most blazars, we find that two sources (2MASS J09343014-1721215 and PKS 0312-770) are well-modelled with a simple power law, while the remaining two (1RXS J225146.9-320614 and PKS 2126-15) exhibit curvature in the form of a log-parabolic power law. We also find that, in addition to the continuum, PKS 2126-15 requires significant absorption at the soft X-rays (≲1 keV) to fully describe the observed curvature, possibly due to absorption from the intergalactic medium.