Investigating the X-ray temporal and spectral properties of blazars and beamed AGN in the Swift-BAT Hard X-ray Survey
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Blazars are generally known to exhibit high-amplitude, rapid variations in flux, polarization, and in their spectra across most timescales and wavelengths. While the consensus for these objects is that their emission is indeed ``highly variable", a more specific characterization of the variability may depend on the timescales considered. In this dissertation, I investigate the nature of the variability of these objects and the physical processes involved in producing it, through the lens of blazars that have been detected by the Swift Burst Alert Telescope.
My foray into the high-energy astrophysics of blazars begins with a case study of a blazar-like AGN. For the first time for this source, I definitively measure X-ray reflection features and help determine the origin of its broadband X-ray emission, suggesting that the X-rays from this object predominantly come from regions in the vicinity of the black hole, while also finding evidence of jetted emission in the hard X-rays. I further explore blazar X-ray emission by investigating the rest of the blazars in the Swift-BAT survey, and in doing so I conduct the first study in the time domain dedicated to the hard X-ray variability behavior of blazars on long timescales based on ~13 years of continuous X-ray data in the 14-195 keV band. In this study, I find that a significant portion of the blazars in the sample (~37%) do not show statistically significant variability on monthly timescales, which is in tension with the expected high variability of blazars seen in previous studies. In addition, I show that for some of the brightest blazars, the long-term spectra in the hard X-rays may be described in a relatively simple way, with a power law that changes slope on monthly timescales.
Since the BAT data are not sensitive to changes on shorter timescales, or to low-amplitude variability on monthly timescales, I follow up on the supposedly ``non-variable" blazars from the previous investigation by using recent NICER observations of a sub-sample of 4 such “quiescent” BAT blazars over 5 months, allowing for insight into the short-timescale and lower amplitude variability while also representing some of the longer timescales sampled by the BAT survey. I show that variations in the NICER band are in fact detected on several timescales, but that the fractional variability appears to decrease with longer timescales, implying generally low-amplitude variability across all sources and showing very low variability on monthly timescales, which is once again at odds with studies that have shown that blazars are highly variable in the X-rays on a wide range of timescales. I also show through a spectral analysis that the broadband X-ray spectra (0.3-195 keV) of these sources can be described with different power law models, with one source requiring significant absorption in the soft X-rays to fully describe its observed curvature, possibly due to absorption in the intergalactic medium. Additional observations from a new follow-up NICER campaign will further facilitate probing the variability of these BAT blazars for up to timescales of a year, serving as an additional stepping stone towards our ultimate goal of characterizing the X-ray variability of blazars and beamed AGN.