Mon. Not. R. Astron. Soc., 481, 3563-3572 (2018/December-2)
Classifying AGN by X-ray hardness variability.
PERETZ U. and BEHAR E.
Abstract (from CDS):
The physics behind the dramatic and unpredictable X-ray variability of active galactic nuclei (AGN) has eluded astronomers since it was discovered. We present an analysis of Swift XRT observations of 44 AGN with at least 20 Swift observations. We define HR-slope as the change of Hardness Ratio (HR) with luminosity (L). This slope is measured for all objects in order to: (1) classify different AGN according to their HR-HR-slope relation and (2) compare HR-L/LEdd trends with those observed in X-ray binaries for the 27 AGN with well-measured black hole masses. We compare results using a count-based HR definition and an energy-based HR definition. We observe a clear dichotomy between Seyferts and radio-loud galaxies when considering count-based HR, which disappears when considering energy-based HR. This, along with the fact no correlation is observed between HR parameters and radio loudness, implies radio-loud and radio-quiet AGN should not be discriminated by their HR behaviour. We discuss schematic physical models to explain the observed transition between energy defined HR states. We find Seyferts populate the high, hard, phase of the HR-L/LEdd diagram as well as do three radio-loud objects. Two LINERs populate the low, soft, phase part of this diagram. Finally, radio-loud objects are concentrated around small positive HR-slopes, while Seyferts tentatively follow a track in the HR phase diagram which may provide clues to the geometry of the corona.