Classifying AGN by X-ray Hardness Variability

TL;DR

This study analyzes X-ray variability in 44 AGN using Swift data to classify their spectral states and compare them with X-ray binaries, revealing differences between Seyferts and radio loud galaxies and insights into corona geometry.

Contribution

It introduces the HR-slope as a new metric for classifying AGN spectral states and compares HR definitions, providing physical models for state transitions.

Findings

Seyferts and radio loud galaxies show a dichotomy in count-based HR but not in energy-based HR.

No correlation between HR parameters and radio loudness was observed.

Seyferts and some radio loud objects occupy distinct regions in the HR-L/L_Edd diagram.

Abstract

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-$L/L_\mathrm{Edd}$ 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 behavior. We provide schematic physical models to explain the observed transition between energy defined HR states. We find Seyferts populate the high, hard, phase of the HR-$L/L_\mathrm{Edd}$ 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 track the HR phase diagram which may provide clues to the geometry of the corona.