Long-term X-Ray Spectral Variability in AGN from the Palomar sample observed by Swift

TL;DR

This study investigates long-term X-ray spectral variability in 24 nearby AGN, revealing a transition from harder-when-brighter at low accretion rates to softer-when-brighter at high accretion rates, linked to changes in seed photon sources.

Contribution

It provides new insights into the accretion rate dependence of spectral variability and the analogy between AGN states and black hole X-ray binary states.

Findings

Low accretion rate AGN show harder-when-brighter behaviour.

Spectral changes are mainly driven by luminosity-dependent photon index variations.

Transition in seed photon sources explains the variability pattern.

Abstract

We present X-ray spectral variability of 24 local active galactic nuclei (AGN) from the Palomar sample of nearby galaxies, as observed mainly by Swift. From hardness ratio measurements, we find that 18 AGN with low accretion rates show hardening with increasing count rate, converse to the softer-when-brighter behaviour normally observed in AGN with higher accretion rates. Two AGN show softening with increasing count rate, two show more complex behaviour, and two do not show any simple relationship. Sufficient data were available for the spectra of 13 AGN to be summed in flux-bins. In 9 of these sources, correlated luminosity-dependent changes in the photon index ($\Gamma$) of a power-law component are found to be the main cause of hardness variability. For 6 objects, with a low accretion rate as a fraction of the Eddington rate (\.m$\mathrm{_{Edd}}$), $\Gamma$ is anticorrelated with \.m$\mathrm{_{Edd}}$, i.e. `harder-when-brighter' behaviour is observed. The 3 higher-\.m$\mathrm{_{Edd}}$-rate objects show a positive correlation between $\Gamma$ and \.m$\mathrm{_{Edd}}$. This transition from harder-when-brighter at low \.m$\mathrm{_{Edd}}$s to softer-when-brighter at high \.m$\mathrm{_{Edd}}$s can be explained by a change in the dominant source of seed-photons for X-ray emission from cyclo-synchrotron emission from the Comptonising corona itself to thermal seed-photons from the accretion disc. This transition is also seen in the `hard state' of black hole X-ray binaries (BHXRBs). The results support the idea that LINERs are analogues of BHXRBs in the hard state and that Seyferts are analogues of BHXRBs in either the high-accretion-rate end of the hard state or in the hard-intermediate state.