X-Ray Variability of AGN and Relationship to Galactic Black Hole Binary Systems

Abstract

Over the last 12 years, AGN monitoring by RXTE, has revolutionised our understanding of the X-ray variability of AGN, of the relationship between AGN and Galactic black hole X-ray binaries (BHBs) and hence of the accretion process itself, which fuels the emission in AGN and BHBs and is the major source of power in the universe. In this chapter I review our current understanding of these topics.

I begin by considering whether AGN and BHBs show the same X-ray spectral-timing “states” (e.g. low flux, hard spectrum or “hard” and high flux, soft spectrum or “soft”). Observational selection effects mean that most of the AGN which we have monitored will probably be “soft-state” objects, but AGN are found in the other BHB states, although possibly with different critical transition accretion rates.

I examine timescale scaling relationships between AGN and BHBs. I show that characteristic power spectral “bend” timescales, T _B , scale approximately with black hole mass, M _BH , but inversely with accretion rate ṁ _E , (in units of the Eddington accretion rate), probably signifying that T _B , arises at the inner edge of the accretion disc. The relationship T _B ∝ M _BH /ṁ _E is a good fit, implying that no other potential variable, e.g. black hole spin, varies significantly. Lags between hard and soft X-ray bands as a function of Fourier timescale follow similar patterns in AGN and BHBs.

I show how our improved understanding of X-ray variability enables us to understand larger scale properties of AGN. For example, the width of the H _β optical emission line, V, scales as T _B ^1/4, providing a natural explanation of the observed small black hole masses in Narrow Line Seyfert Galaxies; if M _BH were large then, as T _B ∝ M _BH /ṁ _E , we would require ṁ _E > 1 to obtain narrow lines.

I note that the rms X-ray variability scales linearly with flux in both AGN and BHBs, indicating that the amplitude of the shorter timescale variations is modulated by that of the longer timescale variations, ruling out simple shot-noise variability models. Blazars follow approximately the same pattern. The variations may therefore arise in the accretion disc and propagate inwards until they hit, and modulate, the X-ray emission region which, in the case of blazars, lies in a relativistic jet.

Short timescale (weeks) optical variability arises from reprocessing of X-rays in the accretion disc, providing a diagnostic of X-ray source geometry. On longer timescales, variations in the disc accretion rate may dominate optical variations.

AGN X-ray monitoring has greatly increased our understanding of the accretion process and there is a strong case for continued monitoring with future observatories.