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 paper I review our current understanding of these topics.I begin by considering whether AGN and BHBs show the same X-ray spectraltiming '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, TB, scale approximately with black hole mass, MBH , but inversely with accretion rate,ṁE (in units of the Eddington accretion rate) probably signifying that TB arises at the inner edge of the accretion disc. The relationship TB ∝ MBH /ṁE 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 1/4 B , providing a natural explanation of the observed small black hole masses in Narrow Line Seyfert Galaxies; if MBH were large then, as TB ∝ MBH /ṁE, we would requireṁE> 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.

Variability and the X-ray/UV ratio of active galactic nuclei

“…It is generally believed that variable X-ray irradiation can drive optical variations by means of variable heating of the internal parts of the disk on relatively short timescales, days to weeks, while intrinsic disk instabilities in the outer parts of the disk dominate on longer timescales, months to years, propagating inwards and modulating X-ray variations in terms of Compton up-scattering in the corona (Czerny 2004;Arévalo 2006Arévalo , 2009Papadakis et al 2008;McHardy 2010).…”

X-ray variability of 104 active galactic nuclei

“…Therefore, we expect to find similarities between the properties of luminous accretion flows around stellar mass BHs in X-ray binaries (BH-XRBs; M BH ∼ 10 M ) and supermassive black holes (SMBHs) in Active Galactic Nuclei (AGN; with M BH ∼ 10 6 −10 9 M ). However, the details and reliability of the proposed scaling relations are currently not well understood (see Merloni et al 2003;Körding et al 2007;McHardy 2010, for a review).…”

“…Uttley et al 2002;Markowitz et al 2003;Vaughan et al 2003bVaughan et al , 2011McHardy et al 2006;McHardy 2010). Both AGN and BH-XRBs show "red noise" power spectra (or power spectral density, PSD) that decrease Tables 1-4 and Appendices A and B are available in electronic form at http://www.aanda.org steeply at high frequencies (short timescales) as a power law, P(ν) ∼ ν −α (where ν is temporal frequency), typically with α ≈ 2.…”

The X-ray Power Density Spectrum of the Seyfert 2 Galaxy NGC 4945: Analysis and Application of the Method of Light Curve Simulations

“…[12]). The focus on a variable matter accretion rate shows particular promise, however [13]. Specifically, the model in which the variations are produced by disturbances propagating inward in the accretion disk and setting up fluctuations at progressively shorter time scales [14] is a good fit to the available observational evidence, foremost the finding that the amplitude of the variability is proportional to the flux (the so-called "rms-flux relation" [15,16]).…”

“…It is known, however, that the features in the PDS (breaks and QPO peaks) in galactic X-ray binaries shift in frequency depending on the (instantaneous) mass accretion rate. The question then becomes: Are these two parameters sufficient to explain the observed differences in both galactic sources and AGN, and the evidence so far seems to indicate that they are, even across the five to six orders of magnitude in mass difference [13]. Overall, while important physical characteristics might be different between these two classes of sources, such as the temperature and density of the inner accretion flow [17], it appears that AGN, in many of their observational properties, are really just scaled-up versions of galactic accretion systems.…”