Searching For Breaks In The Hard X-ray Power Spectral Density Functions Of AGN



The physics of accretion disks in active galactic nuclei (AGN) have been the subject of intensive recent work. While their huge energy output over a broad range of wavelengths (i.e. radio to X-ray) allows for detailed spectral characterization, their extremely small sizes are not resolved by even the best observatories. The Event Horizon Telescope is the current best hope for directly imaging the accretion disk of black holes however it will only be able to observe the two nearest black holes Sgr A\(^{*}\) and M87. A large resolved survey of extragalactic AGN is not possible currently or even in the near future. Time variability analysis in different spectral bands is one of the best ways to study the structure of accretion disks as well as the mechanisms controlling the mass accretion rate crucial to understanding the physics of AGN.

We propose to combine short timescale NuSTAR observations with the latest Swift/Burst Alert Telescope (BAT) long timescale light curves (Shimizu et al., 2013) to study the time variability of a sample of AGN designed to span a wide range of black hole mass, AGN luminosity and Eddington ratio in the previously unexplored hard X-ray band. We will use a new maximum likelihood technique (Zoghbi et al., 2013) to construct the power spectral density functions (PSD) over a large range of timescales (years to hundreds of seconds) as well as measure the important break frequency in the PSD for the first time at high energies. The PSDs will allow us to test relationships between the observed properties of the AGN (e.g. \(M_{BH}\) and \(L/L_{Edd}\)) and time variability (Kelly et al., 2013) and to provide important constraints on models of accretion.

Background and Motivation

Accretion onto super-massive black holes (SMBH) is a complex process that produces an extremely large amount of radiation spanning nearly the entire electromagnetic spectrum. As material spirals into the SMBH, the loss of gravitational potential energy is converted into thermal energy and radiated away producing the characteristic optical-UV AGN SED (i.e. “Big Blue Bump”). Due to the high mass of the SMBH though, the accretion disk itself does not heat to high enough temperatures to produce X-ray emission. Rather, theoretical models must invoke a hot corona to explain the large amounts of X-ray emission seen in AGN. In particular X-rays are thought to be produced through thermal Comptonization of seed photons from the accretion disk by the hot plasma sitting above the disk (Haardt et al., 1993). Further a so-called “Compton hump” that peaks around 30 keV can be produced through reflection of the original X-rays by the relatively cold accretion disk (George et al., 1991; Nandra et al., 1994; Magdziarz et al., 1995; Rivers et al., 2011).

Apart from their spectral characteristics, AGN also exhibit large amounts of variability at all wavelengths. This variability is highly aperiodic over a broad range of timescales from years down to hours. The variability power as measured by the power spectral density function (PSD) is seen to decrease as a function of increasing temporal frequency with the shape normally consistent with a power law, \(P\propto\nu^{-\alpha}\) (Lawrence et al., 1987; Lawrence et al., 1993; Green et al., 1993). The value of