X-Ray Variability and the Inner Region in Active Galactic Nuclei

Abstract

We present theoretical models of X-ray variability attributable to orbital signatures from an accretion disk including emission region size, quasi-periodic oscillations (QPOs) and its quality factor $Q$, and the emergence of a break frequency in the power spectral density shape. We find a fractional variability amplitude of $F_{var}\propto M_{\bullet }^{-0.4}$. We conduct a time series analysis on X-ray light curves ($0.3-10$ keV) of a sample of active galactic nuclei (AGNs). A statistically significant bend frequency is inferred in 9 of 58 light curves (16 %) from 3 AGNs for which the break timescale is consistent with the reported BH spin but not with the reported BH mass. Upper limits of $2.85\times 10^7{M}_{\odot }$ in NGC 4051, $8.02\times 10^7{M}_{\odot }$ in MRK 766 and $4.68\times 10^7{M}_{\odot }$ in MCG-6-30-15 are inferred for maximally spinning BHs. For REJ 1034+396, where a QPO at 3733 s was reported, we obtain an emission region size of $(6-6.5)M$ and a BH spin $a\lesssim$ 0.08. The relativistic inner region of a thin disk, dominated by radiation pressure and electron scattering is likely to host the orbital features as the simulated $Q$ ranges from $6.3\times 10^{-2}$ to $4.25\times 10^6$, containing the observed $Q$. The derived value of $Q\sim$ 32 for REJ 1034+396 therefore suggests that the AGN hosts a thin disk.

32 pages, 11 figures (9 sub-figures); 4 Tables; 1 appendix; published in the Astrophysical Journal

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