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 . 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 × 107 MO in NGC 4051, 8.02 × 107 M O in MRK 766, and 4.68 × 107 M O 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 of a ≲ 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 × 10-2 to 4.25 × 106, containing the observed Q. The derived value of Q ∼ 32 for REJ 1034+396 therefore suggests that the AGN hosts a thin disk. © 2014. The American Astronomical Society. All rights reserved.