A Variable Partial Covering Model for the Seyfert 1 Galaxy MCG-6-30-15

Abstract

We propose a simple spectral model for the Seyfert 1 Galaxy MCG-6-30-15 that can explain most of the 1 - 40 keV spectral variation by change of the partial covering fraction, similar to the one proposed by Miller et al. (2008). Our spectral model is composed of three continuum components; (1) a direct power-law component, (2) a heavily absorbed power-law component by mildly ionized intervening matter, and (3) a cold disk reflection component far from the black hole with moderate solid-angle (Ω/2π \approx 0.3) accompanying a narrow fluorescent iron line. The first two components are affected by the surrounding highly ionized thin absorber with N_H \approx 10^{23.4}cm-2 and log ξ \approx 3.4. The heavy absorber in the second component is fragmented into many clouds, each of which is composed of radial zones with different ionization states and column densities, the main body (N_H \approx 10^24.2cm-2, log ξ \approx 1.6), the envelope (N_H \approx 10^22.1cm-2, log ξ \approx 1.9) and presumably a completely opaque core. These parameters of the ionized absorbers, as well as the intrinsic spectral shape of the X-ray source, are unchanged at all. The central X-ray source is moderately extended, and its luminosity is not significantly variable. The observed flux and spectral variations are mostly explained by variation of the geometrical partial covering fraction of the central source from 0 (uncovered) to \sim0.63 by the intervening ionized clouds in the line of sight. The ionized iron K-edge of the heavily absorbed component explains most of the seemingly broad line-like feature, a well-known spectral characteristic of MCG-6-30-15. The direct component and the absorbed component anti-correlate, cancelling their variations each other, so that the fractional spectral variation becomes the minimum at the iron energy band; another observational characteristic of MCG-6-30-15 is thus explained.