Suzaku Discovery of a Hard Component Varying Independently of the Power-Law Emission in MCG -6-30-15

Abstract

Focusing on hard X-ray variability, we reanalyzed Suzaku data of the Type I Seyfert galaxy MCG -6-30-15 obtained in 2006. Intensity-sorted spectroscopy and a principal-component analysis consistently revealed a very hard component that varies independently of the dominant power-law emission. Although the exact nature of this hard component has not yet been identified, it can be modeled as a power-law with a photon index of ∼2 affected by a partial covering absorption, or as a thermal Comptonization emission with a relatively large optical depth. When this component is included in the fitting model, the time-averaged 2.5-55 keV spectrum of MCG -6-30-15 can be reproduced successfully by invoking a mildly broadened iron line with its emission region located at gtrsim8 times the gravitational radii from the central black hole, and a moderate reflection with a covering fraction of ∼3.4π. This result implies that the solution of a highly spinning black hole in MCG -6-30-15, obtained by Miniutti et al. (2007, PASJ, 59, S315) using the same Suzaku data as ours, is a model-dependent result.