Suzaku Discovery of a Hard Component Varying Independently of the
  Power-Law Emission in MCG-6-30-15
  Focusing on hard X-ray variability, we reanalyzed Suzaku data of 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 is not yet identified, it can be modeled as a
power-law with a photon index ~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 > 8 times the
gravitational radii from the central black hole, and a moderate reflection with
a covering fraction of ~3.4. 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, is a model dependent result.
