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

TL;DR

This study reanalyzed Suzaku X-ray data of MCG-6-30-15, revealing a hard component varying independently of the main emission, which impacts interpretations of the black hole's spin and accretion environment.

Contribution

It identifies and models a previously unrecognized hard X-ray component in MCG-6-30-15, challenging prior assumptions about black hole spin measurements.

Findings

Discovery of a hard X-ray component varying independently of the primary emission

Successful spectral modeling including the hard component with a broad iron line

Implication that black hole spin estimates are model-dependent

Abstract

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.