GR models of the X-ray spectral variability of MCG--6-30-15

TL;DR

This paper evaluates general relativistic models of X-ray spectral variability in MCG--6-30-15, comparing predictions with Suzaku data, and suggests the source is close to a rapidly rotating black hole, with some discrepancies in variability features.

Contribution

It introduces and tests GR models of X-ray variability with different geometries, identifying the most consistent scenario involving a source near a rapidly rotating black hole.

Findings

Models with the source moving vertically or corotating are disfavored.

The best-fit model involves a source close to a rapidly rotating black hole.

Some predicted variability features are not observed in Suzaku data.

Abstract

We study in detail the GR models of the X-ray spectral variability for various geometries of the X-ray source and with various relativistic effects being the dominant cause of spectral variability. The predicted properties are compared with the Suzaku observational data of the Seyfert 1 galaxy MCG--6-30-15. The data disfavor models with the X-ray source (1) moving vertically on the symmetry axis or (2) corotating with the disc and changing height not far above the disc surface. The most likely explanation for the observed variability is given by the model involving the X-ray source located at a very small, varying distance from a rapidly rotating black hole. This model predicts some enhanced variations in the red wing of the Fe line, which are not seen in the Suzaku observations. However, the enhanced variability of the red wing, while ruled out by the Suzaku data, is consistent with an excess RMS variability, between 5 and 6 keV, reported for some previous ASCA and XMM observations. We speculate that the presence or lack of such a feature is related to the change of the ionization state of the innermost part of the disc, however, investigation of such effects is currently not possible in our model (where a neutral disc is assumed). If the model, completed by description of ionization effects, proves to be fully consistent with the observational data, it will provide a strong indication that the central black hole in MCG--6-30-15 rotates rapidly, supporting similar conclusions derived from the Fe line profile.