General relativistic models of the X-ray spectral variability of MCG–6-30-15

¹ University of Łódź, Department of Astrophysics, Pomorska 149/153, 90-236 Łódź, Poland e-mail: niedzwiecki@uni.lodz.pl
² The Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
³ Department of Astronomy, Graduate School of Science, University of Tokyo,7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Received: 21 February 2009
Accepted: 9 October 2009

Abstract

Context. The extremely relativistic Fe lines, detected from some active galactic nuclei (AGN), indicate that generation and reprocessing of the X-ray emission takes place in the immediate vicinity of the event horizon. Recently, general relativistic (GR) effects, in particular light bending which is very strong in that region, have been considered to be the cause of complex variability patterns observed in these AGNs.

Aims. 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 observational data of the Seyfert 1 galaxy MCG–6-30-15, which is currently the best studied AGN with signatures of strong gravity effects.

Methods. We focus on modeling the root mean square (rms) spectra. We compute the rms spectra for the GR models using a Monte Carlo method, and compare them with the rms spectra from the Suzaku observations of MCG–6-30-15 on January 2006.

Results. 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 fractional 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 a 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.