The flare model for X-ray variability of NGC 4258

¹ Institute of Physics, Jagiellonian University, Reymonta 4, 30059 Krakow, Poland
² Copernicus Astronomical Center, Bartycka 18, 00716 Warsaw, Poland
³ Astronomical Institute, Academy of Sciences, Boční II 1401, 14131 Prague, Czech Republic
e-mail: vladimir.karas@cuni.cz
⁴ Observatoire Astronomique de Strasbourg, 67000 Strasbourg, France
⁵ Faculty of Physics, University of Białystok, Lipowa 41, 15424 Białystok, Poland

Received: 20 December 2010
Accepted: 18 April 2011

Abstract

Aims. We study the variability mechanism of active galactic nuclei (AGN) within the framework of the flare model. We examine the case of Seyfert/LINER galaxy NGC 4258, which is observed at high inclination angle and exhibits rapid fluctuations in its X-ray light curve.

Methods. We construct a model light curve based on the assumption of magnetic flares localized in the equatorial plane and orbiting with Keplerian speed at each given radius. We calculate the level of variability as a function of the inclination of an observer, taking into account all effects of general relativity near a rotating supermassive black hole.

Results. The variability level is a monotonic function of the source inclination. It rises more rapidly for larger values of the black hole spin (Kerr parameter a) and for steeper emissivity (index β of the radial profile). We compare the expected level of variability for the viewing angle 81.6 deg, as inferred for NGC 4258, with the case of moderate viewing angles of about 30 deg, which are typical of Seyfert type-1 galaxies.

Conclusions. Highly inclined sources such as this one are particularly suitable to test the flare model because the orbital motion, Doppler boosting, and light bending are all expected to have maximum effect when the accretion disk is seen almost edge-on. The model is consistent with the NGC 4258 variability, where the obscuring material is thought to be localized mainly toward the equatorial plane rather than forming a geometrically thick torus. Once the intrinsic timescales of the flare duration are determined with higher precision, this kind of highly inclined objects with a precisely known mass of the black hole can be used to set independent constraints on the spin parameter.