Relativistic Iron Features from X-Ray Illuminated Spots and the Measure of the Black Hole Mass in AGN

Abstract

Narrow spectral features in the 5-6 keV range were recently discovered in the X-ray spectra of a few Active Galactic Nuclei. We propose that these features originate from localized spots occurring on the surface of an accretion disc following its illumination by flares. Detailed calculations of the temporal and spectral properties of these features in our proposed model can be found in Dovciak et al. (2004). Comparison of the computed profiles with observed features can help to estimate parameters of the system. In principle this method can provide a powerful tool to measure the mass of super-massive black holes in Active Galactic Nuclei. By comparing our calculations with the Chandra and XMM-Newton results, we show, however, that spectra from present generation X-ray satellites are not of good enough quality to fully exploit the method and determine the black hole mass with sufficient accuracy. This task has to be deferred to future missions with both large sensitivity and high energy resolution, such as Constellation-X and Xeus. §1. The method Relativistic iron line profiles may provide a powerful tool to measure the mass of the black hole in Active Galactic Nuclei (AGNs) and Galactic black hole systems. To this aim, Stella (1990) proposed to use temporal changes in the line profile following variations of the illuminating primary source. Along the same line, Matt and Perola (1992) proposed to employ, instead, variations of the integrated line properties such as equivalent width, centroid energy and line width. These methods are very similar conceptually to the classical reverberation mapping method, widely and successfully applied to optical broad lines in AGNs. Sufficiently long monitoring of the continuum and of the line emission is however required, as well as large enough signal-to-noise ratio, and in practice these methods have not provided many results yet. A simple, direct and potentially robust way to measure the black hole mass would be available if the line emission originates at a given radius and azimuth, as expected if the disc illumination is provided by a localized flare just above the disc (possibly due to magnetic reconnection), rather than a central illuminator or an extended corona. If the resulting 'hot spot' co-rotates with the disc and lives for at least a significant part of an orbit, by fitting the light curve and centroid energy of