Relativistic spectral features from X-ray-illuminated spots and the measure of the black hole mass in active galactic nuclei

Narrow spectral features in the 5-6 keV range were recently discovered in the X-ray spectra of a few active galactic nuclei. We discuss the possibility that these features are due to localized spots which occur on the surface of an accretion disc following its illumination by flares. We present detailed line profiles as a function of orbital phase of the spot and its radial distance from a central black hole. Comparison of these 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 simulations 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 exploit the method fully and determine the black hole mass with sufficient accuracy. This task has to be deferred to future missions with high throughput and high energy resolution, such as Constellation-X and Xeus.