THE IRON K-ALPHA RESPONSE IN AN X-RAY ILLUMINATED RELATIVISTIC DISK AND A BLACK-HOLE MASS ESTIMATE

A method is proposed for the estimate of the black hole mass in an active galactic nucleus which is based on the response of the intensity, centroid energy and width of the iron Kalpha fluorescence line to variations of the ionizing continuum. The method is illustrated for the case of a geometrically thin and optically thick, X-ray illuminated disc, in Keplerian rotation around a Schwarzschild black hole, using a detailed model of the line emissivity. This method is complementary to the one that makes use of variations in detailed line profiles, and could be applied also to measurements obtained at moderate energy resolution.