The Distribution and Cosmic Density of Relativistic Iron Lines in Active Galactic Nuclei

Abstract

X-ray observations of several active galactic nuclei show prominent iron K-shell fluorescence lines that are sculpted due to special and general relativistic effects. These observations are important because they probe the space-time geometry close to distant black holes. However, the intrinsic distribution of Fe line strengths in the cosmos has never been determined. This uncertainty has contributed to the controversy surrounding the relativistic interpretation of the emission feature. Now, by making use of the latest multi-wavelength data, we show theoretical predictions of the cosmic density of relativistic Fe lines as a function of their equivalent width and line flux. We are able to show unequivocally that the most common relativistic iron lines in the universe will be produced by neutral iron fluorescence in Seyfert galaxies and have equivalent widths < 100 eV. Thus, the handful of very intense lines that have been discovered are just the bright end of a distribution of line strengths. In addition to validating the current observations, the predicted distributions can be used for planning future surveys of relativistic Fe lines. Finally, the predicted sky density of equivalent widths indicate that the X-ray source in AGNs can not, on average, lie on the axis of the black hole.