Iron fluorescence from within the innermost stable orbit of black hole accretion disks

Abstract

The fluorescent iron Ka line is a powerful observational probe of the inner regions of black holes accretion disks. Previous studies have assumed that only material outside the radius of marginal stability can contribute to the observed line emission. Here, we show that fluorescence by material inside the radius of marginal stability, which is in the process of spiralling towards the event horizon, can have a observable influence on the iron line profile and equivalent width. For concreteness, we consider the case of a geometrically thin accretion disk, around a Schwarzschild black hole, in which fluorescence is excited by an X-ray source placed at some height above the disk and on the axis of the disk. Fully relativistic line profiles are presented for various source heights and efficiencies. It is found that the extra line flux generally emerges in the extreme red wing of the iron line, due to the large gravitational redshift experienced by photons from the region within the radius of marginal stability. We apply our models to the variable iron line seen in the ASCA spectrum of the Seyfert nucleus MCG-6-30-15. It is found that the change in the line profile, equivalent width, and continuum normalization, can be well explained as being due to a change in the height of the source above the disk. We discuss the implications of these results for distinguishing rapidly-rotating black holes from slowly rotating holes using iron line diagnostics.