Iron Fluorescence from within the Innermost Stable Orbit of Black Hole Accretion Disks

Abstract

The fluorescent iron Kα line is a powerful observational probe of the inner regions of black hole accretion disks. Previous studies have assumed that only material outside the radius of marginal stability (r = 6m for a Schwarzschild hole) 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 spiraling toward the event horizon, can have an 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, because of 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. Thus, we can explain the iron line properties of MCG -6-30-15 within the context of an accretion disk around a nonrotating black hole. This contrasts with previous studies that, due to the absence of fluorescence from within the radius of marginal stability, have indicated that this object possesses a rapidly rotating (i.e., near-extremal Kerr) black hole. This is an important issue since it has a direct bearing on the spin paradigm for the radio-loud/radio-quiet dichotomy seen in accreting black hole systems. We discuss some future observational tests that could help distinguish slowly rotating black holes from rapidly rotating holes.