X-Ray Iron Line Reverberation from Black Hole Accretion Disks

The relativistically broad X-ray iron line seen in many active galactic nucleus (AGN) spectra is thought to originate from the central regions of the putative black hole accretion disk. Both the line profile and strength will vary in response to rapid variability of the primary X-ray continuum source. The temporal response of the line contains information on the accretion disk structure, the X-ray source geometry, and the spin of the black hole. Since the X-ray source will have a size comparable with the fluorescing region of the accretion disk, the general reverberation problem is not invertible. However, progress can be made, since, empirically, AGN light curves are seen to undergo dramatic short timescale variability that presumably corresponds to the creation of a single new active region within the distributed X-ray source. The iron-line response to these individual events can be described using linear transfer theory. We consider the line response to the activation/flaring of a new X-ray emitting region. Most of our detailed calculations are performed for the case of an X-ray source on the symmetry axis and at some height above the disk plane around a Kerr black hole. We also present preliminary calculations for off-axis flares. We suggest ways in which future, high-throughput X-ray observatories such as XMM and the Constellation X-ray Mission may use these reverberation signatures to probe both the mass and spin of AGN black holes as well as the X-ray source geometry.