X-ray iron line reverberation from black hole accretion disks

The relativistically broad X-ray iron line seen in many active galactic nuc leus (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 sourc e. 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 reg ion of the accretion disk, the general reverberation problem is not inverti ble. However, progress can be made, since, empirically, AGN light curves ar e seen to undergo dramatic short timescale variability that presumably corr esponds to the creation of a single new active region within the distribute d X-ray source. The iron-line response to these individual events can be de scribed using linear transfer theory. We consider the line response to the activation/flaring of a new X-ray emitting region. Most of our detailed cal culations are performed for the case of an X-ray source on the symmetry axi s 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 w hich future, high-throughput X-ray observatories such as XMM and the Conste llation 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.