X‐Ray Iron Line Reverberation from Black Hole Accretion Disks

The relativistically broad X-ray iron line seen in many AGN spectra isthought to originate from the central regions of the putative black holeaccretion disk. Both the line profile and strength will vary in response torapid variability of the primary X-ray continuum source. The temporal responseof the line contains information on the accretion disk structure, the X-raysource geometry, and the spin of the black hole. Since the X-ray source willhave a size comparable to the fluorescing region of the accretion disk, thegeneral reverberation problem is not invertible. However, progress can be madesince, empirically, AGN light curves are seen to undergo dramatic shorttimescale variability which presumably corresponds to the creation of a singlenew active region within the distributed X-ray source. The iron line responseto these individual events can be described using linear transfer theory. Weconsider the line response to the activation/flaring of a new X-ray emittingregion. Most of our detailed calculations are performed for the case of anX-ray source on the symmetry axis and at some height above the disk planearound a Kerr black hole. We also present preliminary calculations for off-axisflares. We suggest ways in which future, high-throughput X-ray observatoriessuch as XMM and the Constellation X-ray Mission may use these reverberationsignatures to probe both the mass and spin of AGN black holes, as well as theX-ray source geometry.