X-ray iron line variability: constraints on the inner accretion disk

After reviewing the basic physics of X-ray reflection in AGN, we present three case studies which illustrate the current state of X-ray reflection studies. For the low-luminosity AGN NGC 4258, we find that the iron line is much narrower than is typically found in higher luminosity AGN. We argue that this is evidence for either a truncated cold accretion disk (possibly due to a transition to an advection dominate accretion flow at r ∼ 100GM/c) or a large (r ∼ 100GM/c) X-ray emitting corona surrounding the accretion disk. We also present results for the higher luminosity Seyfert nuclei in NGC 5548 and MCG–6-30-15. In both of these sources, RXTE shows that the iron line equivalent width decreases with increasing luminosity. Furthermore, the iron line equivalent width is found to be anticorrelated with the relative strength of the reflection continuum, contrary to all simple reflection models. It is proposed that continuum-flux correlated changes in the ionization of the accretion disk surface can explain this spectral variability. Finally, we address the issue of X-ray iron line reverberation in the light of these complicating factors.