On Time‐dependent X‐Ray Reflection by Photoionized Accretion Disks: Implications for Fe Kα Line Reverberation Studies of Active Galactic Nuclei

We perform a first study of time-dependent X-ray reflection in photo-ionizedaccretion disks. We assume a step-functional change in the X-ray flux and use asimplified prescription to describe the time evolution of the illuminated gasdensity profile in response to changes in the flux. We find that the dynamicaltime for re-adjustment of the hydrostatic balance is an important relaxationtime scale of the problem since it affects evolution of the ionization state ofthe reflector. Because of this the Fe K-alpha line emissivity depends on theshape and intensity of the illuminating flux in prior times, and hence it isnot a function of the instantaneous illuminating spectrum. Moreover, during thetransition, a prominent Helium-like component of the Fe K-alpha line mayappear. As a result, the line flux may appear to be completely uncorrelatedwith X-ray continuum flux on time scales shorter than the dynamical time. Inaddition, the time-dependence of the illuminating flux may leave imprints evenon the time-averaged line spectra, which may be used as an additional test ofaccretion disk geometry. Our findings appear to be important for the proposedFe K-alpha line reverberation studies in lamppost-like geometries for accretionrates exceeding about $\sim 1%$ of the Eddington value. However, most AGN donot show Helium-like lines that are prominent in such models, probablyindicating that these models are not applicable to real sources.Comment: submitted to ApJ May 200