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

Abstract

We perform the first study of time-dependent X-ray reflection in photoionized accretion disks. We assume a step-functional change in the X-ray flux and use a simplified prescription to describe the time evolution of the illuminated gas density proÐle in response to changes in the flux. We find that the dynamical time for readjustment of the hydrostatic balance is an important relaxation timescale of the problem since it affects the evolution of the ionization state of the reflector. If the variations of the X-ray flux occur on timescales shorter than this time, then the Fe Ka line emissivity is not a function of the instantaneous illuminating spectrum since it depends on the shape and intensity of the illuminating flux in prior times. Moreover, during the transition, a prominent Helium-like component of the Fe Ka line may appear. As a result, the Fe Ka line flux may appear to be completely uncorrelated with X-ray con- tinuum flux on timescales shorter than the dynamical time. In addition, the time dependence of the illu- minating flux may leave imprints even on the time-averaged Fe Ka line spectra, which may be used as an additional test of accretion disk geometry. Our findings appear to be important for the proposed Fe Ka line reverberation studies in lamppost-like geometries for accretion rates exceeding about ~1% of the Eddington value. However, most active galactic nuclei do not show Helium-like lines that are promi- nent in such models, probably indicating that these models are not applicable to real sources.