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 profile 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 Kα 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 Kα line may appear. As a result, the Fe Kα line flux may appear to be completely uncorrelated with X-ray continuum flux on timescales shorter than the dynamical time. In addition, the time dependence of the illuminating flux may leave imprints even on the time-averaged Fe Kα line spectra, which may be used as an additional test of accretion disk geometry. Our findings appear to be important for the proposed Fe Kα 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 prominent in such models, probably indicating that these models are not applicable to real sources.