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-ionized 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 re-adjustment of the hydrostatic balance is an important relaxation time scale of the problem since it affects evolution of the ionization state of the reflector. Because of
... ctor. Because of this the Fe K-alpha line emissivity depends on the shape and intensity of the illuminating flux in prior times, and hence it is not a function of the instantaneous illuminating spectrum. Moreover, during the transition, a prominent Helium-like component of the Fe K-alpha line may appear. As a result, the line flux may appear to be completely uncorrelated with X-ray continuum flux on time scales shorter than the dynamical time. In addition, the time-dependence of the illuminating flux may leave imprints even on the time-averaged line spectra, which may be used as an additional test of accretion disk geometry. Our findings appear to be important for the proposed Fe K-alpha line reverberation studies in lamppost-like geometries for accretion rates exceeding about $\sim 1%$ of the Eddington value. However, most AGN do not show Helium-like lines that are prominent in such models, probably indicating that these models are not applicable to real sources.