X‐Ray Reflection from Inhomogeneous Accretion Disks. II. Emission‐Line Variability and Implications for Reverberation Mapping

One of the principal scientific objectives of the upcoming Constellation-Xmission is to attempt to map the inner regions of accretion disks around blackholes in Seyfert galaxies by reverberation mapping of the Fe K fluorescenceline. This area of the disk is likely radiation pressure dominated and subjectto various dynamical instabilities. Here, we show that density inhomogeneitiesin the disk atmosphere resulting from the photon bubble instability (PBI) cancause rapid changes in the X-ray reflection features, even when theilluminating flux is constant. Using a simulation of the development of thePBI, we find that, for the disk parameters chosen, the Fe K and O VIII Ly\alphalines vary on timescales as short as a few hundredths of an orbital time. Inresponse to the changes in accretion disk structure, the Fe K equivalent width(EW) shows variations as large as ~100 eV. The magnitude and direction(positive or negative) of the changes depends on the ionization state of theatmosphere. The largest changes are found when the disk is moderately ionized.The O VIII EW varies by tens of eV, as well as exhibiting plenty of rapid,low-amplitude changes. This effect provides a natural explanation for someobserved instances of short timescale Fe K variability which was uncorrelatedwith the continuum (e.g., Mrk 841). New predictions for Fe K reverberationmapping should be made which include the effects of this accretion disk drivenline variability and a variable ionization state. Reflection spectra averagedover the evolution of the instability are well fit by constant density modelsin the 2-10 keV region.Comment: 20 pages, 3 figures. Accepted by Ap