Modelling the variability of the Fe Ka line in accreting black holes

Abstract

The variability of the Fe Ka line near 6.5 keV seems to be reduced compared to the variability of the hard X-rays which presumably drive the line emission. This is observed in both active galactic nuclei and galactic black hole binaries. We point out that such reduced variability, as well as lack of coherence between the variations of the line and the continuum, are a natural prediction of a propagation model of variability in the geometry of inner hot accretion flow. We compute detailed model predictions of the variability characteristics, which could be compared with current and future data. We also point out that the model requires a gradual disappearance of the cold disc, rather than a sharp transition from the cold disc to a hot flow.