A light bending model for the X-ray temporal and spectral properties of accreting black holes

Abstract

Uncorrelated variability between direct X-ray continuum and reflection components (including the Fe line, if present) has been reported in many accreting black hole systems, challenging theoretical models of disc reflection. We explore a model based on gravitational light bending in the near vicinity of the central black hole in which the X-ray variability is controlled by the height of the primary source of X-rays above the accretion disc. The relationship between the direct power law component (PLC) and the reflection-dominated component (RDC) is not trivial and allows to identify three different regimes in which the RDC (and the Fe line) is correlated, anti-correlated or almost independent with respect to the PLC. X-ray observations of the active galaxy MCG-6-30-15 and the black hole candidate XTE J1650-500 (and possibly other sources, e.g. NGC 4051) reveal that a series of predictions are actually observed: the behaviour of the Fe line flux, profile, and EW and of the reflection fraction with respect to the continuum supports our model which may explain what are otherwise puzzling properties of these sources.