Light bending and the hard X-ray background

摘要

Light bending due to strong gravity has recently been invoked to explain variability and flux correlations between different bands in some accreting black holes. A characteristic feature of light bending is reflection-dominated spectra, especially if photon sources lie in the deepest parts of the gravitational potential within a few gravitational radii of the event horizon. We use the spectrum of the hard X-ray background in order to constrain the prevalence of such reflection-dominated sources. We first explore the broad-band properties of realistic spectra that incorporate light bending and then use these spectra, in conjunction with the observed 2-10 keV AGN luminosity functions in order to predict the hard X-ray background spectrum over 3-100 keV, and provide limits on the fraction of reflection-dominated sources, dependent on the flare height. Our results allow for a cosmologically- significant fraction of sources that incorporate strong light bending. The luminosity function based on intrinsic flare luminosities is derived. We discuss prospects for missions such as NeXT and Simbol-X that can image such sources as well as confirm the precise spectral shape of the background near its peak, important for constraining the universal relevance of light bending.