Constraints on light bending and reflection from the hard X-ray background

TL;DR

This paper uses the hard X-ray background spectrum to constrain the prevalence of reflection-dominated sources caused by light bending near black holes, providing limits on their fraction and implications for future observations.

Contribution

It introduces a method to constrain the fraction of light-bending influenced sources using the X-ray background spectrum and models their spectral properties and evolution.

Findings

Allowed a significant fraction of sources with strong light bending.

Provided limits on the reflection-dominated source fraction based on spectral modeling.

Discussed how future X-ray missions can further test these constraints.

Abstract

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 emphasize the need for reflection and explore the broad-band properties of realistic spectra that incorporate light bending. We then use these spectra, in conjunction with the observed 2-10 keV AGN distribution, evolutionary and obscuration functions in order to predict the hard X-ray background spectrum over 3-100 keV, and provide limits on the fraction of reflection-dominated objects, dependent on the height of the photon sources. 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 and implications discussed. We discuss prospects for future hard X-ray 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.