The (un)resolved X-ray background in the Lockman Hole

Abstract

Most of the soft and a growing fraction of the harder X-ray background has been resolved into emission from point sources, yet the resolved fraction above 7 keV has only been poorly constrained. We use ~700 ks of XMM-Newton observations of the Lockman Hole and a photometric approach to estimate the total flux attributable to resolved sources in a number of different energy bands. We find the resolved fraction of the X-ray background to be ~90 per cent below 2 keV but it decreases rapidly at higher energies with the resolved fraction above ~7 keV being only ~50 per cent. The integrated X-ray spectrum from detected sources has a slope of Γ~ 1.75, much softer than the Γ= 1.4 of the total background spectrum. The unresolved background component has the spectral signature of highly obscured active galactic nuclei.