Resolving the Galactic ridge X-ray background

¹ Max-Planck-Institute für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany e-mail: mikej@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 30 November 2006
Accepted: 24 July 2007

Abstract

We use Chandra deep observations of the Galactic Center (GC) region to improve the constraints on the unresolved fraction of the Galactic ridge X-ray background in the 4–8 keV band. We emphasize the importance of correcting the measured source counts at low fluxes for bias associated with Poisson noise. We find that at distances of – from Sgr A^* at least ~% of the total X-ray emission in the energy band 4–8 keV originates from point sources with luminosities  erg s^-1. From a comparison of the source number-flux function in the GC region with the known luminosity function of faint X-ray sources in the Solar vicinity, we infer that Chandra has already resolved a large fraction of the cumulative contribution of cataclysmic variables to the total X-ray flux from the GC region. This comparison further indicates that most of the yet unresolved X-ray flux from the GC region (~60%) is likely produced by weak cataclysmic variables and coronally active stars with erg s^-1. We conclude that the bulk of the high energy Galactic X-ray background in this direction is produced by discrete sources, and that this is likely to be the case for the rest of the Galactic ridge.