Hard X-ray emission from the Galactic ridge^*

¹ Max-Planck-Institute für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany e-mail: krivonos@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

Received: 17 May 2006
Accepted: 13 October 2006

Abstract

We present results of a study of the Galactic ridge X-ray emission (GRXE) in hard X-rays performed with the IBIS telescope aboard INTEGRAL. The imaging capabilities of this coding aperture telescope make it possible to account for the flux from bright Galactic point sources whereas the wide field of view permits us to collect large flux from the underlying GRXE. Extensive study of the IBIS/ISGRI detector background allowed us to construct a model that predicts the detector count rate with ~% accuracy in the energy band 17–60 keV. The derived longitude and latitude profiles of the ridge emission are in good agreement with the Galactic distribution of stars obtained from infrared observations. This, along with the measured hard X-ray spectrum of the Galactic ridge emission strongly indicates its stellar origin. The derived unit stellar mass emissivity of the ridge in the energy band 17–60 keV,   10²⁷ erg s^-1  (assuming a bulge mass of   ), agrees with that of local (in the Solar neigborhood) accreting magnetic white dwarf binaries – dominant contributors to the GRXE at these energies. In addition, the shape of the obtained GRXE spectrum can be used to determine the average mass of white dwarfs in such systems in the Galaxy as ~. The total hard X-ray luminosity of the GRXE is   10³⁷ erg s^-1 in the 17–60 keV band. At energies 70–200 keV no additional contribution to the total emission of the Galaxy apart from the detected point sources is seen.