Broadband Spectral Analysis of the Galactic Ridge X-Ray Emission

Abstract

Detailed spectral analysis of the Galactic X-ray background emission, or the Galactic Ridge X-ray Emission (GRXE), is presented. To study the origin of the emission, broadband and high-quality GRXE spectra were produced from 18 pointing observations with Suzaku in the Galactic bulge region, with a total exposure of 1 Ms. The spectra were successfully fitted by a sum of two major spectral components: a spectral model of magnetic accreting white dwarfs with a mass of 0.66^+0.09 _-0.07 M _⊙ and a softer optically thin thermal emission with a plasma temperature of 1.2-1.5 keV that is attributable to coronal X-ray sources. When combined with previous studies that employed high spatial resolution of the Chandra satellite, the present spectroscopic result gives stronger support to the scenario that the GRXE is essentially an assembly of numerous discrete faint X-ray stars. The detected GRXE flux in the hard X-ray band was used to estimate the number density of the unresolved hard X-ray sources. When integrated over a luminosity range of ~10³⁰-10³⁴ erg s^-1, the result is consistent with a value that was reported previously by directly resolving faint point sources.