Broad-band spectral analysis of the Galactic Ridge X-ray Emission
  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, broad-band and high-quality GRXE spectra were produced from 18
pointing observations with Suzaku in the Galactic bulge region, with the 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 (0.59-0.75) solar, and a softer optically-thin thermal emission
with a plasma temperature of 1.2-1.5 keV which is attributable to coronal X-ray
sources.
  When combined with previous studies which employed high spatial resolution of
the Chandra satellite (e.g. Revnivtsev et al. 2009, Nature), the present
spectroscopic result gives another strong support to a 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 ~10^30-10^34 erg/s, the result is consistent with a value which was
reported previously by directly resolving faint point sources.
