Distance limits on the bright X-ray emission toward the Galactic center: Evidence for a very hot interstellar medium in the Galactic X-ray bulge

Observations of the diffuse X-ray background at energies similar to3/4 and 1.5 keV show a large region of enhanced emission around the Galactic center. The origin of this X-ray enhancement is not known, but the best candidates are the nearby Loop I superbubble and a Galactic X-ray bulge. To differentiate between these two possibilities, the distance scales to the X-ray-emitting material along the line of sight must be established. A mosaic of 13 ROSAT PSPC pointings in the direction of l similar to 337 degrees, b similar to4 degrees reveals X-ray shadows in the and 1.5 keV bands cast by a distant molecular cloud complex. Analysis of the shadows indicates that a large fraction (45% <^>9%) of the observed emission in this direction originates beyond the cloud complex, located at d similar to2 kpc. The implied surface brightness of this distant emission source can account for similar to 70% of the enhanced emission away from the absorption trough in the Galactic plane. This result indicates that the Loop I bubble cannot be the principal source of the enhanced X-ray emission, and suggests the existence of a bright X-ray source occupying the central region of the Galaxy, with a radial extent of similar to6 kpc and an X-ray luminosity of similar to 10(39) ergs s(-1). We examine some simple models of the emission region and compare them to the ROSAT all-sky survey. A thermal origin for the emission implies a plasma temperature of similar to 4x10(6) K and a total thermal energy in the range of 6-9x10(55) ergs.