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 ~ 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 ~ 337°, b ~ 4° 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 ~ 2 kpc. The implied surface brightness of this distant emission source can account for ~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 ~6 kpc and an X-ray luminosity of ~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 ~4 × 10 6 K and a total thermal energy in the range of 6-9 × 10 55 ergs.