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

Abstract

Observations of the di †use X-ray background at energies and 1.5 keV show a large region of D3 4 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 di †erentiate between these two possibilities, the distance scales to the X-rayÈemitting material along the line of sight must be established. A mosaic of 13 ROSAT PSPC pointings in the direction of l D 337¡, b D 4¡ reveals X-ray shadows in the and 1.5 keV bands cast by a distant molecular cloud complex. Analysis of the 3 4 shadows indicates that a large fraction (45% ^ 9%) of the observed emission in this direction originates beyond the cloud complex, located at d D 2 kpc. The implied surface brightness of this distant emission source can account for D70% of the enhanced emission away from the absorption trough in the Galac-tic 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 D6 kpc and an X-ray luminosity of D1039 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 D4 ] 106 K and a total thermal energy in the range of 6È9 ] 1055 ergs.