Evidence for an X-Ray-emitting Galactic Bulge: Shadows Cast by Distant Molecular Gas

A mosaic of seven ROSAT PSPC pointed observations in the direction of (l, b ~ 10°, 0°) reveals deep X-ray shadows in the 0.5-2.0 keV band cast by dense molecular gas. The comparison between the observed on-cloud and off-cloud X-ray fluxes indicates that ~43% of the diffuse X-ray background in this direction in both the 0.75 and 1.5 keV bands originates behind the molecular gas, which is located at 2-4 kpc from the Sun. Given the short mean free path of X-rays in the 0.75 keV band in the Galactic plane (~1 kpc assuming an average space density of 1 cm-3), this large percentage of the observed flux that originates beyond the molecular gas most likely indicates a strong enhancement in the distribution of X-ray-emitting gas in the Galactic center region, possibly associated with a Galactic X-ray bulge.