Evidence for an X–ray Emitting Galactic Bulge: Shadows Cast by Distant Molecular Gas

A mosaic of 7 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 34 keV 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 34 keV band in the Galactic plane (∼ 1 kpc assuming an average space density of 1 cm−3), this large percentage of the observed flux which 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. Subject headings: Galaxy: structure — ISM: structure — X-rays: galaxies, ISM