ASCA Observation of an "X-ray Shadow" in the Galactic Plane

Abstract

The diffuse X-ray background (DXB) emission near the Galactic plane ($l,b\sim 25.6^{\circ },0.78^{\circ }$) has been observed with $ASCA$. The observed region is toward a Galactic molecular cloud which was recently reported to cast a deep X-ray shadow in the 0.5 $-$ 2.0 keV band DXB. The selection of this particular region is intended to provide a constraint on the spatial distribution of the DXB emission along the line of sight: i.e., the molecular cloud is optically thick at $<$2 keV and so the bulk of the observed soft X-rays {\it must} originate in the foreground of the cloud, which is at $\sim$3 kpc from the Sun. In the 0.8 $-$ 9.0 keV band, the observed spectrum is primarily from multiple components of thermal plasmas. We here report a detection of soft X-ray (0.5 $-$ 2 keV) emission from an $\sim 10^7$ K thermal plasma. Comparisons with the {\it ROSAT} data suggest that this soft X-ray emission is absorbed by $N_H$ = 1 $-$ 3 $\times$ 10${}^{21}$ cm${}^{-2}$, which implies a path-length through the soft X-ray emitting regions of $\la$1 kpc from the Sun.

24 pages including 8 figures, accepted for ApJ

References in corpus (3)

• RXTE Measurement of the Diffuse X-ray Emission From the Galactic Ridge: Implications for the Energetics of the Interstellar Medium
• On the Origin of the Iron K Line in the Spectrum of the Galactic X-ray Background
• Evidence for the Galactic X-ray Bulge II

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