ASCAObservation of an “X‐Ray Shadow” in the Galactic Plane

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 observedregion is toward a Galactic molecular cloud which was recently reported to casta deep X-ray shadow in the 0.5 $-$ 2.0 keV band DXB. The selection of thisparticular region is intended to provide a constraint on the spatialdistribution of the DXB emission along the line of sight: i.e., the molecularcloud is optically thick at $<$2 keV and so the bulk of the observed softX-rays {\it must} originate in the foreground of the cloud, which is at $\sim$3kpc from the Sun. In the 0.8 $-$ 9.0 keV band, the observed spectrum isprimarily from multiple components of thermal plasmas. We here report adetection of soft X-ray (0.5 $-$ 2 keV) emission from an $\sim10^{7}$ K thermalplasma. Comparisons with the {\it ROSAT} data suggest that this soft X-rayemission is absorbed by $N_H$ = 1 $-$ 3 $\times$ 10$^{21}$ cm$^{-2}$, whichimplies a path-length through the soft X-ray emitting regions of $\la$1 kpcfrom the Sun.Comment: 24 pages including 8 figures, accepted for Ap