SOFT-X-RAY AND HI SURVEYS OF THE LOW NH REGION IN URSA MAJOR

A region of almost 300 deg2 in Ursa Major around the direction of the lowest neutral hydrogen column density in the sky has been surveyed in the 1/4 keV X-ray band (approximately 0.12-0.284 keV at 10% of the pe ak response) using the ROSAT PSPC and has been fully mapped in the 21 cm line of neutral hydrogen with an angular resolution of 21' or bette r. We present maps of these two data sets and an analysis of the spati al correlation between them. Over the entire field, the 1/4 keV band X -ray count rate shows a strong negative correlation with the column de nsity of Galactic neutral hydrogen. We discuss the local, extragalacti c, and halo contributions to the observed diffuse X-ray intensity and derive an upper limit to the Galactic X-ray emission originating beyon d this H I distribution that is more than a factor of 7 smaller than t he inferred flux beyond the Draco nebula, an apparent halo object loca ted approximately 41-degrees away. This implies that while there may b e significant amounts of hot gas in the Galactic halo, its properties must vary strongly across the sky.