Soft X-ray and H I surveys of the low N[SUB]H[/SUB] region in Ursa Major

Abstract

A region of almost 300 deg squared 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 to 0.284 keV at 10% of the peak response) using the ROSAT position sensitive proportional counter (PSPC) and has been fully mapped in the 21 cm line of neutral hydrogen with an angular resolution of 21' or better. We present maps of these two data sets and an analysis of the spatial correlation between them. Over the entire field, the 1/4 keV band X-ray count rate shows a strong negative correlation with the column density of Galactic neutral hydrogen. We discuss the local, extragalactic, and halo contributions to the observed diffuse X-ray intensity and derive an upper limit to the Galactic X-ray emission originating beyond this H I distribution that is more than a factor of 7 smaller than the inferred flux beyond the Draco nebula, an apparent halo object located approximately 41 degs away. This implies that while there may be significant amounts of hot gas in the Galactic halo, its properties must vary strongly across the sky.