Neutral hydrogen in the direction of the Small Magellatic Cloud and the limits of an extragalactic soft X-ray flux

Abstract

Previously reported X-ray data from the vicinity of the Small Magellanic Cloud (SMC) have been reanalyzed using H I data from a more detailed and sensitive 21-cm survey of this region. The results support the previous conclusion: assuming that the interstellar material absorbs according to the cross sections of Brown and Gould (1970), at least 75 per cent of the observed 0.25-keV X-ray flux is of local origin. The corollary problem of placing a cosmologically useful upper limit on the extragalactic flux will be difficult to solve until the behavior of the local component is better understood; but if the local flux is isotropic, a 3-sigma upper limit of 240 photons per (sq cm s sr keV) at 0.25 keV may be placed on a flux originating beyond the SMC. Tables of H I column density are given for an area 30 deg by 30 deg about the SMC. The high-velocity component, presumably associated with the Magellanic Cloud system, and the galactic disk component are tabulated separately.