UNDER THE SHADOW OF THE MAGELLANIC-BRIDGE - A MEASUREMENT OF THE EXTRAGALACTIC BACKGROUND AT SIMILAR-TO-0.7 KEV

We measured the extragalactic 0.7 keV X-ray background by observing th e X-ray shadow of a neutral gas cloud in the Magellanic Bridge region. Two ROSAT PSPC observations of total 104 ks were complemented by a de tailed H I mapping of the cloud with both the Parkes 64 m telescope an d the Australia Telescope Compact Array. From the detected anti-correl ation between the observed background intensity and the H I column den sity of the cloud, we derived the unabsorbed extragalactic background intensity as similar to 28 keV s(-1) cm(-2) keV(-1) sr(-1) at similar to 0.7 keV. The 95% confidence lower limit 18 keV s(-1) cm(-2) keV(-1) sr(-1) is greater than the expected point-like source contribution le ss than or similar to 14 keV s(-1) cm(-2) keV(-1) sr(-1), constrained by the mean source spectrum together with the total background intensi ty in the 1-2 keV band. A significant fraction of the 0.7 keV backgrou nd likely arises in a diffuse hot intergalactic medium of a few millio n degrees, as has been predicted in hydrodynamic simulations of cosmol ogical structure formation.