SHADOWING OF THE 0.25-KEV EXTRAGALACTIC X-RAY-BACKGROUND BY THE DISC OF NGC-55

ROSAT observations are used to search for a shadow in the 0.25-keV X-r ay background cast by the disc of the nearby spiral galaxy NGC55. Seve ral factors, including the close to edge-on aspect of this galaxy, its extensive HI disc and its location in a direction of relatively low G alactic foreground column density, make NGC55 an excellent target in w hich to search for such effects. The ROSAT PSPC image shows a clear de ficit of 0.25-keV counts coincident with the outer disc of NGC55. From the depth of the shadow we obtain an estimate of the total extragalac tic background signal at 0.25 keV of 29.4 +/- 7.2 keV cm(-2) s(-1) sr( -1) keV(-1) We compare this measurement with other recent estimates of the 0.25-keV background intensity, and briefly discuss the implicatio ns of the result in the context of the source populations which may pr oduce the X-ray background radiation.