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-ray background cast by the disc of the nearby spiral galaxy NGC55. Several factors, including the close to edge-on aspect of this galaxy, its extensive HI disc and its location in a direction of relatively low Galactic foreground column density, make NGC55 an excellent target in which to search for such effects. The ROSAT PSPC image shows a clear deficit 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 extragalactic 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 implications of the result in the context of the source populations which may produce the X-ray background radiation.