DIFFUSE SOFT-X-RAY EMISSION IN ROSAT OBSERVATIONS OF M31

We present the results of a search for diffuse 0.5-2.0 keV band X-ray emission in the Andromeda Galaxy (M31) using data acquired with the RO SAT PSPC and HRI during pointed observations with a total exposure of more than 600 ks. For the most part we concentrate out study in a regi on of radius 19.3 kpc (100 arcmin) in the plane of M31 and greater tha n similar to 1 kpc (5 arcmin) from the care. After removing detected p oint-like sources and other effects of instrumental and cosmic backgro und, we find a residual signal within the survey region which exceeds the extrapolation of the distribution of observed point-like sources t o luminosities below our sensitivity threshold. Fits to the surface br ightness maps have allowed us to identify two distinct spatial compone nts in the residual: a spherical core and a disc with a scalelength of similar to 5 kpc. Our study yields the first detection of emission in the disc of M31 that cannot be explained by the detected point-source population. The disc contributes around 45 per cent of our total diff use signal. We shaw that this emission from the disc is consistent wit h that expected from the normal stellar population. and confirm the ex cess observed in the care by other workers. We place Limits on the mas s of hot diffuse gas in the disc of M31 and on the luminosity of an ex tended halo component in our survey region. We discuss the implication s for models of the 0.5-2.0 keV X-ray background within the Milky Way.