Rosat X-ray observations of the spiral galaxy M81

We present results from the analysis of deep ROSAT HRI and PSPC observation s of the spiral galaxy M81. The inferred total (0.5-2 keV band) luminosity of M81 is similar to3 x 10(40) ergs s(-1), excluding the contribution from identified interlopers found within the D-25 ellipse. The nucleus of the ga laxy alone accounts for about 65% of this luminosity. The rest is due to 26 other X-ray sources (contributing similar to 10%) and to apparently diffus e emission, which is seen across much of the galactic disk and is particula rly bright in the bulge region around the nucleus. Spectral analysis furthe r gives evidence for a soft component, which can be characterized by a two- temperature optically thin plasma with temperature at similar to0.15 and 0. 60 keV and an absorption of the galactic foreground only. These components, accounting for similar to 13% of the X-ray emission from the region, appar ently arise in a combination of hot gas and faint discrete sources. We find interesting spatial coincidences of luminous (10(37)-10(40) ergs s(-1)) an d variable X-ray sources with shock-heated optical nebulae. Three of them a re previously classified as supernova remnant candidates. The other one is far off the main body of M81 but is apparently associated with a dense H I concentration produced most likely by the tidal interactions of the galaxy with its companions. These associations suggest that such optical nebulae m ay be powered by outflows from luminous X-ray binaries, which are comparabl e to, or more luminous than, Galactic "microquasars."