ROSATX‐Ray Observations of the Spiral Galaxy M81

We present results from the analysis of deep ROSAT HRI and PSPC observationsof the spiral galaxy M81. The inferred total (0.5-2 keV band) luminosity of M81is ~3x10^40 ergs/s, excluding the contribution from identified interlopersfound within the D_25 ellipse. The nucleus of the galaxy alone accounts forabout 65% of this luminosity. The rest is due to 26 other X-ray sources(contributing ~10%) and to apparently diffuse emission, which is seen acrossmuch of the galactic disk and is particularly bright in the bulge region aroundthe nucleus. Spectral analysis further gives evidence for a soft component,which can be characterized by a two-temperature optically thin plasma withtemperature at ~0.15 keV and 0.60 keV and an absorption of the Galacticforeground only. These components, accounting for ~13% of the X-ray emissionfrom the region, apparently arise in a combination of hot gas and faintdiscrete sources. We find interesting spatial coincidences of luminous(10^37-10^40 ergs/s) and variable X-ray sources with shock-heated opticalnebulae. Three of them are previously classified as supernova remnantcandidates. The other one is far off the main body of M81, but is apparentlyassociated with a dense HI concentration produced most likely by the tidalinteractions of the galaxy with its companions. These associations suggest thatsuch optical nebulae may be powered by outflows from luminous X-ray binaries,which are comparable to, or more luminous than Galactic `micro-quasars'.Comment: 15 pages, 8 tables, 9 figures, accepted for publication in the Astrophysical Journa