The X-ray emission of M81 and its nucleus

X-ray observations of M81 made with the Einstein satellite are analyzed, and the results are reported. Eight bright sources other than the nucleus were detected, with luminosities in excess of about 2 x 10 to the 38th ergs/s. These are probably young, massive, accreting binary systems. A comparison of M81 with the morphologically similar galaxy M31 shows that the former is overluminous in both the radio continuum and the X-ray emission and to a lesser extent the far-infared emission, relative to the optical luminosity. This points to a difference in the star-formation histories of the two galaxies. The individual X-ray sources detected in M81 are all more luminous than the most luminous sources of M31. The nuclear source in M81 has a very soft and absorbed X-ray spectrum which cannot be explained by either inverse Compton radiation of the compact nuclear radio source or by an extension of the radio synchrotron emission.