The X-Ray Emission of M81 and Its Nucleus

M81 was observed in X-ray with the imaging detectors on board the Einstein Observatory. The discovery of a pointlike nuclear source in its low-luminosity Seyfert nucleus was reported by Elvis and Van Speybroeck. Here we report the results of a complete spatial and spectral analysis of this galaxy and of its nuclear source. We detect nine individual sources in M81, including the one at the nucleus, with X-ray luminosities greater than ~ 2 x 10^38^ ergs s^-1^, assuming a distance of 3.5 Mpc. Five of these are associated with the spiral arms, and three of them are associated with resolved thermal radio emission and/or giant H II regions. We also find that about two-thirds of the nonnuclear emission is not resolved in detected sources. The radial distribution of the X-ray surface brightness closely resembles that of the blue light, as found for other spiral galaxies. A comparison in different energy ranges of the total non-nuclear emission of M81 with that of M31 suggests a more active star formation history in M81. The X-ray sources detected in M81 are also more luminous than those of M31, but we cannot tell if this is due to an increased probability of X-ray source formation in M81 or also to a shift of the luminosity function of X-ray sources in this galaxy toward intrinsically higher luminosities. The X-ray spectral data of the nuclear source are well fitted with a steep power-law spectrum with energy index α_E_ ~ 3 and intrinsic absorption column ~ 10^21^-10^22^ cm^-2^. The fit to a thermal bremsstrahlung model is similarly good and gives a relatively low temperature kT ~ 1 keV. The soft spectrum of the nuclear source could be related to the soft spectral components seen in more luminous active nuclei. If we interpret this emission in terms of the accretion disk model of Czerny and Elvis and Bechtold et al., we find a mass for the central compact object of <=10^4^-10^5^ M_sun_ and accretion rates <=10^-4^-10^-3^ M_sun_ yr^-1^. The measured N_H_ is far less than those predicted from an extrapolation of the properties of brighter nuclei, suggesting that the explanation of the properties of active nuclei in terms of obscuration is not always applicable. The soft spectrum, if typical of low- luminosity active nuclei, also suggests that the contribution of these objects to the 2-10 keV X-ray background might be less than previously thought.