THE BROAD-BAND X-RAY-SPECTRUM OF THE NUCLEUS OF M81

We report on the results of a 2000 s observation of the nearby spiral galaxy M81 using the Broad-Band X-Ray Telescope (BBXRT). The observed X-ray emission is dominated by the flux from the nucleus, classified optically as a low-ionization nuclear emission-line region (LINER) (Filippenko & Sargent 1988). The 0.5-10 keV spectrum is well fit by a simple power law of photon index 2.20 (+0.3, -0.15), distinct from the flatter index observed in Seyfert galaxies. An upper limit of 90 eV has been placed on the equivalent width of any Fe K fluorescent feature. Taken together with the nondetection in the optical and UV of any nonthermal continuum emission, this argues strongly for the absence of an accretion disk around the M81 nucleus. Given the inferred low accretion rate in M81 and the very high X-ray-to-UV-optical luminosity ratio compared with the more X-ray-luminous Seyfert galaxies, it is very likely that the X-ray production mechanism is qualitatively different. The lack of strong X-ray lines leads us to reject a starburst origin of the X-ray flux. Based on the similarity of the M81 X-ray spectrum to that of other LINERs having broad Halpha emission, such as Pic A, NGC 3998, and M87, some of these conclusions may be applicable, in general, to this type of LINER.