Y. Ishisaki, K. Makishima, N. Iyomoto, K. Hayashida, Y. Kohmura, R.F. Mushotzky, R. Petre, P.J. Serlemitsos, and Y. Terashima
Due to the appearance of SN 1993J, the region of M81 (NGC 3031) was observed with ASCA ten times between 1993 April and 1995 April. Data from eight of these observations have been used to study the 0.5-10 keV spectral and temporal properties of the source designated X-5, coincident with the M81 nucleus. The 2-10 keV luminosity of X-5 varied by a factor of 1.7 over a period of two years, around a mean of ~2x1040 erg s-1. Variations by ~20% on a time scale of one day were also detected. The average spectrum of X-5 can be represented in the 2-10 keV range by a power-law continuum of the photon index, Gamma = 1.85+/- 0.04, and a relatively low intrinsic absorption column of NH ~ 1x1021 cm-2. In the soft (< 2 keV) energy range, an additional thermal component with a temperature of 0.6-0.8 keV has been detected. An apparently broad or complex Fe-K emission line centered at 6.6-6.7 keV was also detected at an equivalent width of 170 +/- 60 eV. These results indicate that the M81 nucleus is a low-luminosity active galactic nucleus, and suggest that similar objects are relatively numerous. A comparison is made between the nucleus of M81 and those of Seyfert galaxies.