The nuclear X-ray source in M 81.

X-ray observations of the M81 galaxy were taken by the high resolution imager and the monitor proportional counter aboard the Einstein observatory. The luminosity is about 1.7 X 10 to the 40th ergs/s. It is suggested that M81 probably has an active nucleus two orders of magnitude less luminous than any other well-established case. Any critical 'turn-on' luminosity is thus very low and approaches that of compact X-ray binary systems. The agreement of the alpha hydrogen/X-ray ratio of M81 suggests that the physics of the activity is not strongly different at these low luminosities from that in the normal range