Two very luminous variable X-ray sources in M82

We report on a variability study of X-ray sources in the starburst galaxy M82, based on ROSAT and Einstein High-Resolution Imager observations. In particular, we concentrate our analysis on two bright sources which exhibit significant variability. The brightest source in M82 is located in the central core of the galaxy, is variable within individual ROSAT observations, and indication are that it might have varied between the Einstein and the ROSAT observations. It is the most luminous X-ray binary candidate yet known. The other source is located outside the crowded central region and was very bright in the Einstein observation, but was not detected by ROSAT, despite the larger effective area of the instrument and the much longer exposure. The detection of variability poses strong constraints on the X-ray luminosity of individual components of the X-ray sources. Even if both sources are radiating at the Eddington limit, their mass would be at least several solar masses, making them candidates for extragalactic black holes.