Abstract
We present and discuss the results of a long term sporadic monitoring of the H2O masers observed toward the nucleus of M 82 between 1984 and 1991. Time variability is clearly established in several narrow features. In 1988 we observed a strong burst in one single component centered around 109 kms-1. This component had an isotropic luminosity ot' about 0.17 L. at the epoch of the flare, was as narrow as approximately 0.3 km s-1, and showed flux variations on a time scale of 12 days. Our observations suggest that the 109 km s-1 H2O maser is partly saturated with physical properties similar to the brightest H2O clumps of the Galaxy. The results of a search for H2O masers in selected positions along the arms of M 81 and toward the center of the two other major members of the M 81 group of galaxies, NGC 3077 and NGC 2976, are presented. The rms flux limit achieved during this search was about 10 to 40 mJy from which we deduce that we had no masers brighter than 0.01-0.03 L.. Our results are discussed in the light of the star formation rate for these three galaxies. Finally, the H2O and CO luminosities of the four major members of the M 81 group of galaxies and of some other galaxies are compared to conclude that higher CO luminosities in external galaxies imply better chances to detect H2O maser activity.