A search for flux density variations in 24 compact radio sources in M82

We report results of the most extensive and accurate monitoring yet undertaken for a supernova population in a starburst galaxy. The number of sources being monitored in M82's starburst nuclear region has been increased to 24, all of which now have detailed radio spectra. An improved calibration method was applied to 11 observation epochs at lambda 6 cm, beginning shortly after the VLA's inauguration in 1980, up to 1992 December. They show that about 25% of the sources have some degree of measurable variability over the 11.8 yr period. In contrast, approximately 75% were remarkably constant over the period between 1981 and 1993. The range of variability was from similar to-100% yr(-1) to less than similar to 0.1% yr(-1). A small number of the sources have a degree of flux consistency that is, at least collectively, comparable with the best extragalactic radio source flux density calibrators for radio interferometers. The average luminosity decay over the above monitoring period of 41.9 + 58 has been determined to be 8.4% +/- 0.1% yr(-1), with an excellent fit to an exponential decay form.