What is the compact radio source in the nucleus of M82

Observations of M82 taken over a 4 1/2 year interval with the NRAO interferometer have been analyzed to search for any variation in the compact (28 light-day) radio source, 41.9+58, at the nucleus. A decrease of 14 +- 10% between 1971 and 1975 was found, and there is no evidence for variations on a time scale of months. The relative stability of this source in view of its small size, combined with the predominance of relativistic over thermal electrons within it (/sub r//n/sub c/> or approx. =4000), indicates an acceleration mechanism that is very efficient, yet does not involve highly relativistic bulk motions. This suggests a massive rotating object with strong magnetic fields. Our very low upper limit (p/sub L/< or =0.3%) on the linear polarization of 41.9+58 in the optically thin part of its spectrum suggests that the magnetic field in this source is quite tangled.A new 8 GHzmap at 2''.2 resolution has been made, incorporating the second-epoch data, which establishes new upper limits to any linear polarization elsewhere in the nucleus of M82