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R.A. Sramek (NRAO/VLA), L.C. Ho (Carnegie Obs.), S.D. Van Dyk (IPAC), G.G. Pooley (Mullard Radio Astronomy Observatory), K. W. Weiler (Naval Research Laboratory)
The low-luminosity active galactic nucleus of M81 has been monitored at centimeter wavelengths since early 1993 as a by-product of radio programs to study the radio emission from Supernova 1993J. The extensive data set reveals that the nucleus experienced several radio outbursts during the monitoring period. At 3.6 cm, the main outburst occurred in the middle of May 1993 and lasted for approximately eight months; at longer wavelengths, the maximum flux density decreases, and the onset of the burst is delayed. These characteristics qualitatively resemble the standard model for adiabatically expanding radio sources, although certain discrepancies between the observations and the theoretical predictions suggest that the model is too simplistic. In addition to the large-amplitude variations, we also detected milder (30%-50%) changes in the flux density at 3.6 cm on short (less than ~1 day) timescales. We discuss a possible association between the radio activity and an optical flare discovered during the period that the nucleus was monitored in the radio.