Discovery of Radio Outbursts in the Active Nucleus of M81

The low-luminosity active galactic nucleus of M81 has been monitored atcentimeter wavelengths since early 1993 as a by-product of radio programs tostudy the radio emission from Supernova 1993J. The extensive data sets revealthat the nucleus experienced several radio outbursts during the monitoringperiod. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of1993 September and lasted for approximately three months; at longerwavelengths, the maximum flux density decreases, and the onset of the burst isdelayed. These characteristics qualitatively resemble the standard model foradiabatically expanding radio sources, although certain discrepancies betweenthe observations and the theoretical predictions suggest that the model is toosimplistic. In addition to the large-amplitude, prolonged variations, we alsodetected milder changes in the flux density at 3.6 cm and possibly at 6 cm onshort (less than 1 day) timescales. We discuss a possible association betweenthe radio activity and an optical flare observed during the period that thenucleus was monitored at radio wavelengths.Comment: To appear in The Astronomical Journal. Latex, 18 pages including embedded figures and table