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Published August 1999 | Published + Submitted
Journal Article Open

Discovery of Radio Outbursts in the Active Nucleus of M81


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 sets reveal that the nucleus experienced several radio outbursts during the monitoring period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of 1993 September and lasted for approximately 3 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, prolonged variations, we also detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on short (≾1 day) timescales. We discuss a possible association between the radio activity and an optical flare observed during the period that the nucleus was monitored at radio wavelengths.

Additional Information

© 1999. The American Astronomical Society. Received 1997 July 9; accepted 1999 May 6. During the course of this work, L. C. H. was supported by a postdoctoral fellowship from the Harvard-Smithsonian Center for Astrophysics, by NASA grant NAG 5-3556, and by NASA grants GO-06837.01-95A and AR-07527.02-96A from the Space Telescope Science Institute (operated by AURA, Inc., under NASA contract NAS 5-26555). K. W. W. wishes to acknowledge the Office of Naval Research for the 6.1 funding that supports his work. We thank Norbert Bartel, Michael Eracleous, Heino Falcke, and the referee for helpful comments and Rick Perley and Michael Rupen for advice concerning pointing errors of the VLA.

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