Discovery of an entire population of radio supernova candidates in the nucleus of messier 82
Observations with the full resolution of the NRAO Very Large Array in New Mexico by Kronberg, Biermann and Schwab have revealed about 40 discrete radio sources in the inner, visually obscured nucleus of the enigmatic galaxy Messier 82. These radio sources, more luminous than any comparable objects in our Galaxy, are comparable to those of the half dozen radio supernovae found to date in other nearby galaxies. Their physical characteristics are presumably similar to those in more distant galaxies which might be used as distance scale indicators.
Repeated monitoring of the M82 nuclear region since 1981 by R.A. Sramek and myself reveals that most of the brightest of these sources are declining in intensity on the scale of months to years. Both their radio luminosity and variability strongly suggest that M82 contains an entire population of radio supernovae, whose radio structure and variability we can compare at the same distance. Striking individual differences in variability between 1981 and 1983 already suggest that M82's population of radio supernovae contains both Type I and Type II supernovae, and possibly other classes as yet unrecognized.
Most of the other 10 strongest sources are declining so rapidly that they will fade into the background within .35 years. Thus, we expect new supernovae to appear in M82's nucleus every few years. This will provide the opportunity for VLBI size and structure evolution measurements over the next few years. The high degree of asymmetry seen for many of the M82 sources suggests that the radio size growth of this class of source (presumably radio SN) may not be usable as a distance scale indicator until we acquire a better understanding of their physical nature.
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