Discovery of a bright radio transient in M 82: a new radio supernova?

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Abstract
In this Letter, we report the discovery of a new bright radio transient in M 82. Using the Very Large Array, we observed the nuclear region of M 82 at several epochs at 22 GHz and detected a new bright radio source in this galaxy's central region. We find a flux density for this flaring source that is ~300 times larger than the upper limits determined in previous observations. The flare must have started between 2007 October 29 and 2008 March 24. Over the past year, the flux density of this new source has decreased from ~100 mJy to ~11 mJy. The lightcurve (based on only three data points) can be fitted better with an exponential decay than with a power law. Based on the current data we cannot identify the nature of this transient source. However, a new radio supernova seems to be the most natural explanation. With its flux density of more than 100 mJy, it is at least 1.5 times brighter than SN1993J in M 81 at the peak of its lightcurve at 22 GHz.