A Revised View of the Transient Radio Sky

Abstract

We report on a re-analysis of archival data from the Very Large Array for a sample of 10 long-duration radio transients reported by Bower and others. These transients have an implied all-sky rate that would make them the most common radio transient in the sky and yet most have no quiescent counterparts at other wavelengths and therefore no known progenitor (other than Galactic neutron stars). We find that more than half of these transients are due to rare data artifacts. The remaining sources have lower signal-to-noise ratio (S/N) than initially reported by 1σ-1.5σ. This lowering of S/N matters greatly since the sources are at the threshold. We are unable to decisively account for the S/N differences. By two orthogonal criteria one source appears to be a good detection. Thus the rate of long-duration radio transients without optical counterparts is, at best, comparable to that of the class of recently discovered Swift J1644+57 nuclear radio transients. We revisit the known and expected classes of long-duration radio transients and conclude that the dynamic radio sky remains a rich area for further exploration. Informed by the experience of past searches for radio transients, we suggest that future surveys pay closer attention to rare data errors and ensure that a wealth of sensitive multi-wavelength data be available in advance of the radio observations and that the radio searches should have assured follow-up resources.