2006ApJ...639..331G

We present the first full characterization of the transient radio sky via radio and optical follow-up observations of all the possible radio transients we have discovered in a survey covering ~1/17 of the sky. The two confirmed radio transients turn out to be an optically obscured radio supernova (SN) in the nearby galaxy NGC 4216, the first such event to be discovered by a wide-field radio survey, and a source not associated with a bright host galaxy. We speculate that this second source may be a flare from a peculiar radio-loud AGN, or a burst from an unusual Galactic compact object, but its nature merits further study. We place an upper limit of 65 radio transients above 6 mJy over the entire sky (95% confidence level). The implications are as follows. First, we derive a limit on the typical beaming of GRBs; we find f^–1_b≳60, ∼5 times higher than our earlier results [f^–1_b≡(θ²_jet/2)^–1]. Second, our results impose an upper limit on the rate of events that eject ≳10⁵¹ ergs in unconfined relativistic ejecta, whether or not accompanied by detectable emission in wavebands other than the radio. Our estimated rate, n{dot}≤1000/yr/Gpc, is about 2 orders of magnitude smaller than the rate of core-collapse SNe (and Type Ib/c events in particular), indicating that only a minority of such events eject significant amounts of relativistic material, which are required by fireball models of long-soft GRBs. Finally, we show that wider and/or deeper radio variability surveys are expected to detect numerous orphan radio GRB afterglows and illustrate the great potential of new radio instruments to revolutionize the study of nearby SNe.