RADIO AND OPTICAL FOLLOW-UP OBSERVATIONS OF A UNIFORM RADIO TRANSIENT SEARCH: IMPLICATIONS FOR GAMMA-RAY BURSTS AND SUPERNOVAE

摘要

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_b~(-1) approx > 60, ～5 times higher than our earlier results [f_b~(-1) ≡ (θ_(jet)~2/2)~(-1)]. Second, our results impose an upper limit on the rate of events that eject approx > 10~(51) ergs in unconfined relativistic ejecta, whether or not accompanied by detectable emission in wavebands other than the radio. Our estimated rate, n ≤ 1000 yr~(-1) Gpc~(-1), 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.