Radio and Optical Follow‐up Observations of a Uniform Radio Transient Search: Implications for Gamma‐Ray Bursts and Supernovae

We had previously reported on a survey for radio transients, used to set an\nupper limit on the number of orphan gamma-ray burst (GRB) radio afterglows, and\nthus a lower limit on the typical GRB beaming factor. Here we report radio and\noptical follow-up observations of these possible transients, achieving the\nfirst full characterization of the transient radio sky. We find that only two\nsource are likely to be real radio transients, an optically obscured radio\nsupernova (SN) in the nearby galaxy NGC 4216, and a source not associated with\na bright host galaxy, which is too radio luminous to be a GRB afterglow. We\nspeculate that this may be a flare from a peculiar active galactic nucleus, or\na burst from an unusual Galactic compact object. We place an upper limit of 65\nradio transients above 6 mJy over the entire sky at the 95% confidence level.\nThe implications are as follows. First, we derive a limit on the typical\nbeaming of GRBs; we find f_b^{-1} >~ 60, ~5 times higher than our earlier\nresults. Second, we impose an upper limit on the rate of events that eject >~\n10^{51} erg in unconfined relativistic ejecta, whether or not accompanied by\ndetectable emission in wavebands other than the radio. Our estimated rate,\n<=1000/y/Gpc, is about two orders of magnitude smaller than the rate of\ncore-collapse SNe (and type Ib/c events in particular), indicating that only a\nminority of such events eject significant amounts of relativistic material,\nwhich are required by fireball models of long-soft GRBs. Finally, we show that\nfuture wider and/or deeper radio variability surveys are expected to detect\nnumerous orphan radio GRB afterglows. Our survey also illustrates the great\npotential of sensitive surveys with new instruments to revolutionize the study\nof nearby SNe (abridged).\n