2002ApJ...576..923L

The number of orphan radio afterglows associated with gamma-ray bursts (GRBs) that should be detected by a flux-limited radio survey is calculated. It is shown that for jetted GRBs, this number is smaller for a smaller jet opening angle θ, contrary to naive expectation. For a beaming factor f^–1_b≡(θ²/2)^–1≃500, roughly the value inferred by Frail et al. from analysis of afterglow light curves, we predict that between several hundred to several thousand orphan radio afterglows should be detectable (over all sky) above 1 mJy at GHz frequencies at any given time. This orphan population is dominated by sources lying at distances of a few hundred Mpc and having an age of ∼1 yr. A search for pointlike radio transients with flux densities greater than 6 mJy was conducted using the FIRST and NVSS surveys, yielding a list of nine orphan candidates. We argue that most of the candidates are unlikely to be radio supernovae. However, the possibility that they are radio-loud active galactic nuclei cannot be ruled out without further observation. Our analysis sets a conservative 95% CL upper limit for the all-sky number of radio orphans, which corresponds to a lower limit f^–1_b>13 on the beaming factor. Rejection of all candidates found in our search would imply f^–1_b>80. This, and the possibility that some candidates may indeed be radio afterglows, strongly motivate further observations of these transients.