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 smaller jet opening angle
(theta), contrary to naive expectation. For a beaming factor
f_b^{-1}=(theta^2/2)^{-1} = 500, roughly the value inferred by Frail et al.
(2001) from analysis of afterglow light curves, we predict that between several
hundreds to several thousands 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 point-like radio transients with flux
densities greater than 6 mJy was conducted using the FIRST and NVSS surveys,
yielding a list of 25 orphan candidates. We argue that most of the candidates
are unlikely to be radio supernovae. However, the possibility that they are
radio loud AGNs cannot be ruled out without further observations. Our analysis
sets an upper limit for the all sky number of radio orphans, which corresponds
to a lower limit f_b^{-1}>10 on the beaming factor. Rejection of all candidates
found in our search would imply f_b^{-1}>100. This, and the fact that some
candidates may indeed be radio afterglows, strongly motivate further
observations of these transients.Comment: 18 pages, including 2 figure