The detectability of orphan afterglows

The realization that gamma-ray bursts (GRBs) release a constant amount of energy implies that post-jet-break afterglow evolution is largely universal. For a given redshift, all afterglows should be detected up to a fixed observer angle, We estimate the observed magnitude and the implied detectability of orphan afterglows. We show that for reasonable limiting magnitudes (m _lim = 25), orphan afterglows will typically be detected from small (∼10°) angles away from the GRB jet axis. A detected orphan afterglow generally corresponds to a "near miss" of a GRB whose jet is pointing just slightly away from us. With our most optimistic parameters, we expect that 15 orphan afterglows will be recorded in the Sloan Digital Sky Survey, and 35 transients will be recorded in a dedicated 2 m class telescope operating full time for a year in an orphan afterglow search. The rate is smaller by a factor of 15 for our "canonical" parameters. We show that for a given facility, an optimal survey should be shallower, covering a larger area, rather than deeper. The limiting magnitude should not be, however, lower than ∼23, as in this case, more transients from on-axis GRBs will be discovered than orphan afterglows. About 15% of the transients could be discovered with a second exposure of the same area provided that it follows after 3, 4, and 8 days for m_lim = 23, 25, and 27, respectively.