Constraining the Beaming of γ-Ray Bursts With Radio Surveys

The degree of beaming in Gamma-Ray Bursts (GRBs) is currently unknown. The uncertainty in the γ-ray beaming angle, θb, leaves the total energy release (∝ θ2 b) and the event rate per galaxy (∝ θ b ) unknown to within orders of magnitude. Since the delayed radio emission of GRB sources originates from a mildly relativistic shock and receives only weak relativistic beaming, the rate of radio-selected transients with no GRB counterparts can be used to set an upper limit on θ b . We find that a VLA survey with a sensitivity of ∼ 0.1 mJy at 10 GHz could identify ∼> 2 × 10(θb/10) radio afterglows across the sky if each source is sampled at least twice over a period of one month or longer. From the total number of ∼> 0.1 mJy sources observed at 8.44 GHz and the fraction of fading sources at 1.44 GHz, we get the crude limit θb ∼> 6. Subject headings: gamma rays: bursts