Optical and Radio Observations of the Afterglow from GRB 990510: Evidence for a Jet

We present multi-color optical and two-frequency radio observations of the bright SAX event, GRB 990510. The well-sampled optical decay, and the radio observations are both inconsistent with simple spherical afterglow models. The achromatic optical steepening and the decay of the radio afterglow both occuring at t ∼ 1 day are evidence for hydrodynamical evolution of the source, and can be most easily interpreted by models where the GRB ejecta are collimated in a jet. Employing a simple jet model to interpret the observations, we derive a jet opening angle of θo = 0.08, reducing the isotropic gamma-ray emission of 2.9 × 10 53 erg by a factor ∼ 300. If the jet interpretation is correct, we conclude that GRB observations to-date are consistent with an energy for the central source of E ∼ < 10 52 erg. 1