The jet and circumburst stellar wind of GRB 9805191

We present extensive multicolor (UBVRCIC) photometry of the optical aftergl ow of GRB 980519. Upon discovery, 8.3 hr after the burst, the source was de caying as a power law, with a rapid (t - t(GRB))(alpha), decay rate alpha ( 1) = -1.73 +/- 0.04. About 13 hr after the burst a steepening of the light curve to alpha (2) = -2.22 +/- 0.04 was observed. Within the framework of c urrent afterglow models, we argue that the rapid initial decline, the "brea k" in the light curve, and the spectral properties of the afterglow are bes t interpreted as being due to a collimated ultrarelativistic jet of fixed o pening angle expanding into an inhomogeneous medium. In this scenario, we f ind that the circumburst medium has a density structure that goes as r(-2.0 5+0.22). This is characteristic of a preexisting wind expelled from a massi ve star. A possible physical scenario is that the progenitor star collapsed to form a black hole (i.e., a "collapsar"), producing the observed burst a nd afterglow. However, the supernova signature expected in the light curve in such a scenario is not detected. This either implies that the redshift o f GRB 980519 is greater than 1.5 or that supernovae accompanying GRBs are n ot standard candles.