The Jet and Circumburst Stellar Wind of GRB 980519

We present extensive multicolor (UBVRCIC) photometry of the optical afterglow of GRB 980519. Upon discovery, 8.3 hr after the burst, the source was decaying as a power law, (t - tGRB)α, with a rapid decay rate α1 = -1.73 ± 0.04. About 13 hr after the burst a steepening of the light curve to α2 = -2.22 ± 0.04 was observed. Within the framework of current afterglow models, we argue that the rapid initial decline, the "break" in the light curve, and the spectral properties of the afterglow are best interpreted as being due to a collimated ultrarelativistic jet of fixed opening angle expanding into an inhomogeneous medium. In this scenario, we find that the circumburst medium has a density structure that goes as r-2.05±0.22. This is characteristic of a preexisting wind expelled from a massive star. A possible physical scenario is that the progenitor star collapsed to form a black hole (i.e., a "collapsar"), producing the observed burst and afterglow. However, the supernova signature expected in the light curve in such a scenario is not detected. This either implies that the redshift of GRB 980519 is greater than 1.5 or that supernovae accompanying GRBs are not standard candles.