The Broadband Afterglow of GRB 980329

Abstract

We present radio observations of the afterglow of the bright gamma-ray burst\nGRB980329 made between one month and several years after the burst, a\nre-analysis of previously published submillimeter data, and late-time optical\nand near-infrared (NIR) observations of the host galaxy. From the absence of a\nspectral break in the optical/NIR colors of the host galaxy, we exclude the\nearlier suggestion that GRB980329 lies at a redshift of z >~5. We combine our\ndata with the numerous multi-wavelength observations of the early afterglow,\nfit a comprehensive afterglow model to the entire broadband dataset, and derive\nfundamental physical parameters of the blast-wave and its host environment.\nModels for which the ejecta expand isotropically require both a high\ncircumburst density and extreme radiative losses from the shock. No low density\nmodel (n << 10 cm^{-3}) fits the data. A burst with a total energy of ~ 10^{51}\nerg, with the ejecta narrowly collimated to an opening angle of a few degrees,\ndriven into a surrounding medium with density ~ 20 cm^{-3}, provides a\nsatisfactory fit to the lightcurves over a range of redshifts.\n