The Rapidly Fading Optical Afterglow of GRB 980519

Abstract

GRB 980519 had the most rapidly fading of the well-documented GRB afterglows, consistent with t^{-2.05 +/- 0.04} in BVRI as well as in X-rays during the two days in which observations were made. We report VRI observations from the MDM 1.3m and WIYN 3.5m telescopes, and we synthesize an optical spectrum from all of the available photometry. The optical spectrum alone is well fitted by a power law of the form nu^{-1.20 +/- 0.25}, with some of the uncertainty due to the significant Galactic reddening in this direction. The optical and X-ray spectra together are adequately fitted by a single power law nu^{-1.05 +/- 0.10}. This combination of steep temporal decay and flat broad-band spectrum places a severe strain on the simplest afterglow models involving spherical blast waves in a homogeneous medium. Instead, the rapid observed temporal decay is more consistent with models of expansion into a medium of density n(r) proportional to r^{-2}, or with predictions of the evolution of a jet after it slows down and spreads laterally. The jet model would relax the energy requirements on some of the more extreme GRBs, of which GRB 980519 is likely to be an example because of its large gamma-ray fluence and faint host galaxy.