A Radio Flare from GRB 020405: Evidence for a Uniform Medium around a Massive Stellar Progenitor

Abstract

We present observations of GRB 020405 starting 1.2 days after the burst that reveal a rapidly fading radio flare. Based on its temporal and spectral properties, we interpret the flare as emission from the reverse shock. This scenario rules out a circumburst medium with a radial density profile ρ ∝ r-2 expected around a mass-losing massive star since in that case, the reverse-shock emission decays on the timescale of the burst duration t ~ 102 s. Using published optical and X-ray data, along with the data presented here, we further show that a self-consistent model requires collimated ejecta with an opening angle, θj ~ 6° (tj ≈ 0.95 days). As a consequence of the early jet break, the late-time (t > 10 days) emission measured with the Hubble Space Telescope significantly deviates from an extrapolation of the early, ground-based data. This, along with an unusually red spectrum, Fν ∝ ν-3.9, strengthens the case for a supernova that exploded at about the same time as GRB 020405, thus pointing to a massive stellar progenitor for this burst. This is the first clear association of a massive progenitor with a uniform medium, indicating that a ρ ∝ r-2 profile is not a required signature and, in fact, may not be present on the length scales probed by the afterglow in the majority of bursts.