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

This is the authors' abstract. We don't add key points for this paper.

We present radio 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 radio 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 ~ 10 2 s. Using published optical and X-ray data, along with the radio data presented here, we further show that a self-consistent model requires collimated ejecta with an opening angle, θ j ~ 6° ( t j ≈ 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, F ν ∝ 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.