Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-01-31
Astrophys.J. 587 (2003) L5-L8
Astronomy and Astrophysics
Astrophysics
Submitted to ApJ; 14 pages, 2 tables, 3 figures
Scientific paper
10.1086/375158
We present radio observations of GRB 020405 starting 1.2 days after the burst, which 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 \rho ~ 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~100 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 of 6 degrees (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_\nu \~ \nu^{-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 \rho ~ r^{-2} profile is not a required signature, and in fact may not be present on the lengthscales probed by the afterglow in the majority of bursts.
Berger Edmond
Frail Dale A.
Kulkarni Sanjeev R.
Soderberg Alicia Margarita
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