A REVERSE SHOCK IN GRB 130427A

We present extensive radio and millimeter observations of the unusually bright GRB 130427A at z = 0.340, spanning 0.67-12 days after the burst. We combine these data with detailed multi-band UV, optical, NIR, and Swift X-ray observations and find that the broadband afterglow emission is composed of distinct reverse shock and forward shock contributions. The reverse shock emission dominates in the radio/millimeter and at 0.1 days in the UV/optical/NIR, while the forward shock emission dominates in the X-rays and at 0.1 days in the UV/optical/NIR. We further find that the optical and X-ray data require a wind circumburst environment, pointing to a massive star progenitor. Using the combined forward and reverse shock emission, we find that the parameters of the burst include an isotropic kinetic energy of E K, iso 2 × 1053 erg, a mass loss rate of M ☉ yr–1 (for a wind velocity of 1000 km s–1), and a Lorentz factor at the deceleration time of Γ(200 s) 130. Due to the low density and large isotropic energy, the absence of a jet break to 15 days places only a weak constraint on the opening angle, θj 2.°5, and therefore a total energy of E γ + EK 1.2 × 1051 erg, similar to other gamma-ray bursts (GRBs). The reverse shock emission is detectable in this burst due to the low circumburst density, which leads to a slow cooling shock. We speculate that this property is required for the detectability of reverse shocks in radio and millimeter bands. Following on GRB 130427A as a benchmark event, observations of future GRBs with the exquisite sensitivity of the Very Large Array and ALMA, coupled with detailed modeling of the reverse and forward shock contributions, will test this hypothesis.