A Reverse Shock in GRB 130427A

Abstract

We present extensive radio and millimeter observations of the unusually bright GRB 130427A at z = 0.340, spanning 0.67 to 12 d after the burst. Taken in conjunction with detailed multi-band UV, optical, NIR, and X-ray observations we find that the broad-band 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 d in the UV/optical/NIR, while the forward shock emission dominates in the X-rays and at & 0.1 d 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 are an isotropic kinetic energy of EK,iso ≈ 2×1053 erg, a mass loss rate of M˙ ≈ 3×10−8 M⊙ yr−1 (for a wind velocity of 1,000 km s−1), and a Lorentz factor at the deceleration time of Γ(200s) ≈ 130. Due to the low density and large isotropic energy, the absence of a jet break to ≈ 15 d places only a weak constraint on the opening angle, θj & 2.5◦ , and therefore a total energy of Eγ + EK & 1.2×1051 erg, similar to other 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 is a required property for the detectability of reverse shocks in the radio and millimeter bands. Following on GRB 130427A as a benchmark event, observations of future GRBs with the exquisite sensitivity of VLA and ALMA, coupled with detailed modeling of the reverse and forward shock contributions will test this hypothesis.