A Reverse Shock in GRB 130427A

TL;DR

This paper reports detailed multi-wavelength observations of GRB 130427A, identifying distinct reverse and forward shock emissions, and derives key physical parameters, highlighting the importance of low circumburst density for reverse shock detectability.

Contribution

It provides the first comprehensive analysis of reverse shock emission in GRB 130427A using radio, millimeter, UV, optical, and X-ray data, and links reverse shock detectability to circumburst environment properties.

Findings

Reverse shock dominates in radio/millimeter and early UV/optical/NIR emission.

Optical and X-ray data indicate a wind-like circumburst environment.

Derived burst parameters include isotropic kinetic energy ~2e53 erg and Lorentz factor ~130.

Abstract

We present extensive radio and millimeter observations of the unusually bright GRB 130427A at z=0.340, spanning 0.67 to 12 days 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 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 are an isotropic kinetic energy of E_Kiso~2e53 erg, a mass loss rate of Mdot~3e-8 Msun/yr (for a wind velocity of 1,000 km/s), and a Lorentz factor at the deceleration time of Gamma(200s)~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 of theta_j>2.5 deg, and therefore a total energy of E_gamma+E_K>1.2e51 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.