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 \lesssim 0.1 d in the UV/optical/NIR , while the forward shock emission dominates in the X-rays and at \gtrsim 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 E _ { K,iso } \approx 2 \times 10 ^ { 53 } erg , a mass loss rate of \dot { M } \approx 3 \times 10 ^ { -8 } M _ { \odot } yr ^ { -1 } ( for a wind velocity of 1 , 000 km s ^ { -1 } ) , and a Lorentz factor at the deceleration time of \Gamma ( 200 { s } ) \approx 130 .
Due to the low density and large isotropic energy , the absence of a jet break to \approx 15 d places only a weak constraint on the opening angle , \theta _ { j } \gtrsim 2.5 ^ { \circ } , and therefore a total energy of E _ { \gamma } + E _ { K } \gtrsim 1.2 \times 10 ^ { 51 } 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 .