We present one of the best sampled early time light curves of a gamma-ray burst ( GRB ) at radio wavelengths .
Using the Arcminute Mircrokelvin Imager ( AMI ) we observed GRB 130427A at the central frequency of 15.7 GHz between 0.36 and 59.32 days post-burst .
These results yield one of the earliest radio detections of a GRB and demonstrate a clear rise in flux less than one day after the \gamma -ray trigger followed by a rapid decline .
This early time radio emission probably originates in the GRB reverse shock so our AMI light curve reveals the first ever confirmed detection of a reverse shock peak in the radio domain .
At later times ( about 3.2 days post-burst ) the rate of decline decreases , indicating that the forward shock component has begun to dominate the light-curve .
Comparisons of the AMI light curve with modelling conducted by Perley et al .
show that the most likely explanation of the early time 15.7 GHz peak is caused by the self-absorption turn-over frequency , rather than the peak frequency , of the reverse shock moving through the observing bands .