Probing the bright radio flare and afterglow of GRB 130427A with the Arcminute Microkelvin Imager

ABSTRACT 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 GRB130427A at the central frequency of 15.7 GHz between 0.36 and 59.32 days post-burst. Theseresults yield one of the earliest radio detections of a GRB and demonstrate a clear rise influx less than one day after the -ray trigger followed by a rapid decline. This early time radioemission probably originates in the GRB reverse shock so our AMI light curve reveals the firstever confirmed detection of a reverse shock peak in the radio domain. At later times (about 3:2days post-burst) the rate of decline decreases, indicating that the forward shock componenthas begun to dominate the light-curve. Comparisons of the AMI light curve with modellingconducted by Perley et al. show that the most likely explanation of the early time 15.7 GHzpeak is caused by the self-absorption turn-over frequency, rather than the peak frequency, ofthe reverse shock moving through the observing bands.Key words: gamma-ray burst: individual: GRB 130427A – radio continuum: stars

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