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11th of September 2023
Rapid radio brightening of GRB 210702A
by Anderson et al.
Anderson et al. report the detection of the rapid radio brightening of GRB 210702A with the ATCA just 11 h post-burst. Early-time radio variability was tracked over a 5 hr period on ~15 min timescales at 9.0, 16.7, and 21.2 GHz. A broken power law fit to the 9.0 GHz light curve showed that the 5 hr flare peaked at a flux density of 0.4 mJy at ∼13 h post-burst. The observed temporal and spectral evolution is not expected in the standard internal-external shock model, where forward and reverse shock radio emission evolves on much longer timescales. The authors suggest that the early-time radio flare is likely due to weak interstellar scintillation (ISS), which boosted the radio afterglow emission above the ATCA sensitivity limit on minute timescales. This represents the earliest ISS size constraint on a GRB blast wave to date, demonstrating the importance of rapid (< 1 d) radio follow-up of GRBs using several-hour integrations to capture the early afterglow evolution and to track the scintillation over a broad frequency range.

The figure above shows the radio light curves of GRB 210702A at 9 GHz (15 min bins), 16.7 GHz (12.5 min bins) and 21.2 GHz (12.5 min bins). Filled data points are considered detections (SNR ≥ 3). The dashed line represents the detection threshold (marked as an SNR of 3). At 9.0 GHz the flux density of a check source in the field is also plotted to demonstrate that the observed transient nature of source identified as the radio afterglow to GRB 210702A was not an instrumental artefact.

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