| 11th of September 2023 |
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| Rapid radio brightening of GRB 210702A |
| by Anderson et al. |
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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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