1. Rapid radio brightening of GRB 210702A
- Author
-
Anderson, GE, Russell, TD, Fausey, HM, van der Horst, AJ, Hancock, PJ, Bahramian, A, Bell, ME, Miller-Jones, JCA, Rowell, G, Sammons, MW, Wijers, RAMJ, Galvin, TJ, Goodwin, AJ, Konno, R, Rowlinson, A, Ryder, SD, Schüssler, F, Wagner, SJ, Zhu, SJ, Anderson, GE, Russell, TD, Fausey, HM, van der Horst, AJ, Hancock, PJ, Bahramian, A, Bell, ME, Miller-Jones, JCA, Rowell, G, Sammons, MW, Wijers, RAMJ, Galvin, TJ, Goodwin, AJ, Konno, R, Rowlinson, A, Ryder, SD, Schüssler, F, Wagner, SJ, and Zhu, SJ
- Abstract
We observed the rapid radio brightening of GRB 210702A with the Australia Telescope Compact Array (ATCA) just 11 h post-burst, tracking early-time radio variability over a 5 h period on ∼15 min time-scales 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 h flare peaked at a flux density of 0.4 ± 0.1 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 time-scales. The early-time (<1 d) optical and X-ray light curves from the Neil Gehrels Swift Observatory demonstrated typical afterglow forward shock behaviour, allowing us to use blast wave physics to determine a likely homogeneous circumburst medium and an emitting electron population power-law index of p = 2.9 ± 0.1. We 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 time-scales. Using relations for ISS in the weak regime, we were able to place an upper limit on the size of the blast wave of ≤6 × 1016 cm in the plane of the sky, which is consistent with the theoretical forward shock size prediction of 8 × 1016 cm for GRB 210702A at ∼13 h post-burst. This represents the earliest ISS size constraint on a gamma-ray burst (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.
- Published
- 2023