000586816 001__ 586816 000586816 005__ 20240112093107.0 000586816 0247_ $$2INSPIRETeX$$aAnderson:2022qrc 000586816 0247_ $$2inspire$$ainspire:2513184 000586816 0247_ $$2arXiv$$aarXiv:2211.11212 000586816 0247_ $$2datacite_doi$$a10.3204/PUBDB-2023-04030 000586816 037__ $$aPUBDB-2023-04030 000586816 041__ $$aEnglish 000586816 082__ $$a520 000586816 088__ $$2arXiv$$aarXiv:2211.11212 000586816 1001_ $$0P:(DE-H253)PIP1092871$$aAndersson, Gabriella$$b0$$eCorresponding author 000586816 245__ $$aRapid radio brightening of GRB 210702A 000586816 260__ $$c2023 000586816 3367_ $$0PUB:(DE-HGF)25$$2PUB:(DE-HGF)$$aPreprint$$bpreprint$$mpreprint$$s1688555184_3479717 000586816 3367_ $$2ORCID$$aWORKING_PAPER 000586816 3367_ $$028$$2EndNote$$aElectronic Article 000586816 3367_ $$2DRIVER$$apreprint 000586816 3367_ $$2BibTeX$$aARTICLE 000586816 3367_ $$2DataCite$$aOutput Types/Working Paper 000586816 500__ $$aSubmitted to MNRAS 13 pages, 6 figures, 2 tables 000586816 520__ $$aWe observed the rapid radio brightening of GRB 210702A with the Australian Telescope Compact Array (ATCA) just 11hr post-burst, tracking early-time radio variability over a 5hr period on ~15min timescales at 9.0, 16.7, and 21.2GHz. A broken power-law fit to the 9.0GHz light curve showed that the 5hr flare peaked at a flux density of 0.4+/-0.1mJy at ~13hr post-burst with a steep rise and decline. The observed temporal and spectral evolution are not expected in the standard internal-external shock model, where forward and reverse shock radio emission evolves on much longer timescales. The early-time (<1day) 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 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. Using weak ISS relations, we were able to place an upper limit on the size of the blast wave of $\leq6 \times 10^{16}$cm in the plane of the sky, which is consistent with the theoretical forward shock size prediction of $8\times10^{16}$cm for GRB 210702A at ~13h post-burst. This represents the earliest ISS size constraint on a GRB blast wave to date, demonstrating the importance of rapid (<1day) radio follow-up of GRBs using several-hour integrations to capture the early afterglow evolution, and to track scintillation over a broad frequency range. 000586816 536__ $$0G:(DE-HGF)POF4-613$$a613 - Matter and Radiation from the Universe (POF4-613)$$cPOF4-613$$fPOF IV$$x0 000586816 588__ $$aDataset connected to CrossRef, INSPIRE, Journals: bib-pubdb1.desy.de 000586816 693__ $$0EXP:(DE-H253)HESS-20170101$$5EXP:(DE-H253)HESS-20170101$$eHigh Energy Stereoscopic System$$x0 000586816 7001_ $$aRussell, T. D.$$b1 000586816 7001_ $$aFausey, H. M.$$b2 000586816 7001_ $$avan der Horst, A. J.$$b3 000586816 7001_ $$aHancock, P. J.$$b4 000586816 7001_ $$aBahramian, A.$$b5 000586816 7001_ $$aBell, M. E.$$b6 000586816 7001_ $$aMiller-Jones, J. C. A.$$b7 000586816 7001_ $$aRowell, G.$$b8 000586816 7001_ $$aSammons, M. W.$$b9 000586816 7001_ $$aWijers, R. A. M. J.$$b10 000586816 7001_ $$aGalvin, T. J.$$b11 000586816 7001_ $$aGoodwin, A. J.$$b12 000586816 7001_ $$0P:(DE-H253)PIP1084239$$aKonno, R.$$b13 000586816 7001_ $$aRowlinson, A.$$b14 000586816 7001_ $$aRyder, S. D.$$b15 000586816 7001_ $$aSchussler, F.$$b16 000586816 7001_ $$aWagner, S. J.$$b17 000586816 7001_ $$0P:(DE-H253)PIP1089041$$aZhu, S. 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