Rapid radio brightening of GRB 210702A

Abstract

We 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 $\le 6\times 10^{16}$cm in the plane of the sky, which is consistent with the theoretical forward shock size prediction of $8\times 10^{16}$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.

Submitted to MNRAS 13 pages, 6 figures, 2 tables

References in corpus (31)

• The NumPy array: a structure for efficient numerical computation
• Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A
• The Supernova -- Gamma-Ray Burst Connection
• CASA, the Common Astronomy Software Applications for Radio Astronomy
• An online repository of Swift/XRT light curves of GRBs
• Discerning the physical origins of cosmological Gamma-ray bursts based on multiple observational criteria: the cases of z=6.7 GRB 080913, z=8.3 GRB 090423, and some short/hard GRBs
• The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. VI. Radio Constraints on a Relativistic Jet and Predictions for Late-Time Emission from the Kilonova Ejecta
• Teraelectronvolt emission from the $\gamma$-ray burst GRB 190114C
• A very-high-energy component deep in the Gamma-ray Burst afterglow
• Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow
• Discovery and confirmation of the shortest gamma ray burst from a collapsar
• Gamma Ray Bursts Are Observed Off-Axis
• A Peculiarly Short-duration Gamma-Ray Burst from Massive Star Core Collapse
• Electrons' energy in GRB afterglows implied by radio peaks
• A Comprehensive Radio View of the Extremely Bright Gamma-Ray Burst 130427A
• Gamma-Ray Burst Afterglows as Probes of Environment and Blastwave Physics II: the Distribution of p and Structure of the Circumburst Medium
• Detailed study of the GRB 030329 radio afterglow deep into the non-relativistic phase
• A Reverse Shock in GRB 160509A
• Clustering of LAT light curves: a clue to the origin of high-energy emission in Gamma-Ray Bursts
• Probing the Bright Radio Flare and Afterglow of GRB 130427A with the Arcminute Microkelvin Imager
• The Arcminute Microkelvin Imager Catalogue of Gamma-ray Burst afterglows at 15.7 GHz
• A search for variable and transient radio sources in the extended Chandra Deep Field South at 5.5 GHz
• LOFAR early-time search for coherent radio emission from GRB 180706A
• Detailed Afterglow Modeling and Host Galaxy Properties of the Dark GRB 111215A
• Constraints on Electron Acceleration in Gamma-Ray Bursts Afterglows from Radio Peaks
• Murchison Widefield Array rapid-response observations of the short GRB 180805A
• Early-time Searches for Coherent Radio Emission from Short GRBs with the Murchison Widefield Array
• Radio data challenge the broadband modelling of GRB160131A afterglow
• Rapid-response radio observations of short GRB 181123B with the Australia Telescope Compact Array
• High time resolution search for prompt radio emission from the long GRB 210419A with the Murchison Widefield Array
• Jet-Cocoon Geometry in the Optically Dark, Very High Energy Gamma-ray Burst 201216C

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