The Radio Flare and Multiwavelength Afterglow of the Short GRB 231117A: Energy Injection from a Violent Shell Collision

Anderson, GE ORCID: 0000-0001-6544-8007, Lamb, GP ORCID: 0000-0001-5169-4143, Gompertz, BP ORCID: 0000-0002-5826-0548, Rhodes, L ORCID: 0000-0003-2705-4941, Martin-Carrillo, A ORCID: 0000-0001-5108-0627, van der Horst, AJ ORCID: 0000-0001-9149-6707, Rowlinson, A ORCID: 0000-0002-1195-7022, Bell, ME ORCID: 0000-0003-1767-5277, Chen, T-W ORCID: 0000-0002-1066-6098, Fausey, HM ORCID: 0000-0002-2927-2398, Ferro, M ORCID: 0009-0007-5708-7978, Hancock, PJ ORCID: 0000-0002-4203-2946, Oates, SR ORCID: 0000-0001-9309-7873, Schulze, S ORCID: 0000-0001-6797-1889, Starling, RLC ORCID: 0000-0001-5803-2038, Yang, S ORCID: 0000-0002-2898-6532, Ackley, K ORCID: 0000-0002-8648-0767, Anderson, JP ORCID: 0000-0003-0227-3451, Andersson, A ORCID: 0000-0003-2734-1895, Agüí Fernández, JF ORCID: 0000-0001-6991-7616 et al (2025) The Radio Flare and Multiwavelength Afterglow of the Short GRB 231117A: Energy Injection from a Violent Shell Collision. The Astrophysical Journal, 994 (1). p. 5. ISSN 0004-637X

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Abstract

We present the early radio detection and multiwavelength modeling of the short gamma-ray burst (GRB) 231117A at redshift z = 0.257. The Australia Telescope Compact Array automatically triggered a 9 hr observation of GRB 231117A at 5.5 and 9 GHz following its detection by the Neil Gehrels Swift Observatory just 1.3 hr post-burst. Splitting this observation into 1 hr time bins, the early radio afterglow exhibited flaring, scintillating and plateau
phases. The scintillation allowed us to place the earliest upper limit (<10 hr) on the size of a GRB blast wave to date, constraining it to <1 × 1016 cm. Multiwavelength modeling of the full afterglow required a period of significant energy injection between ∼0.02 and 1 day. The energy injection was modeled as a violent collision of two shells: a reverse shock passing through the injection shell explains the early radio plateau, while an X-ray flare is consistent with a shock passing through the leading impulsive shell. Beyond 1 day, the blast wave evolves as a classic decelerating forward shock with an electron distribution index of p = 1.66 ± 0.01. Our model also indicates a jet break at ∼2 days, and a half-opening angle of 0j = 16°.6 ± 1°.1. Following the period of injection, the total energy is ζ ∼ 18 times the initial impulsive energy, with a final collimation corrected energy of EKf ∼ 5.7 × 1049 erg. The minimum Lorentz factors this model requires are consistent with constraints from the early radio measurements of Γ > 35 to Γ > 5 between ∼0.1 and 1 day. These results demonstrate the importance of rapid and sensitive radio follow-up of GRBs for exploring their central engines and outflow behaviour.

Item Type:	Article
Uncontrolled Keywords:	0201 Astronomical and Space Sciences; 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics; 0306 Physical Chemistry (incl. Structural); Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	American Astronomical Society
Date of acceptance:	21 August 2025
Date of first compliant Open Access:	17 November 2025
Date Deposited:	17 Nov 2025 13:22
Last Modified:	17 Nov 2025 13:30
DOI or ID number:	10.3847/1538-4357/adfed7
URI:	https://researchonline.ljmu.ac.uk/id/eprint/27577

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