The Radio Flare and Multiwavelength Afterglow of the Short GRB 231117A: Energy Injection from a Violent Shell Collision
The Astrophysical Journal American Astronomical Society 994:1 (2025) 5-5
Authors:
GE Anderson, GP Lamb, BP Gompertz, L Rhodes, A Martin-Carrillo, AJ van der Horst, A Rowlinson, ME Bell, T-W Chen, HM Fausey, M Ferro, PJ Hancock, SR Oates, S Schulze, RLC Starling, S Yang, K Ackley, JP Anderson, A Andersson, JF Agüí Fernández, R Brivio, E Burns, KC Chambers, T de Boer, V D’Elia, M De Pasquale, A de Ugarte Postigo, Dimple, R Fender, MD Fulton, H Gao, JH Gillanders, DA Green, M Gromadzki, A Gulati, DH Hartmann, ME Huber, NJ Klingler, NPM Kuin, JK Leung, AJ Levan, C-C Lin, E Magnier, DB Malesani, P Minguez, KP Mooley, T Mukherjee, M Nicholl, PT O’Brien, G Pugliese, A Rossi, SD Ryder, B Sbarufatti, B Schneider, F Schüssler, SJ Smartt, KW Smith, S Srivastav, D Steeghs, NR Tanvir, CC Thoene, SD Vergani, RJ Wainscoat, Z-N Wang, RAMJ Wijers, D Williams-Baldwin, I Worssam, T Zafar
Abstract:
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 × 10
16
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
θ
j
=
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
E
Kf
∼ 5.7 × 10
49
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.
