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

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 x 10(16) cm. Multiwavelength modeling of the full afterglow required a period of significant energy injection between similar to 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 similar to 2 days, and a half-opening angle of theta(j) = 16.degrees 6 +/- 1.degrees 1 . Following the period of injection, the total energy is zeta similar to 18 times the initial impulsive energy, with a final collimation-corrected energy of E-Kf similar to 5.7 x 10(49) erg. The minimum Lorentz factors this model requires are consistent with constraints from the early radio measurements of Gamma > 35 to Gamma > 5 between similar to 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.