GRB 260924A: Extreme Energetics, Bright GeV Emission, and Exceptional X-ray and Optical Luminosity

Abstract

We present a multiwavelength study of GRB~260924A at $z=3.51\pm0.10$ using \textit{Fermi}, \textit{Swift}, and ground-based optical and near-infrared observations. Its isotropic-equivalent gamma-ray energy, $E_{γ,\mathrm{iso}}\simeq9.79\times10^{54}$~erg, places it among the most energetic GRBs observed. The prompt emission comprised two bright episodes separated by weaker activity. The X-ray afterglow ranked among the most luminous in our comparison sample at selected post-flare epochs, with a luminosity comparable to that of GRB~221009A at a rest-frame time of 3.5~ks. At a common redshift of $z=1$, the optical afterglow outshone GRB~221009A by approximately 2~mag at 0.3~d and 1~mag near 1~d. LAT emission extended to approximately 1100~s and showed an early excess above the subsequent decay. The highest-energy photon, 43.3~GeV, arrived at 82~s during the first episode; a 14.2~GeV photon arrived at 304~s between episodes. We describe the afterglow light curves through forward-shock synchrotron and synchrotron self-Compton (SSC) emission, with reverse-shock SSC accounting for the early LAT excess. We connect the early and late afterglow phases by modelling the X-ray rebrightening at several kiloseconds through energy injection from trailing ejecta and evolving shock microphysics. The late afterglow suggests a stratified medium, modelled as $n(R)\propto R^{-1.2}$. Within this interpretation, the magnetic contrast $R_B\equiv(\varepsilon_{B,r}/\varepsilon_{B,f})^{1/2} \simeq2.6$ suggests magnetized ejecta, while the highest-energy LAT photons exceed the predicted synchrotron limit, supporting an SSC contribution.