Thermal Electrons in an Ultra-Relativistic Shock Shape the Optical Afterglow of GRB 250702F

TL;DR

This paper presents early optical observations of GRB 250702F, revealing a steep decay phase explained by thermal electrons in the shock, providing evidence for thermal electron signatures in GRB afterglows.

Contribution

It introduces the first observational evidence linking thermal electrons in ultra-relativistic shocks to optical afterglow features of GRBs.

Findings

Detection of optical counterpart 27.8 seconds after trigger

Identification of a steep decay phase in the optical light curve

Evidence supporting thermal electron populations in GRB shocks

Abstract

Observing early optical emission from gamma-ray bursts (GRBs) contemporaneous with the MeV prompt emission phase remains rare, requiring rapid-response robotic facilities. The Ond\v{r}ejov D50 telescope detected the optical counterpart of GRB 250702F at z = 1.520 only 27.8 s after trigger, enabling high-cadence monitoring during the brightest prompt emission pulses. The optical light curve reveals two distinct flares. The first (30 - 100 s) is spectrally consistent with the MeV prompt emission. The second flare (100 - 1400 s) exhibits an unusual morphology (F_nu ~ t^-alpha): a rapid rise to a plateau, followed by a steep decay (alpha ~ 1.6) before transitioning to a standard power-law afterglow (alpha = 0.79). This steep decay phase cannot be explained by nonthermal electrons accelerated at the forward shock, and reverse-shock scenario is disfavored due to the long duration of the flare and the temporal offset from the underlying deceleration time. We interpret the steep decay as the synchrotron frequency of a thermal (Maxwellian) electron population sweeping through the optical band. Modeling yields a non-thermal energy fraction delta ~ 0.8 with the remaining energy heating electrons at characteristic Lorentz factor gamma_th ~ 900. These observations provide evidence for thermal electron signatures in GRB afterglows, consistent with predictions from particle-in-cell simulations of ultra-relativistic collisionless shocks.