Thermal electrons in an ultra-relativistic shock shape the optical afterglow of GRB 250702F

Observing early optical emission from gamma-ray bursts (GRBs) contemporaneous with the MeV prompt emission phase remains rare. Such observations require rapid-response robotic facilities. The Ondřejov 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 revealed 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_ν ∝ -α ): a rapid rise turning gradually into a steep decay (α ∼ 1.6) before transitioning to a standard power-law afterglow (α = 0.79). This steep decay phase cannot be explained by non-thermal electrons accelerated at the forward shock, and a reverse-shock scenario is disfavoured 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. Modelling yields a non-thermal energy fraction of δ ≈ 0.8, with the remaining energy heating electrons at a characteristic Lorentz factor of γ_ ̊m th ∼ 900. These observations provide evidence of thermal electron signatures in GRB afterglows consistent with predictions from particle-in-cell simulations of ultra-relativistic collisionless shocks.