Letter to the Editor

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

¹ Astronomical Institute of the Czech Academy of Sciences (ASU-CAS), Fričova 298, 251 65, Ondřejov, Czech Republic
² Gran Sasso Science Institute, Viale F. Crispi 7, I-67100, L’Aquila (AQ), Italy
³ INFN – Laboratori Nazionali del Gran Sasso, I-67100, L’Aquila (AQ), Italy
⁴ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, Brno, 611 37, Czech Republic
⁵ Department of Physics and Astronomy, Purdue University, West Lafayette, IN, USA
⁶ Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic
⁷ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047, Selargius (CA), Italy
⁸ Charles University, Faculty of Mathematics and Physics, Astronomical Institute, V Holešovičkách 2, Prague, 180 00, Czech Republic
⁹ Cahill Center for Astronomy & Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA, 91125, USA
¹⁰ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, Granada, 18008, Spain

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 March 2026
Accepted: 7 April 2026

Abstract

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 ∝ t^−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_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.