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

Martin Jelínek, Annarita Ierardi, Filip Novotný, Gor Oganesyan, Biswajit Banerjee, Dimitrios Giannios, Sergey Karpov, Martin Topinka, Elias Kammoun, Jan Štrobl, Alberto J. Castro-Tirado

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 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 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.

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

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 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 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.
Paper Structure (19 sections, 5 equations, 4 figures, 6 tables)

This paper contains 19 sections, 5 equations, 4 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Prompt emission
  4. Optical light curve analysis
  5. Empirical characterization
  6. Thermal electron interpretation
  7. Discussion
  8. From prompt emission to X-ray flares
  9. Physics of thermal electrons
  10. Conclusions
  11. Optical observations
  12. High-energy observations and data analysis
  13. Fermi GBM
  14. Swift XRT and BAT
  15. Fermi LAT
  16. ...and 4 more sections

Figures (4)

  • Figure 1: Multi-wavelength light curve of GRB 250702F. Optical data from D50 are shown together with Swift/BAT and XRT observations. Multi-filter optical points are color-corrected to the $r$-band using the measured spectral slope. Open symbols represent GCN points. The solid line shows an empirical fit to the optical data (see Section \ref{['sec:lcmodel']}).
  • Figure 2: Multi-wavelength light curves (top panel), spectral energy distributions during the prompt phase (bottom left panel) and during the X-ray flares (bottom right panel). The optical data points from D50 and the LAT upper limits are shown together with the best-fit spectral models. During the prompt phase (SP1, SP2), the extrapolated gamma-ray spectrum is consistent with the observed optical flux. During the X-ray flares (SP3--SP8), the optical flux was set as an upper limit for the joint fit.
  • Figure 3: Optical afterglow fit with a hybrid Maxwellian--power-law electron distribution model (top) and the corresponding X-ray prediction based on the optical fit (bottom). The steep decay arises as the synchrotron frequency of thermal electrons sweep through the optical band.
  • Figure 4: Two-component (RS + FS) fit to the optical light curve. The inferred RS peak time does not coincide with the FS peak, contrary to the expectation that both components originate from the same deceleration radius.