Radiation-mediated shocks in gamma-ray bursts: Spectral evolution

¹ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris (IAP), 98 bis boulevard Arago, 75014 Paris, France
² KTH Royal Institute of Technology, Department of Physics, SE-10691 Stockholm, Sweden
³ The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, SE-10691 Stockholm, Sweden
⁴ Institut Universitaire de France, Ministère de l’Enseignement Supérieur et de la Recherche, 1 rue Descartes, 75231 Paris Cedex F-05, France

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

Received: 11 November 2025
Accepted: 9 March 2026

Abstract

Radiation-mediated shocks (RMSs) occurring below the photosphere in a gamma-ray burst (GRB) jet could play a crucial role in shaping the prompt emission. In this paper, we study the time-resolved signal expected from such early shocks. We model an internal collision using a 1D special relativistic hydrodynamical simulation, and we follow the photon distributions in the resulting forward and reverse shocks as well as in the common downstream region to well above the photosphere using a designated RMS simulation code. We compute the light curve and time-resolved spectrum of the resulting single pulse taking into account the emission at different optical depths and angles to the line of sight. For the specific case considered, we find a light curve consisting of a short pulse lasting ∼0.1 s for an assumed redshift of z = 1, which could constitute a whole short GRB or be a building block within a highly variable longer GRB light curve. The efficiency is large, with ≈23% of the total burst energy being radiated. The spectrum has a complex shape at very early times, after which it settles into a more generic shape with a smooth curvature below the peak energy, E_p, and a clear high-energy power law that cuts off at ∼5 MeV in the observer frame. The spectrum becomes narrower and softer at late times with E_p steadily decreasing during the pulse decay from E_p ≈ 250 keV to E_p ≈ 100 keV. The low-energy index, α, decreases during the bright part of the pulse from α ≈ −0.5 to α ≈ −1, although the low-energy part is better fit with a broken power law when the signal-to-noise ratio is high. The high-energy power law is generated by the reverse shock at low optical depths (τ < 30) and has an index that decreases from β ≈ −2 to β ≈ −2.4. These results provide support for RMSs as potential candidates for the prompt emission in GRBs.