Monte Carlo simulations of relativistic shock breakout from a stellar wind

Abstract We present Monte Carlo simulations of relativistic radiation-mediated shocks (RRMS) in the photon-starved regime, incorporating photon escape from the upstream region—characterized by the escape fraction, fesc—under a steady-state assumption. These simulations, performed for shock Lorentz factors Γu = 2, 3.5, 6, 10, and 15, are applicable to RRMS breakouts in shallowly declining density profiles such as stellar winds. We find that vigorous pair production acts as a thermostat, regulating the downstream temperature to ~100-200 keV, largely independent of fesc. A subshock forms and strengthens with increasing fesc. The escaping spectra peak at Ep ≈ 300-600 keV in the shock frame and deviate from a Wien distribution, exhibiting low-energy flattening (fν∝ν0) due to free-free emission and high-energy extensions caused by inverse Compton scattering from subshock-heated pairs. While an earlier analytical model reproduces the velocity structure well at Γu = 2, it significantly overestimates the shock width at higher Lorentz factors, particularly for fesc ≳ a few %. Based on this finding, we provide updated predictions for breakout observables in wind environments for Γu ≳ 6. Notably, the duration of the relativistic breakout becomes largely insensitive to the explosion energy and ejecta mass, typically exceeding analytical predictions by orders of magnitude and capable of producing a ~300 s flash of MeV photons with a radiated energy of ~1050 erg for an energetic explosion yielding Γbo ~ 6. We also discuss limitations of our modelling assumptions and their implications for the predicted breakout observables.