2025ApJ...993...46W

Observations indicate that an optically thick circumstellar medium (CSM) at radii of 10¹⁴-10¹⁵ cm around Type II core-collapse supernova (SN) progenitors is common (and may be present in other types of massive star explosions). The breakout of the SN radiation-mediated shock (RMS) through such a CSM leads to the formation of a collisionless shock (CLS). We analyze the evolution of the shock structure and associated radiation field during and after the RMS-CLS transition for nonrelativistic shock breakout velocities (v_bo = 10⁹v₉ cm s^–1 < 0.1c) through a hydrogen-rich CSM “wind” density profile, ρ ∝ r^–2, with breakout radius R_bo = 10¹⁴R₁₄ cm, much larger than the progenitor radius. An analytic description of the key properties of the emitted optical to X-ray radiation is provided, supported by numeric radiation hydrodynamics calculations self-consistently describing the time-dependent spatial distribution of the plasma and radiation, governed by bremsstrahlung emission/absorption and inelastic Compton scattering. The characteristic energy of the photons carrying most of the luminosity, $\approx 1{0}^{43}\,{R}_{14}\,{v}_{9}^{2}\,$erg s^–1, shifts from UV to X-rays, reaching 1 keV as the shock reaches ≈3 R_bo, in ≈3 R₁₄/v₉ days. The X-ray signal is not suppressed by propagation through the upstream wind, and its absence may suggest that the dense CSM does not extend much beyond R_bo. Our results provide the basis for a quantitative calculation of the high-energy γ-ray and neutrino emission that is expected from particles accelerated at the CLS, and will allow using data from upcoming surveys that will systematically detect large numbers of young SNe, particularly ULTRASAT, to infer the preexplosion mass-loss history of the SN progenitor population.