2025ApJ...991..180C

The radiation reprocessing model, in which an optically thick outflow absorbs the high-energy emission from a central source and reemits in longer wavelengths, has been frequently invoked to explain some optically bright transients, such as fast blue optical transients (FBOTs), whose progenitor and explosion mechanism are still unknown. Previous studies on this model did not take into account the frequency dependence of the opacity. We study the radiative reprocessing and calculate the UV-optical-near-infrared (NIR) band spectra from a spherical outflow composed of pure hydrogen gas, for a time-dependent outflowing mass rate. Electron scattering and frequency-dependent bound-free and free-free opacities are considered. The spectrum deviates from the blackbody at NIR and UV frequencies; in particular, it has νL_ν ∝ ν^1.5 at NIR frequencies, because at these frequencies the absorption optical depth from the outflow's outer edge to the so-called photon trapping radius is large and is frequency dependent. We apply our model to the prototype FBOT AT2018cow by fitting the spectra to the observed spectral energy distribution. The best-fit mass-loss rate suggests that the total outflow mass in AT2018cow is ${M}_{{\rm{out}}}\approx 5.{7}_{-0.4}^{+0.4}\,{M}_{\odot }$. If that equals the total mass lost during an explosion, and if the progenitor is a blue supergiant (with a preexplosion mass of ∼20 M_⊙), then it will suggest that the central compact remnant mass is at least ≈14 M_⊙. This would imply that the central remnant is a black hole.