Double-shock Radiation in Supernova Wind Breakout and EP260321a

Abstract We study the supernova (SN) shock breakout (SBO) from an optically thick stellar wind. The SN ejecta–wind interaction forms a forward shock (FS) plus reverse shock (RS) structure. The RS’s internal energy and shock velocity are both a fraction μ ≲ 1/6 of the FS’s. The lower RS velocity makes it easier to build up local thermal equilibrium (LTE) and generate blackbody (BB) emission. In the same time a μ −1 ≳ 6 times larger non-BB emission may be produced by the likely non-LTE FS. We apply the theory to analyze the recent detected type Ic-BL SN 2026gzf associated X-ray flash EP260321a, and find it is naturally accounted for by an SN wind breakout: the ∼0.13 keV, thermal SBO emission can be produced by the RS during the interaction of a typical type Ic-BL SN ejecta ( E k ∼ 10 52 erg and M e ∼ 1 M ⊙ ) and a stellar wind with a mass M w ∼ 10 −4 M ⊙ confined within a radius r w ∼ 10 13 cm. The X-ray rise time is mainly determined by the RS emission time, whereas the long duration and decrease time are due to the scatterings of SBO photons by a distant shell-like medium that has a large photosphere, r ph ∼ 10 14 cm. The early (within 30 ks) optical emission can be naturally explained by the cooling emission of a shocked pure C/O envelope, since the observed SBO onset is delayed by ≳ a few kiloseconds due to the large medium photosphere. BB+non-BB components from RS and FS, respectively, may be common in SN wind breakouts. SBO observations can be crucial in the study of progenitor stars by providing information for the ejecta from the SN explosion and the nearby wind from the pre-SN activities.