Shock breakout from mildly relativistic ejecta in a dense wind: the case of EP260321a/SN~2026gzf

Abstract

We present shock breakout (SBO) modeling of the recently discovered X-ray transient EP260321a detected by the \textit{Einstein Probe} mission. Our semi-analytic model, based on our previous work, follows the interaction between a supernova ejecta with a mildly relativistic outer envelope and a dense, wind-like circumstellar medium (CSM) by using a thin-shell approximation that incorporates relativistic effects. We find that the observed properties of the X-ray emission are well explained by the breakout emission powered by a high-velocity envelope with a kinetic energy of $\sim 3.5\times 10^{49}\mathrm{erg}$ (excluding the supernova ejecta) and a dense wind characterized by a mass-loss rate of $\dot{M}\simeq 1.2\times 10^{-3}(v_{\mathrm{w}}/10^3{\mathrm{km}\mathrm{s}}^{-1})M_{\odot }{\mathrm{yr}}^{-1}$, where $v_{\mathrm{w}}$ is the wind velocity. The observed burst duration requires the dense CSM to extend up to $\sim 350R_{\odot }$, corresponding to a CSM mass of $\sim 10^{-5}{M}_{\odot }$. These results demonstrate that SBO observations provide a sensitive probe of mass loss from stripped-envelope supernova progenitors shortly before core collapse.

14 pages, 3 figures, submitted