Shock Breakout from Mildly Relativistic Ejecta in a Dense Wind: The Case of EP260321a/SN 2026gzf

We present shock breakout (SBO) modeling of the recently discovered X-ray transient EP260321a detected by the Einstein Probe mission. Our semianalytic 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 ∼3.5 × 10 49 erg (excluding the supernova ejecta) and a dense wind characterized by a mass-loss rate of M ̇ ≃ 1.2 × 1 0 − 3 ( v w / 1 0 3 km s − 1 ) M ⊙ yr − 1 , where v w is the wind velocity. The observed burst duration requires the dense CSM to extend up to ∼350 R ⊙ , corresponding to a CSM mass of ∼10 −5 M ⊙ . These results demonstrate that SBO observations provide a sensitive probe of mass loss from stripped-envelope supernova progenitors shortly before core collapse.