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The explosion of a star is first marked by the shock wave breaking out of the stellar surface, producing a burst of ultraviolet and X-ray radiation. These events are observationally rare, despite likely accompanying the majority of supernovae (SNe). Here, we report on our multiwavelength observing campaign of the closest Einstein Probe fast X-ray transient (FXT) EP260321a at z = 0.0344. The thermal (kT = 130 eV) X-ray emission with peak luminosity of 1.0 × 10<SUP>45</SUP> erg s<SUP>−1</SUP> points to a shock breakout origin. We demonstrate that EP260321a is accompanied by a broad-lined Type Ic supernova, SN 2026gzf. The supernova (SN) properties, including its spectral evolution, lightcurve evolution, and expansion velocities, are all typical of the energetic stripped-envelope SNe associated with gamma-ray bursts (GRBs). However, deep X-ray upper limits obtained with the Chandra X-ray Observatory do not detect an X-ray afterglow, and instead exclude the afterglow of known GRBs or FXTs. If the stellar explosion launched a successful relativistic jet, we require that it had both a low Lorentz factor of Γ<SUB>0</SUB> &lt; 30 and a kinetic energy of E<SUB>kin</SUB> &lt; 10<SUP>49</SUP> erg for a stellar wind density of A<SUB>*</SUB> ≳ 1. We propose that EP260321a originated from a mildly relativistic, weak outflow that was choked by the progenitor star. This scenario is capable of naturally explaining its low X-ray luminosity and lack of prompt gamma-ray emission. EP260321a bridges the gap between SN 2008D and low-luminosity GRBs, suggesting a greater diversity in the physical parameters of stripped stars as they undergo terminal collapse.

The complex radio light curves of tidal disruption events (TDEs) challenge our understanding of the properties of both the outflows and the circumnuclear medium (CNM) surrounding supermassive black holes. In this work, we explore outflow─CNM interactions across a broad parameter space using three-dimensional hydrodynamic simulations, adopting a broken power-law CNM density profile with a transition near the Bondi radius. The outflow─CNM interaction inside the Bondi radius produces an early radio flare (≲2 yr) once the emitting region becomes optically thin. A second radio rebrightening can appear a few years later if the outflow decelerates beyond the Bondi radius. We also find that either a very dense inner CNM, which causes rapid deceleration, or a rarefied outer CNM suppresses the late rebrightening, which produces a single early-peaked flare. In contrast, a rarefied CNM inside the Bondi radius suppresses the early flare and yields a single late-peaked event. For the case of a very dense CNM at large radii, the interaction will trigger a sharp late-time rise as observed in some TDEs. We further explore the interaction of a relativistic jet with a broken power-law CNM, which can reproduce the characteristic light curves observed in jetted TDEs without invoking a complex jet structure.