EP240414a: Off-axis View of a Jet-Cocoon System from an Expanded Progenitor Star

When a relativistic jet is launched following the core-collapse of a star, its interaction with the stellar envelope leads to the formation of a hot cocoon, which produces various viewing-angle-dependent observational phenomena following the breakout from the surface. We study the observational signatures of fast X-ray transient (FXT) EP240414a, which may originate from a jet-cocoon system viewed slightly off-axis. In our model, (1) the prompt X-ray emission lasting $\sim \text{\hspace{0.17em}⁣}100\mathrm{s}$ is attributed to the cooling emission from the inner cocoon (shocked jet material); (2) the $\sim \text{\hspace{0.17em}⁣}0.1\mathrm{d}$ X-ray emission comes from the inner cocoon’s afterglow; (3) the $\sim \text{\hspace{0.17em}⁣}0.4\mathrm{d}$ thermal-dominated optical emission arises from the cooling of the outer cocoon (shocked stellar material); (4) the $\sim \text{\hspace{0.17em}⁣}3\mathrm{d}$ non-thermal optical component and subsequent radio emission can be explained by the afterglow from a jet with a viewing angle of ${10}^{\circ }\lesssim {\theta }_{\mathrm{v}}\lesssim {15}^{\circ }$; and (5) the associated broad-lined Type Ic supernova only dominates the optical emission after $\sim \text{\hspace{0.17em}⁣}7\mathrm{d}$. Both the jet inferred from the off-axis afterglow and the inner cocoon constrained by the cooling emission are found to have similar kinetic energies, on the order of ${10}^{51}\mathrm{e}\mathrm{r}\mathrm{g}$. We find that the progenitor’s radius is $\sim 3R_{\odot }$ as constrained by the { inner cocoon’s} cooling emissions, indicating that the pre-explosion star may be a massive helium star that is slightly inflated. More FXTs associated with off-axis jets and supernovae will be further examined by the Einstein Probe, leading to a deeper understanding of jet-cocoon systems.