2025ApJ...985...21Z

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 ∼100 s is attributed to the cooling emission from the inner cocoon (shocked jet material); (2) the ∼0.1 day X-ray emission comes from the inner cocoon's afterglow; (3) the ∼0.4 day thermal-dominated optical emission arises from the cooling of the outer cocoon (shocked stellar material); (4) the ∼3 day nonthermal optical component and subsequent radio emission can be explained by the afterglow from a jet with a viewing angle of 10^∘ ≲ θ_v ≲ 15^∘; and (5) the associated broad-lined Type Ic supernova only dominates the optical emission after ∼7 days. 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 10⁵¹ erg. We find that the progenitor's radius is ∼3 R_⊙ as constrained by the inner cocoon's cooling emissions, indicating that the preexplosion 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.