EP240414a: Off-axis View of a Jet-cocoon System from an Expanded Progenitor Star
Jian-He Zheng, Jin-Ping Zhu, Wenbin Lu, Bing Zhang
Abstract
Abstract 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 10 51 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.