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

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 $\sim 100\mathrm{s}$ is attributed to the cooling emission from the inner cocoon (shocked jet material); (2) the $\sim 0.1\mathrm{d}$ X-ray emission comes from the inner cocoon's afterglow; (3) the $\sim 0.4\mathrm{d}$ thermal-dominated optical emission arises from the cooling of the outer cocoon (shocked stellar material); (4) the $\sim 3\mathrm{d}$ 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 $\sim 7\mathrm{d}$. 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 $10^{51}\mathrm{erg}$. 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.

16 pages, 5 figures, 1 table. Revised version, first submitted on March 1st

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• Fast X-ray Transients produced by Off-axis Jet-Cocoons from Long Gamma-Ray Bursts
• Search for the Optical Counterpart of Einstein Probe Discovered Fast X-ray Transients from Lulin Observatory
• EP250108a/SN2025kg: A Magnetar-powered Gamma-Ray Burst Supernova Originating from a Close Helium-star Binary via Isolated Binary Evolution