Fast X-Ray Transients Produced by Off-axis Jet Cocoons from Long Gamma-Ray Bursts

Abstract Fast X-ray transients (FXTs) have been detected for over a decade, yet their origins are still enigmatic. The observed association between FXTs and broad-lined Type Ic supernovae (SNe Ic-BL) suggests that some may share the same progenitor with long gamma-ray bursts. In this work, we numerically simulate the long-term evolution of a relativistic jet propagating from inside the progenitor star up to the photon diffusion radius of the cocoon. Then we post-process the hydrodynamic results and calculate the cocoon cooling emission for various viewing angles from the jet axis. We find that, for viewing angles θ v = 10°–20°, the off-axis cocoon emission can produce FXTs with luminosity L X ≃ 10 47−48 erg s −1 and duration t X ≃ 10–100 s. The observed spectra are quasi-thermal, with a peak energy E peak ≃ 0.8 keV. These properties naturally explain observational features of a fraction of FXTs, including their high luminosity, soft spectra, and lack of gamma-ray counterparts. The Rayleigh–Jeans tail of the FXT spectra extends to the UV, producing an early UV flash simultaneously. As the cocoon expands and cools, the emission peak shifts to UV and optical bands, resulting in a bright optical plateau lasting for ∼1 day with color temperature T UV/opt ≃ (1–3) × 10 4 K and bolometric luminosity L bol ≃ 10 41−42 erg s −1 , before the emergence of supernova emission. Although our model underpredicts the UV/optical luminosity at ∼1 day for some events (e.g., EP 240414a), it still provides useful diagnostics for identifying the origins of FXTs.