The Landscape of Collapsar Outflows: Structure, Signatures, and Origins of Einstein Probe Relativistic Supernova Transients

Abstract The Einstein Probe is revolutionizing time-domain astrophysics through the discovery of new classes of X-ray transients associated with broad-line Type Ic supernovae. These events commonly exhibit bright early-time optical counterparts and sudden afterglow rebrightening within the first week—features that existing models fail to explain. In particular, structured jet and cocoon scenarios are inconsistent with the observed sharp rebrightening and multiday optical emission, while the refreshed shock model is ruled out owing to its inconsistency with collapsar hydrodynamics. Drawing on 3D general relativistic magnetohydrodynamic simulations, we present the multiscale angular and radial structure characterizing collapsar outflows. The resulting morphology features episodic, wobbling jets with a “top-hat” geometry, embedded within a smoother global cocoon and disk ejecta angular structure. The wobbling jets give rise to variations in radiative efficiency that can account for the observed alternation between X-ray-dominated and γ -ray-dominated jet emission. The top-hat structure of individual wobbling jet episodes naturally explains the sudden rebrightening observed when the emission from the top-hat jet cores enters the observer’s line of sight. The radial structure is consistent with that inferred from observations of stripped-envelope supernovae. It comprises a mildly relativistic cocoon (0.3 ≲ β Γ ≲ 3) that may power an early (∼1 day) rapidly decaying emission, followed by slower, black-hole-accretion-disk-driven outflows ( β ≲ 0.3), which dominate the slowly evolving optical emission at t ≳ 1 day. This novel multicomponent outflow structure provides a unified explanation for the multiband light curves observed in Einstein Probe transients and is likely a common feature of broad-line Type Ic supernovae more broadly.