📝 SummarXiv

Abstract

The Einstein Probe is revolutionizing time-domain astrophysics through the discovery of new classes of X-ray transients associated with Type Ic-Broad Line 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 multi-day optical emission, while the refreshed shock model is ruled out due to its inconsistency with collapsar hydrodynamics. Drawing on 3D general-relativistic magnetohydrodynamic simulations, we present the multi-scale 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 $\gamma$-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 ($0.3\lesssim\beta\Gamma\lesssim3$) that may power an early ($\sim1\,{\rm day}$) rapidly decaying emission, followed by slower, black hole accretion disk-driven outflows ($\beta\lesssim0.3$), which dominate the slowly evolving optical emission at $t\gtrsim1\,{\rm day}$. This novel multi-component outflow structure provides a unified explanation for the multiband light curves observed in Einstein Probe transients and is likely a common feature of Type Ic-Broad Line supernovae more broadly.