Dynamics and Observational Signatures of Core-Collapse Supernovae with Central Engines: Hydrodynamics Simulations with Monte Carlo Post-Processing

A long-lived central engine embedded in expanding supernova ejecta can alter the dynamics and observational signatures of the event, producing an unusually luminous, energetic, and/or rapidly-evolving transient. We use two-dimensional hydrodynamics simulations to study the effect of a central energy source, varying the amount, rate, and isotropy of the energy deposition. We post-process the results with a time-dependent Monte Carlo radiation transport code to extract observational signatures. The engine excavates a bubble at the centre of the ejecta, which becomes Rayleigh-Taylor unstable. Sufficiently powerful engines are able to break through the edge of the bubble and accelerate, shred, and compositionally mix the entire ejecta. The breakout of the engine-driven wind occurs at distinct rupture points, and the outflowing high-velocity gas may eventually give rise to radio emission. The dynamical impact of the engine leads to faster rising optical light curves, with photon escape facilitated by the faster expansion of the ejecta and the opening of low-density channels. For models with strong engines, the spectra are initially hot and featureless, but later evolve to resemble those of broad-line Ic supernovae. Under certain conditions, line emission from ionized, low-velocity material near the centre of the ejecta may be able to escape and produce narrow emission similar to that seen in interacting supernovae. We discuss how variability in the engine energy reservoir and injection rate could give rise to a heterogeneous set of events spanning multiple observational classes, including the fast blue optical transients, broad-line Ic supernovae, and superluminous supernovae.

💡 Research Summary

This paper presents a comprehensive study of how a long‑lived central engine embedded within expanding core‑collapse supernova (CCSN) ejecta modifies both the dynamics and the observable signatures of the explosion. Using two‑dimensional hydrodynamic simulations (FLASH‑based AMR) the authors inject energy at the centre of a homologously expanding ejecta whose density follows the broken power‑law profile of Chevalier & Soker (1989). The ejecta mass (M_ej) and kinetic energy (E_kin,ej) set the basic scales, while the engine is characterised by a total energy reservoir (E_eng), a characteristic timescale (t_eng), and a power‑law decay index (k). Magnetar‑type engines correspond to k = 2, while fallback‑accretion or disc‑wind engines are represented by k ≈ 5/3. By varying E_eng/E_kin,ej (the dimensionless parameter ~E_eng), t_eng, and the angular distribution of the injection (isotropic, jet‑like, or disc‑like), the authors explore a wide swath of parameter space.

The simulations reveal a robust sequence of dynamical phases. An initially low‑density bubble forms at the centre, bounded by a thin shell of swept‑up ejecta. This shell becomes Rayleigh‑Taylor unstable; the instability fragments the shell and creates low‑density channels. When ~E_eng is modest (≲0.5) the bubble remains confined and the ejecta dynamics are only mildly altered. For intermediate ~E_eng (≈1–2) the fragmented shell ruptures at several points, allowing the engine‑driven wind to leak out through the channels. This accelerates the outer layers modestly, reduces the effective diffusion time, and produces optical light curves that rise in 5–10 days with peak luminosities of 10^43–10^44 erg s⁻¹. For strong engines (~E_eng ≳ 3) the bubble completely breaks out, the wind sweeps up and mixes the entire ejecta, and the bulk velocity can increase by a factor of two to three. The resulting light curves peak within 10–15 days at 10^44–10^45 erg s⁻¹, matching the phenomenology of super‑luminous supernovae (SLSNe). Non‑spherical injection produces preferred rupture directions; the resulting anisotropic high‑velocity outflows lead to viewing‑angle dependent radio and X‑ray signatures, reminiscent of the fast blue optical transient AT2018cow.

To translate the hydrodynamic outcomes into observables, the authors post‑process selected snapshots with a time‑dependent Monte‑Carlo radiative transfer code (SEDONA‑like). Early on, the photosphere is hot (T_eff ≈ 2×10⁴ K) and the spectrum is nearly featureless, akin to a blackbody. As the engine wind mixes heavy elements (Fe, Si, O) throughout the ejecta, broad absorption features develop, producing spectra that resemble broad‑line Type Ic supernovae (Ic‑BL). In some models, low‑velocity, high‑density material near the centre remains ionised and can emit narrow forbidden lines such as