Black-hole excision scheme for general relativistic core-collapse supernova simulations

Fallback supernovae and the collapsar scenario for long gamma-ray bursts and hypernovae have received considerable interest as pathways to black-hole formation and extreme transient events. Consistent simulations of these scenarios require a general relativistic treatment and need to deal appropriately with the formation of a singularity. Free evolution schemes for the Einstein equations can handle the formation of black holes by means of excision or puncture schemes. However, in constrained schemes, which offer distinct advantages in long-term numerical stability in stellar collapse simulations over well above 104 light-crossing timescales, the dynamical treatment of black-hole spacetimes is more challenging. Building on previous work on excision in conformally flat spacetimes, we here present the implementation of a black-hole excision scheme for supernova simulations with the CoCoNuT-FMT neutrino transport code. We describe in detail a choice of boundary conditions that ensures long-time numerical stability, and also address upgrades to the hydrodynamics solver that are required to stably evolve the relativistic accretion flow onto the black hole. The scheme is currently limited to a spherically symmetric metric, but the hydrodynamics can be treated multidimensionally. For demonstration, we present a spherically symmetric simulation of black-hole formation in an 85M0 star, as well as a two-dimensional simulation of the fallback explosion of the same progenitor. These extend past 9 and 0.3 s after black-hole formation, respectively.