Development of a spherical harmonics based radiation transport code for dynamical spacetime neutron star merger simulations

Motivated by the multimessenger observations of GW170817 and the associated kilonova, accurate modeling of binary neutron star merger and post-merger dynamics has become a major focus of the numerical relativity community. In addition to spacetime evolution and GRMHD, accurate neutrino radiative transport (RT) is widely recognized a key requirement for improving astrophysical realism. To achieve this, a neutrino RT scheme needs to operate in fully dynamical spacetimes, and be stable and accurate in optically thick and optically thin regimes, and, ideally, should be computationally efficient and avoid problematic artifacts such as the "ray effects" that arise in ray-based methods or the beam collision effects that arise in M1 closures. Here, we describe ongoing work to extend the promising FP_N method to dynamical spacetimes. This method uses spherical harmonics to describe the angular structure of intensity at each point, avoiding the inaccuracies of M1 closures and eliminating ray effects entirely. Extending this to the dynamical spacetime case involves a unique formulation of the RT equation employing the spin-weighted spherical harmonics in its derivation. This formulation will be described in detail, and tests of the resulting code will be presented.