Neutrino oscillations in binary neutron star mergers simulations

Gravitational waves (GWs) and electromagnetic (EM) radiation from colliding binary neutron stars (BNS) contain valuable information about the nature of neutron stars and the astrophysical processes that produce heavy elements, through multi-messenger signals. With the advent of next-generation multi-messenger detectors, it becomes crucial to pair these observations with precise models of merger dynamics, which can only be achieved through ab-initio supercomputer simulations. Currently, BNS simulations suffer from significant systematic errors due to the classical treatment of neutrinos. This work addresses these limitations by introducing a Bhatnagar-Gross-Krook (BGK) sub-grid model in the 3D general relativistic magnetohydrodynamic (GRMHD) code WhiskyTHC, enabling neutrino oscillations within the M1 neutrino transport solver. Neutrino mixing is modeled as a relaxation process towards an asymptotic equilibrium state in the lab frame. By adopting reasonable relaxation times and mixing prescriptions, we test our code on single-star and binary neutron star merger simulations. For the first time through BNS simulations, we present detailed comparisons of gravitational wave signals and outflow analyses between classical (no-mixing) and neutrino-mixing scenarios.