Monte-Carlo radiation transport in neutron star merger simulations

Neutron star mergers provide us with a remarkable laboratory to test the laws of physics in dense environments, to constrain the production sites of heavy elements, and to understand high-energy transients like gamma-ray bursts. Neutrinos play an important role in these mergers: they are the main source of cooling in post-merger remnants, and the main drivers of changes to the composition of the matter that they eject. As a result, neutrino-matter interactions significantly impact the outcome of nucleosynthesis in mergers, and the properties of the optical/infrared transients that they power. Most merger simulations to-date have however treated neutrinos using approximate transport methods – either "leakage" of "moment" schemes. Here, I will discuss the implementation in the SpEC code of a Monte-Carlo transport scheme, the challenges associated with the use of such an algorithm in merger simulations, and its application so far to simulations of NSNS and NSBH mergers. These simulations allow us to provide improved estimates for the properties of the baryonic matter ejected by mergers and of the neutrinos escaping merger remnants. Comparisons with simulations using approximate transport algorithms also allow us to estimate uncertainties in these approximate transport schemes.