Calibration of the Advanced Spectral Leakage scheme for neutron star merger simulations, and extension to smoothed-particle hydrodynamics

TL;DR

This paper calibrates the Advanced Spectral Leakage scheme for neutron star merger simulations, extending it to smoothed-particle hydrodynamics, achieving comparable accuracy to M1 with significantly improved computational efficiency.

Contribution

It introduces a calibrated, efficient neutrino transport scheme for neutron star mergers, including a novel particle-based optical depth calculation for SPH.

Findings

ASL recovers luminosities and energies within 25%

ASL achieves similar accuracy to M1 in energy and electron fraction rates

Speed-up factor of 100 in optical depth calculations with SPH

Abstract

We calibrate a neutrino transport approximation, called Advanced Spectral Leakage (ASL), with the purpose of modeling neutrino-driven winds in neutron star mergers. Based on a number of snapshots we gauge the ASL parameters by comparing against both the two-moment (M1) scheme implemented in the FLASH code and the Monte Carlo neutrino code Sedonu. The ASL scheme contains three parameters, the least robust of which results to be a blocking parameter for electron neutrinos and anti-neutrinos. The parameter steering the angular distribution of neutrino heating is re-calibrated compared to the earlier work. We also present a new, fast and mesh-free algorithm for calculating spectral optical depths, which, when using Smoothed Particle Hydrodynamics (SPH), makes the neutrino transport completely particle-based. We estimate a speed-up of a factor of 100 in the optical depth calculation when comparing to a grid-based approach. In the suggested calibration we recover luminosities and mean energies within 25%. A comparison of the rates of change of internal energy and electron fraction in the neutrino-driven wind suggests comparable accuracies of ASL and M1, but a higher computational efficiency of the ASL scheme. We estimate that the ratio between the CPU hours spent on the ASL neutrino scheme and those spent on the hydrodynamics is 0.8 per timestep when considering the SPH code MAGMA2 as source code for the Lagrangian hydrodynamics, to be compared with a factor of 10 from the M1 in FLASH.