Neutrino opacities in magnetic fields for binary neutron star merger simulations

Neutrino interactions play a central role in transport and flavor evolution in the ejecta of binary neutron star mergers. Simulations suggest that neutron star mergers may produce magnetic fields as strong as ${10}^{17}$ G, but computational difficulties have hampered the inclusion of magnetic field effects in neutrino interaction rates. In this paper we give approximate interaction rates for neutrinos in the presence of strong magnetic fields, including the effects of Landau quantization and anomalous magnetic moments with errors of order $\sqrt{T\mathrm{/}M}$. We also comment on a neutrino production channel from individual neutrons that can produce low-energy $\nu \bar{\nu }$ pairs even at low density.