Neutrino opacities in magnetic fields for binary neutron star merger simulations

TL;DR

This work develops approximate, rapid neutrino opacities and emissivities for sub-nuclear matter in binary neutron star mergers under strong magnetic fields, incorporating Landau quantization and nucleon anomalous magnetic moments. It provides analytic forms for charged- and neutral-current interactions, covering both non-degenerate and degenerate nucleon regimes, including finite-temperature and weak-magnetism corrections, with detailed validation against full integrals. The results reveal pronounced low-energy enhancements in charged-current opacities and strong anisotropies in neutral-current scattering, especially for electrons, indicating magnetic fields can significantly alter neutrino transport and fluxes in merger ejecta. While synchrotron neutrino emission from spin-flip processes exists, it generally remains subdominant to Urca processes for the densities and fields relevant to mergers, guiding future transport simulations to incorporate these anisotropic magnetized opacities for more accurate nucleosynthesis predictions.

Abstract

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 $\sqrt{T/M}$. We also comment on a neutrino production channel from individual neutrons that can produce low-energy $ν\barν$ pairs even at low density.

Figures (8)

• Figure 1: Charged current opacities for a range of temperatures, magnetic field strengths, and neutrino energies. Thin lines show opacity for capture on neutrons while thick lines show opacity for capture on protons. The (anti)neutrino momentum is chosen perpendicular to the magnetic field. For discussion of anisotropies in opacities, see Sec. \ref{['sec:anisotropy']}.
• Figure 2: Neutral current opacities for a range of temperatures, magnetic field strengths, and neutrino energies. Thin lines show opacity for scattering on neutrons while thick lines show opacity for scattering on protons. The momentum transfer is chosen perpendicular to the magnetic field. For discussion of anisotropies in opacities, see Sec. \ref{['sec:anisotropy']}.
• Figure 3: Proton scattering kernel at $n_B = 0.1 \, n_{\rm sat}$, $y_p = 0.2$, $T = 20 \, \mathrm{MeV}$, and a variety of directions of momentum transfer with fixed overall magnitude of momentum transfer $q = 15 \, \mathrm{MeV}$ for elastic scattering of $10 \, \mathrm{MeV}$ neutrinos.
• Figure 4: Total opacities as a function of incoming neutrino direction as compared to the magnetic field axis. Opacities are normalized to the value when $k_\nu$ is perpendicular to the magnetic field. Neutral current opacities for nucleons are not shown as they are nearly isotropic.
• Figure 5: Emissivity as a function of density for Urca and synchrotron neutrino emission. Temperature and proton fraction are fixed at $T = 20 \, \mathrm{MeV}$ and $y_p = 0.2$.