Effect of Magnetic Fields on Urca Rates in Neutron Star Mergers

TL;DR

This paper investigates how strong magnetic fields affect Urca process rates in neutron star environments, revealing significant impacts at lower temperatures and near the direct Urca threshold, with implications for neutron star physics.

Contribution

It provides a detailed analysis of temperature and magnetic field effects on Urca rates using full rate integrals for various equations of state, highlighting the magnetic field's role near the direct Urca threshold.

Findings

Magnetic fields significantly influence Urca rates at a few MeV temperatures.

Magnetic fields soften the direct Urca threshold.

Neutron decay rates are more affected by magnetic fields than electron capture rates.

Abstract

Isospin-equilibrating weak processes, called ``Urca" processes, are of fundamental importance in astrophysical environments like (proto-)neutron stars, neutron star mergers, and supernovae. In these environments, matter can reach high temperatures of tens of MeVs and be subject to large magnetic fields. We thus investigate Urca rates at different temperatures and field strengths by performing the full temperature and magnetic-field dependent rate integrals for different equations of state. We find that the magnetic fields play an important role at temperatures of a few MeV, especially close to or below the direct Urca threshold, which is softened by the magnetic field. At higher temperatures, the effect of the magnetic fields can be overshadowed by the thermal effects. We observe that the magnetic field more strongly influences the neutron decay rates than the electron capture rates, leading to a shift in the flavor equilibrium.