Transport in neutron star mergers

TL;DR

This paper investigates transport phenomena in neutron star mergers, focusing on beta equilibrium, bulk viscosity, and axion-induced cooling, revealing their potential impact on merger dynamics and the importance of including these effects in simulations.

Contribution

It provides new calculations of beta equilibrium conditions, bulk viscosity effects, and axion cooling in neutron star mergers, highlighting their significance for accurate modeling.

Findings

Modified beta equilibrium condition with an additional chemical potential.

Bulk viscosity can damp oscillations on ~10 ms timescales.

Axions can escape and cool merger matter within current coupling constraints.

Abstract

After an introduction to the QCD phase diagram, the nuclear equations of state, and neutron star mergers, I discuss three projects related to transport and nuclear matter in neutron star mergers. The first is the nature of beta equilibrium in the portion of a merger that is transparent to neutrinos. We calculate the weak interaction (Urca) rates and find that the beta equilibrium condition needs to be modified by adding an additional chemical potential, which changes slightly the particle content in neutrino-transparent beta equilibrium. Secondly, we calculate the bulk viscosity in neutrino-transparent nuclear matter in conditions encountered in neutron star mergers. Bulk viscosity arises from a phase lag between the pressure and density in the nuclear matter, which is due to the finite rate of beta equilibration. When bulk viscosity is sufficiently strong, which happens when the equilibration rate nearly matches the frequency of the density oscillation, it can noticeably dampen the oscillation. We find that in certain thermodynamic conditions likely encountered in mergers, oscillations in nuclear matter can be damped on timescales on the order of 10 milliseconds, so we conclude that bulk viscosity should be included in merger simulations. Finally, we study thermal transport due to axions in neutron star mergers. We conclude that axions are never trapped in mergers, but instead escape, carrying energy away from the merger. We calculate the cooling time due to the energy carried away by axions and find that within current constraints on the axion-nucleon coupling, axions could cool fluid elements in mergers on timescales which could affect the dynamics of the merger.