Constraints from GW170817 on the bulk viscosity of neutron star matter and the r-mode instability

TL;DR

This paper investigates how the bulk viscosity and r-mode instability in neutron stars depend on the symmetry energy slope parameter L, using equations of state constrained by GW170817 data, revealing the impact of direct Urca processes on neutron star spin-down.

Contribution

It provides a systematic analysis of the influence of the symmetry energy slope parameter L on r-mode phenomenology, incorporating constraints from GW170817 and detailed equations of state.

Findings

Direct Urca processes significantly affect bulk viscosity and r-mode stability.

The L parameter influences the instability boundary and spin-down properties.

GW170817 data constrains the range of L relevant for neutron star models.

Abstract

We perform a systematic study of the dependence of the r-mode phenomenology in normal fluid pulsar neutron stars on the symmetry energy slope parameter $L$. An essential ingredient in this study is the bulk viscosity, which is evaluated consistently for several equations of state corresponding to different values of the slope parameter $L$. Direct Urca processes, which are allowed from a critical $L$-value onwards, enhance the bulk viscosity and have large influence on the $r$-mode features, such as the instability boundary and spin-down properties of newborn neutron stars. The magnitude of the changes in the $r$-mode properties induced by the direct Urca processes are driven by the $L$-value of the equation of state and the mass of the pulsar. The study has been done by using a family of equations of state of $\beta$-equilibrated neutron star matter obtained with the finite range simple effective interaction, which provides realistic results for nuclear matter and finite nuclei properties. These equations of state predict the same properties in symmetric nuclear matter and have the same value of the symmetry energy parameter, $E_s(\rho_0)$, but differ in the slope parameter $L$. The range chosen for the variation of $L$ is decided from the tidal deformability data extracted from the GW170817 event and the maximum mass constraint.