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

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 rmode 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 β-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 (ρ 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.

Extreme matter meets extreme gravity: Ultraheavy neutron stars with phase transitions

“…Another reason for the argument presented above is based on the electromagnetic emission coincident with the GW170817 event, which suggests a maximum mass of ∼ 2.3M [12,24,25]. This conclusion, however, hinges on details of numerical relativity simulations that currently do not contain all of the physics that may be relevant in binary neutron star mergers (like viscosity, neutrino transport, and precise knowledge of the EoS) [26][27][28].…”