Temperature dependent appearance of exotic matter makes nascent neutron stars spin faster

TL;DR

This paper investigates how temperature-dependent exotic matter, such as hyperons, influences the spin evolution of neutron star merger remnants, revealing potential gravitational-wave signatures of exotic phases.

Contribution

It introduces finite-entropy equations of state to demonstrate temperature-dependent spin-up effects and identifies gravitational-wave frequency shifts as signatures of hyperons in neutron stars.

Findings

Hyperons cause a ~540 Hz frequency shift during spin-up

Without hyperons, the shift is ~360 Hz

Temperature-dependent phase transitions affect gravitational-wave signals

Abstract

Neutron stars offer the opportunity to study the behaviour of matter at densities and temperatures inaccessible to terrestrial experiments. Gravitational-wave observations of binary neutron star coalescences can constrain the neutron-star equation of state before and after merger. After the neutron star binary merges, hyperons can form in the remnant, changing the behaviour of the neutron-star equation of state. In this study, we use finite-entropy equations of state to show that a post-merger remnant can spin up due to cooling. The magnitude of the spin-up depends on the neutron-star equation of state. If hyperons are present, the post-merger spin-up changes the peak gravitational-wave frequency by $\sim 540$ Hz, when the entropy per baryon drops from $s=2$ $k_B$ to $s=0$ $k_B$. If hyperons are not present, the post-merger spin-up changes by $\sim 360$ Hz, providing a gravitational-wave signature for exotic matter. We expect the same qualitative behaviour whenever temperature dependent phase transitions are triggered.