# Spin-polarized neutron matter, the maximum mass of neutron stars, and GW170817

## TL;DR

This paper explores how a phase transition to spin-polarized neutron matter influences the maximum mass and structure of neutron stars, aligning theoretical limits with observational data from GW170817.

## Contribution

It introduces the impact of spin-polarized neutron matter phase transition on neutron star properties and constrains maximum mass predictions based on recent gravitational wave observations.

## Key findings

- Phase transition limits neutron star mass to below 2.6-2.9 solar masses.
- Large spin-polarized cores in neutron stars are inconsistent with GW170817 data.
- Constraints depend on microscopic nuclear force models.

## Abstract

We investigate how a phase transition from neutron-star matter to spin-polarized neutron matter affects the equation of state and mass-radius relation of neutron stars. While general extension schemes for the equation of state allow for high pressures inside neutron stars, we find that a phase transition to spin-polarized neutron matter excludes extreme regimes. Hence, such a transition limits the maximum mass of neutron stars to lie below 2.6$-$$2.9 \, M_{\odot}$, depending on the microscopic nuclear forces used, while significantly larger masses could be reached without these constraints. These limits are in good agreement with recent constraints extracted from the neutron-star merger GW170817 and its electromagnetic counterpart. Assuming the description in terms of spin-polarized neutron matter to be valid in the center of neutron stars, we find that stars with a large spin-polarized domain in their core are ruled out by GW170817.