Scalarized neutron stars in massive scalar-tensor gravity: X-ray pulsars and tidal deformability

TL;DR

This paper investigates scalarized neutron stars within massive scalar-tensor gravity theories, analyzing their X-ray pulse profiles and tidal deformability, and establishing universal relations to aid observational constraints.

Contribution

It compares different scalar-tensor theories with massive scalars, computes neutron star properties, and introduces universal relations relevant for observational tests.

Findings

Universal relation between moment of inertia and tidal deformability.

Massive scalar fields significantly affect neutron star scalarization.

X-ray pulse profiles and tidal deformability can constrain scalar-tensor theories.

Abstract

Neutron stars (NSs) in scalar-tensor theories of gravitation with the phenomenon of spontaneous scalarization can develop significant deviations from general relativity. Cases with a massless scalar were studied widely. Here we compare the NS scalarizations in the Damour--Esposito-Far{\`e}se theory, the Mendes-Ortiz theory, and the $\xi$-theory with a massive scalar field. Numerical solutions for slowly rotating NSs are obtained. They are used to construct the X-ray pulse profiles of a pair of extended hot spots on the surface of NSs. We also calculate the tidal deformability for NSs with spontaneous scalarization which is done for the first time with a massive scalar field. We show the universal relation between the moment of inertia and the tidal deformability. The X-ray pulse profiles, the tidal deformability, and the universal relation may help to constrain the massive scalar-tensor theories in X-ray and gravitational-wave observations of NSs, including the Neutron star Interior Composition Explorer (NICER) satellite, Square Kilometre Array (SKA) telescope, and LIGO/Virgo/KAGRA laser interferometers.