Strong-field effects in massive scalar-tensor gravity for slowly spinning neutron stars and application to X-ray pulsar pulse profiles

TL;DR

This paper investigates how massive scalar-tensor gravity affects slowly spinning neutron stars, focusing on scalarization effects and their potential to be tested through X-ray pulse profile observations.

Contribution

It provides a detailed analysis of scalarization in massive scalar-tensor theories and applies the results to model X-ray pulse profiles for observational tests.

Findings

Scalarization occurs within specific parameter ranges.

Spacetime modifications influence neutron star geodesics.

Predicted X-ray profiles can be compared with NICER data.

Abstract

Neutron stars (NSs) in scalar-tensor (ST) theories of gravitation can acquire scalar charges and generate distinct spacetimes from those in General Relativity (GR) through the celebrated phenomenon of spontaneous scalarization. Taking on an ST theory with the mass term of the scalar field, we determine the theory parameter space for spontaneous scalarization by investigating the linearized scalar field equation. Then the full numerical solutions for slowly rotating NSs are obtained and studied in great detail. The resulted spacetime is used to calculate test-particle geodesics. The lightlike geodesics are used to construct the profile of X-ray radiation from a pair of hot spots on the surface of scalarized NSs, which potentially can be compared with the data from the Neutron star Interior Composition Explorer (NICER) mission for testing the ST theory.