Axisymmetric equilibrium models for magnetised neutron stars in Scalar-Tensor Theories

TL;DR

This paper develops models of magnetised neutron stars within Scalar-Tensor Theories, exploring how magnetic fields and scalarisation interact and influence stellar properties like deformability and maximum mass.

Contribution

It extends the formalism for magnetised neutron stars in General Relativity to Scalar-Tensor Theories, analyzing the interplay between magnetic fields and scalarisation effects.

Findings

Magnetic and scalar fields mutually influence neutron star properties.

Scalarisation affects maximum mass and magnetic deformability.

Magnetic field geometry impacts scalarisation behavior.

Abstract

Among the possible extensions of General Relativity that have been put forward in order to address some long standing issues in our understanding of the Universe, Scalar-Tensor Theories have received a lot of attention for their simplicity. Interestingly, some of these predict a potentially observable non-linear phenomenon, known as \textit{spontaneous scalarisation}, in the presence of highly compact matter distributions, like the case of neutron stars. Neutron stars are ideal laboratories to investigate the properties of matter under extreme conditions, and in particular they are known to harbour the strongest magnetic fields in the Universe. Here, for the first time, we present a detailed study of magnetised neutron stars in Scalar-Tensor Theories. First, we show that the formalism developed for the study of magnetised neutron stars in General Relativity, based on the \textit{eXtended Conformally Flat Condition}, can easily be extended in the presence of a non-minimally coupled scalar field, retaining many of its numerical advantages. We then carry out a study of the parameter space considering the two extreme geometries of purely toroidal and purely poloidal magnetic fields, varying both the strength of the magnetic field and the intensity of scalarisation. We compare our results with magnetised general-relativistic solutions and un-magnetised scalarised solutions, showing how the mutual interplay between magnetic and scalar fields affect the magnetic and the scalarisation properties of neutron stars. In particular, we focus our discussion on magnetic deformability, maximum mass and range of scalarisation.