Axisymmetric equilibrium models of magnetised neutron stars in scalar-tensor theories

TL;DR

This paper extends numerical models of magnetised neutron stars to scalar-tensor theories, revealing universal relations that could help test alternative gravity theories and understand neutron star magnetic fields.

Contribution

It updates the XNS code to include scalar-tensor theories with spontaneous scalarisation, uncovering quasi-universal relations in neutron star properties.

Findings

Existence of quasi-universal relations among neutron star properties

These relations are independent of the equation of state

Potential for new tests of scalar-tensor theories

Abstract

General relativity probably is not the definitive theory of gravity, due a number or issues, both from the theoretical and from the observational point of view. Alternative theories of gravity were conceived to extend general relativity and account for such issues. Among the most promising ones are scalar-tensor theories, which predict an enrichment of the phenomenology of compact objects, like neutron stars. We updated the well-tested XNS code to numerically solve the Einstein-Maxwell equations for a stationary, magnetised neutron star in a class of scalar-tensor theories containing the spontaneous scalarisation phenomenon. We found that there exist "quasi-universal relations" among the mass, radius, scalar charge and magnetic deformation of a neutron star that are true independently of the equation of state, both in general relativity and in scalar-tensor theories. This result could potentially provide new tools to test scalar-tensor theories and the magnetic field geometry inside neutron stars.