Magnetic deformation of neutron stars in scalar-tensor theories

TL;DR

This paper explores how spontaneous scalarisation in scalar-tensor theories influences the magnetic deformation of neutron stars and their gravitational wave emissions, providing parameterizations and discussing observational prospects.

Contribution

It introduces simple parameterizations of magnetic deformation and gravitational wave power in scalar-tensor theories, highlighting universal scaling laws independent of magnetic field geometry.

Findings

Scalarisation affects neutron star magnetic deformation and gravitational wave emission.

Universal scaling laws relate deformation and wave power to star parameters.

Potential observability of deviations from general relativity in gravitational wave signals.

Abstract

Scalar-tensor theories are among the most promising alternatives to general relativity that have been developed to account for some long standing issues in our understanding of gravity. Some of these theories predict the existence of a non-linear phenomenon, spontaneous scalarisation, which can lead to the appearance of sizeable modifications to general relativity in the presence of compact matter distributions, namely neutron stars. On the one hand, one of the effects of the scalar field is to modify the emission of gravitational waves, both due to variations in the quadrupolar deformation of the star and to the presence of additional modes of emission. On the other hand, neutron stars are known to harbour extremely powerful magnetic fields which can affect their structure and shape, leading in turn to the emission of gravitational waves, this time due to a magnetic quadrupolar deformation. We investigate here how the presence of spontaneous scalarisation can affect the magnetic deformation of neutron stars and their emission of quadrupolar gravitational waves, both of tensor and scalar nature. We will show that it is possible to provide simple parameterisations of the magnetic deformation and gravitational wave power of neutron stars in terms of their baryonic mass, circumferential radius and scalar charge, and that a universal scaling exists independently of the magnetic field geometry and of the parameters of the scalar-tensor theory. Finally, we comment on the observability of the deviations of the gravitational waves strain from general relativity by current and future observatories.