Magnetic deformation of neutron stars in scalartensor theories
Abstract
Scalartensor theories are among the most promising alternatives to general relativity that have been developed to account for some longstanding issues in our understanding of gravity. Some of these theories predict the existence of a nonlinear phenomenon that is spontaneous scalarisation, which can lead to the appearance of sizable 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 that are due to both variations in the quadrupolar deformation of the star and 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  in this case due to a magnetic quadrupolar deformation. In this work, we investigate 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 show that it is possible to provide simple parametrisations of the magnetic deformation and gravitational wave power of neutron stars in terms of their baryonic mass, circumferential radius, and scalar charge, while also demonstrating that a universal scaling exists independently of the magnetic field geometry and of the parameters of the scalartensor theory. Finally, we comment on the observability of the deviations in the strain of gravitational waves from general relativity by current and future observatories.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 January 2021
 DOI:
 10.1051/00046361/202038826
 arXiv:
 arXiv:2010.14833
 Bibcode:
 2021A&A...645A..39S
 Keywords:

 gravitation;
 stars: magnetic field;
 stars: neutron;
 gravitational waves;
 magnetohydrodynamics (MHD);
 relativistic processes;
 Astrophysics  High Energy Astrophysical Phenomena;
 General Relativity and Quantum Cosmology
 EPrint:
 9 pages, 2 figures, accepted for publication in A&