On the universality of I-Love-Q relations in magnetized neutron stars

Abstract: Recently, general relations among the quadrupole moment (Q), the moment of inertia (I), and the tidal deformability (Love number) of a neutron star were shown to exist. They are nearly independent of the nuclear matter equation of state and would be of great aid in extracting parameters from observed gravitational-waves and in testing general relativity. These relations, however, do not account for strong magnetic fields. We consider this problem by studying the effect of a strong magnetic field on slowly rotating relativistic neutron stars and show that, for simple magnetic field configurations that are purely poloidal or purely toroidal, the relation between Q and I is again nearly universal. However, different magnetic field geometries lead to different I-Q relations, and, in the case of a more realistic twisted-torus magnetic field configuration, the relation depends significantly on the equation of state, losing its universality. I-Love-Q relations must thus be used with very great care, since universality is lost for stars with long spin periods, i.e. P > 10 s, and strong magnetic fields, i.e. B > 10^12 G.