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

General relativity (GR) 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 GR and account for such issues. Among the most promising ones are scalar-tensor theories (STTs), which predict an enrichment of the phenomenology of compact objects, like neutron stars (NSs). We updated the well-tested XNS code to numerically solve the Einstein-Maxwell equations for a stationary, magnetised NS in a class of STTs containing the spontaneous scalarisation phenomenon. We found that there exist “quasi-universal relations” among the mass, radius, scalar charge and magnetic deformation of a NS that are true independently of the equation of state (EoS), both in GR and in STTs. This result could potentially provide new tools to test STTs and the magnetic field geometry inside NSs.