Magnetic field dynamics in neutron stars

• Applicant: Professor Dr. Sebastiano Bernuzzi
• Subject Area: Astrophysics and Astronomy
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 524726453

Neutron stars host some of the strongest magnetic fields in nature and strongly affect its dynamics. Strong magnetic fields are thought to be created at the birth of the star, and lead to high energy electromagnetic and gravitational wave emission. The magnetic field can play a role in phenomena such as gamma ray bursts and fast radio bursts, but can also deform the star, leading to a time varying quadrupole and continuous gravitational wave emission. Furthermore in mature neutron stars the magnetic field configuration affects the emission geometry, and plays a crucial role in attempts to use pulse profile modelling of X-ray pulsars, with instruments such as NICER, to measure the mass and radius of neutron stars, and thus constrain the equation of state of dense matter. The aim of this proposal is to use numerical simulations to understand the magnetic field configuration in neutron star interiors and investigate the role of turbulence in its evolution. Despite its crucial role in many aspects of neutron star physics, the magnetic field topology of a neutron star is still not known. Observationally most constraints on the field are indirect, and are obtained by fitting a magnetic dipole spin-down model to the measured period derivative of the star, with only a handful of models probing the surface field with spectral fits. Theoretically a number of authors have constructed equilibrium models of a magnetized neutron star, but the stability of such configurations is an open question. It has been suggested that no equilibria can exist in barotropic stars, but fluid motions and compositional or thermal stratification are needed to stabilize the configuration. Numerical simulations have provided some insight into the problem, and revealed that while an exact equilibrium does not develop, the ratios of poloidal to toroidal energy in the magnetic field settle down to a stable ratio, allowing to make predictions on the gravitational wave and electromagnetic signals expected from the star. However the simulations show that turbulence develops in the problem and plays a crucial role in the large scale topology of the field, but is currently not fully resolved. This proposal aims to tackle this problem, making use of a multi scale approach. It is in fact numerically intractable to simulate a whole neutron star with the high resolution needed to resolve turbulence on small scales. We thus plan to make use of small scale high resolution simulations of a fluid in a neutron star to calibrate sub-grid methods that allow to include the effect of turbulence in large scale simulations. We will then use these methods in general relativistic magnetohydrodynamics simulations, including also the effects of temperature in the equation of state, to simulate realistic magnetic field configurations in neutron stars, and make realistic predictions for electromagnetic and gravitational wave signal detectability with current and planned gravitational wave detectors.

• DFG Programme: Research Grants
• International Connection: Poland
• Cooperation Partner: Privatdozent Dr. Brynmore Haskell, Ph.D.