Neutron stars are a very particular class of stars: they are the most compact stars and they posses the strongest magnetic fields ever observed. They are the final state of the evolution of massive normal stars, i.e. once the fusion process ceases in the centre of such a star, it explodes as a supernova and leaves a neutron star as a compact remnant. But despite this fact, these stars cannot be considered to be “dead”. Instead, they show high activity related to their strong magnetic fields: emission of the most stable electromagnetic pulses (radio pulsars) and repeated flares in X- and gamma-rays (soft gamma-ray repeater/magnetars). Therefore, they are of major interest for astrophysicists.

While the structure of the solid crust of neutron stars is well constrained from terrestrial experiments, little is known about the state of the matter in their interiors. Various theories predict a different behavior of the core matter. Because neutron stars are held together by very strong gravity their interior is so dense that these conditions cannot be reproduced in any laboratory on Earth. Therefore, observations of neutron stars provide the only opportunity to help us understand the complex nuclear physics of very dense matter, and in particular the interaction of fundamental particles under these conditions.

"Starquakes" of neutron stars with extremely strong magnetic fields (magnetars) may shed light into their exotic interior structure ( see Highlight). In past years, two giant flares were detected in the so-called soft gamma-ray repeaters SGR 1806-20 (2004) and SGR1900+14 (1998). During these events the emitted gamma-ray emission was modulated at different frequencies. Some of the lower oscillation frequencies discovered approximately match the frequencies of magneto-elastic oscillations of magnetars. These pulsations of neutron stars arise from the interaction of the magnetic field in the core with elastic shear oscillations in the solid crust that are similar to earthquakes. However, in this magneto-elastic model the observed high frequencies could not be explained.

With recent numerical simulations, carried out by a collaboration between researchers from the MPA, the University of Valencia and the University of Thessaloniki, it is now possible to identify both low and high frequencies consistently as magneto-elastic oscillations or "starquakes". The crucial ingredient to match all observations at once is another exotic property of the core matter: superfluidity. In this state, there exists no viscosity in the fluid and it has infinite thermal conductivity. On Earth, superfluidity can only be observed at extremely low temperatures, and for just a few elements such as liquid helium.

If one includes superfluid effects in recent neutron star models, only a fraction of the matter (mainly the non-superfluid protons and electrons) is participating in the magneto-elastic oscillations that need to be matched to the observed frequencies. This decoupling leads to a better agreement of our estimates of the magnetic field strength of magnetars with alternative estimates. Moreover, a new family of oscillations appears that is crucial for a complete interpretation of the observed frequencies: a high-frequency shear mode in the crust (that was damped in previous models without superfluidity) resonates with a high overtone of the Alfvén oscillations in the core. (Alfvén oscillations are magnetohydrodynamic waves that are caused by the magnetic field acting as restoring force.) Additionally, superfluid magneto-elastic oscillations should live longer than the oscillations described by previous models, which is important for their detectability.

In future studies the new model can be used to further constrain the state of the matter in neutron stars in general and its superfluid properties in particular.

M. Gabler (MPA, Valencia), E. Müller (MPA), P.Cerdá-Durán (Valencia), T. Font (Valencia) and N. Stergioulas (Thessaloniki)

Original Publication

Michael Gabler, Pablo Cerdá-Durán, Nikolaos Stergioulas, José A. Font, and Ewald Müller, "Imprints of Superfluidity on Magnetoelastic Quasiperiodic Oscillations of Soft Gamma-Ray Repeaters", Phys. Rev. Lett. 111, 211102 (2013) http://de.arxiv.org/abs/1304.3566