Stratification, superfluidity and magnetar QPOs

November 2012

Andrea Passamonti, Sam Lander

Magnetars are slowly rotating neutron stars with a strong magnetic field. Three of these objects have been seen to produce enormously energetic ‘giant flares’. Quasi-periodic-oscillation (QPOs) were detected in the flare’s decaying X-ray tail, between 100 and 400 s after the initial burst, and a number of frequencies were identified within each tail, of varying duration and intensity. The quasiperiodic oscillations of magnetars provide an exciting possibility of probing the interior physics of neutron stars, as they are thought to relate directly to oscillation modes of the underlying star. We have studied the time evolution of non-axisymmetric oscillations of purely two-fluid stars with superfluid neutrons and normal (not superconducting) protons. We have considered various magnetic field geometries, proton fractions, entrainment and composition stratifications and analysed their impact on the Alfvén mode spectrum. We find that a ‘typical’ multifluid magnetar could have QPO frequencies which are roughly a factor of 5 or 6 higher than expected from a single-fluid unstratified model (see equation).

MNRAS 429, 767 (2013) arXiv:1210.2969v1