1303.3270 (Alex T. Deibel et al.)

Mass And Radius Constraints Using Magnetar Giant Flare Oscillations    [PDF]

Alex T. Deibel, Andrew W. Steiner, Edward F. Brown

We study crustal oscillations in magnetars including corrections for a finite Alfv\'en velocity. Our crust model uses a new nuclear mass formula that predicts nuclear masses with an accuracy very close to that of the Finite Range Droplet Model. This mass model for equilibrium nuclei also includes recent developments in the nuclear physics, in particular, shell corrections and an updated neutron-drip line. We perturb our crust model to predict axial crust modes and assign them to observed giant flare quasi-periodic oscillation (QPO) frequencies from SGR 1806-20. The QPOs associated with the fundamental and first harmonic can be used to constrain magnetar masses and radii. We use these modes and the phenomenological equations of state from Steiner et al. to find a magnetar crust which reproduces observations of SGR 1806-20. We find magnetar crusts that match observations for various magnetic field strengths, entrainment of the free neutron gas in the inner crust, and crust-core transition densities. Matching observations with a field-free model we obtain the approximate values of M =1.35 Msun and R = 11.9 km. Matching observations using a model with the surface dipole field of SGR 1806-20 (B=2.4x10^15 G) we obtain the approximate values of M = 1.25 Msun and R = 12.4 km. Without significant entrainment of the free neutron gas the magnetar requires a larger mass and radius to reproduce observations. If the crust-core transition occurs at a lower density the magnetar requires a lower mass and a larger radius to reproduce observations.

View original: http://arxiv.org/abs/1303.3270