Constraining properties of high-density matter in neutron stars with magneto-elastic oscillations

Abstract

We discuss torsional oscillations of highly magnetised neutron stars (magnetars) using two-dimensional, magneto-elastic-hydrodynamical simulations. Our model is able to explain both the low- and high-frequency quasi-periodic oscillations (QPOs) observed in magnetars. The analysis of these oscillations provides constraints on the breakout magnetic-field strength, on the fundamental QPO frequency, and on the frequency of a particularly excited overtone. More importantly, we show how to use this information to generically constraint properties of high-density matter in neutron stars, employing Bayesian analysis. In spite of current uncertainties and computational approximations, our model-dependent Bayesian posterior estimates for SGR 1806-20 yield a magnetic-field strength $\bar{B}\sim 2.1_{-1.0}^{+1.3}\times 10^{15}$G and a crust thickness of $\hat{I}”r=1.6_{-0.6}^{+0.7}$ km, which are both in remarkable agreement with observational and theoretical expectations, respectively (1-$\ddot{I}ƒ$ error bars are indicated). Our posteriors also favour the presence of a superfluid phase in the core, a relatively low stellar compactness, $M/R<0.19$, indicating a relatively stiff equation of state and/or low mass neutron star, and high shear speeds at the base of the crust, $c_s>1.4\times 10^8$cm/s. Although the procedure laid out here still has large uncertainties, these constraints could become tighter when additional observations become available.

14 pages, 8 figures, 6 tables, submitted to MNRAS