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 twodimensional, 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 strengthB ∼ 2.1 +1.3 −1.0 × 10 15 G and a crust thickness of ∆r = 1.6 +0.7 −0.6 km, which are both in remarkable agreement with observational and theoretical expectations, respectively (1-σ 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 × 10 8 cm/s. Although the procedure laid out here still has large uncertainties, these constraints could become tighter when additional observations become available.

Breaking properties of neutron star crust

“…Crystalline properties of NS crust are invoked to explain a number of observed astrophysical phenomena. For instance, crustal elasticity is thought to be important for interpretation of quasi-periodic oscillations observed in magnetars (e.g., Gabler, Cerdá-Durán, Stergioulas, Font & Müller 2018). Solid crust can support asymmetric distributions of density, so-called, mountains, which can produce gravitational wave (GW) emission (Ushomirsky, Cutler & Bildsten 2000;Horowitz 2010;Johnson-McDaniel & Owen 2013).…”

The gravitational Higgs mechanism and resulting smoking gun effects

“…If predictions are made based solely on observations of the external field in the interior of, especially, a strongly magnetized neutron star (magnetar) there would be significant deviations which will affect the interpretation of the quasi-periodic oscillation (QPO) spectra [29,30] of both global magneto-elastic [31][32][33] and/or localized crust oscillations [34] associated with the geometry and dynamics of the magnetic field. During the last two decades, the modelling of the observed QPOs led to significant progress in associating the QPOs with the equation of state (EoS) and the strength and geometry of the magnetic field.…”

Breaking stress of Coulomb crystals in the neutron star crust

“…When a neutron star evolves, its crust undergoes various deformations (e.g., [12,4,13,14,15]). For magnetars, these deformations are primarily associated with the magnetic field (e.g., [16,18,19]); for pulsars they are thought to be connected with glitches (e.g., [20]).…”