Anisotropic neutron star crust, solar system mountains, and gravitational waves

TL;DR

This paper explores how anisotropic crusts in neutron stars could lead to observable gravitational waves and explains their spin-down behavior, drawing analogies with surface features of solar system bodies.

Contribution

It introduces the idea that anisotropic neutron star crusts can produce ellipticity and unique braking indices, offering new insights into neutron star spin evolution.

Findings

Anisotropic crusts can cause ellipticity in neutron stars.

The braking index may differ significantly from 5 due to crust anisotropy.

This mechanism could explain maximum observed neutron star spins.

Abstract

"Mountains" or non-axisymmetric deformations of rotating neutron stars (NS) efficiently radiate gravitational waves (GW). We consider analogies between NS mountains and surface features of solar system bodies. Both NS and moons such as Europa or Enceladus have thin crusts over deep oceans while Mercury has a thin crust over a large metallic core. Thin sheets may wrinkle in universal ways. Europa has linear features, Enceladus has "Tiger" stripes, and Mercury has lobate scarps. NS may have analogous features. The innermost inner core of the Earth is anisotropic with a shear modulus that depends on direction. If NS crust material is also anisotropic this will produce an ellipticity, when the crust is stressed, that grows with spin frequency. This yields a braking index (log derivative of spin down rate assuming only GW spin down) very different from $n=5$ and could explain the maximum spin observed for neutron stars and a possible minimum ellipticity of millisecond pulsars.