Roton instabilities in the superfluid outer core of neutron stars

TL;DR

This paper investigates the superfluid dynamics in neutron star outer cores, revealing rotonic excitations that lead to instabilities potentially explaining pulsar timing anomalies.

Contribution

It introduces a hydrodynamic model with coupled superfluid and superconductor components, uncovering rotonic structures and instabilities not seen in previous studies.

Findings

Identification of rotonic structures below pair-breaking energy

Discovery of dynamical instabilities triggered by relative superfluid motion

Potential link between instabilities and pulsar timing anomalies

Abstract

We study the superfluid dynamics of the outer core of neutron stars by means of a hydrodynamic model made of a neutronic superfluid and a protonic superconductor, coupled by both the dynamic entrainment and the Skyrme SLy4 nucleon-nucleon interactions. The resulting nonlinear equations of motion are probed in the search for dynamical instabilities triggered by the relative motion of the superfluids that could be related to observed timing anomalies in pulsars. Through linear analysis, the origin and expected growth of the instabilities is explored for varying nuclear-matter density. Differently from previous findings, the dispersion of linear excitations in our model shows rotonic structures below the pair-breaking energy threshold, which lies at the origin of the dynamical instabilities, and could eventually lead to emergent vorticity along with modulations of the superfluid density.