Theoretical Constraints on Neutron Star Superfluidity from Her X-1 Precession

TL;DR

This paper explores the implications of neutron star precession observed in Her X-1 for the superfluid dynamics within the star's crust, challenging existing models of vortex pinning and mutual friction.

Contribution

It provides theoretical constraints on neutron star superfluidity based on observational evidence of precession, highlighting the need for weak vortex pinning over decades.

Findings

Precession over 50 years requires unpinned vortices.

Weak mutual friction is necessary for sustained precession.

Standard glitch models may not apply under these conditions.

Abstract

Recent IXPE observations of Her X-1 reveal correlations between flux, polarization degree, and polarization angle across its 35-day superorbital cycle. These measurements have been interpreted as strong evidence that the 35-day period is driven by nearly free precession of the neutron star. We show that this interpretation carries far-reaching implications for the dynamics of the crustal superfluid. In particular, maintaining precession over the $\sim 50$-year observational baseline of Her X-1 would require that superfluid vortices remain unpinned for centuries and experience extremely weak mutual friction while traversing the heavy-ion lattice of the inner crust -- conditions that challenge conventional wisdom and standard models of glitch dynamics. Under the condition of weak pinning, nearly free precession of the crust may be sustained by a balance between the internal and external torques.