A dead star's internal liquid has been spinning freely for centuries, contradicting everything we thought we knew about how neutron stars work.
Neutron stars are the ultra-dense cores of exploded suns, filled with a strange superfluid that usually pins itself to the star's crust. Observations of the star Her X-1 show it wobbling in a way that is only possible if that liquid is completely unpinned and moving freely. This should not happen, as the friction between the liquid and the crust is supposed to stop that wobble very quickly. It suggests that our standard models of starquakes and internal neutron star friction are fundamentally wrong. We are looking at a state of matter that is more slippery and persistent than any theory predicted. This wobbling star is telling us that our understanding of density and friction at its limits is incomplete.
Theoretical Constraints on Neutron Star Superfluidity from Her X-1 Precession
arXiv · 2605.04686
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 w