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A spinning top that wobbles as it rotates: that's roughly how astronomers picture the neutron star in the binary system Hercules X-1. It completes one full turn on its axis every second, but its axis slowly traces a cone with a period of 35 days. Recently, the IXPE space observatory measured how the polarization of the star's X-ray emission (simply put, the direction in which the light waves oscillate) changes over this cycle. It turns out that the polarization rotates in sync with the 35-day rhythm, which is a strong hint of a wobbling star.

If this is indeed free precession, then something strange is happening inside the star. A neutron star is not just a hot ball. Its crust is a rigid crystal lattice of atomic nuclei, while inside flows a superfluid threaded with microscopic vortices. Normally, these vortices pin to the lattice nuclei, and when stress builds up, they break free — causing sudden spin glitches. But for the star to wobble for decades, the vortices must remain free and experience almost no friction against the lattice. Such weak pinning seems almost unbelievable, like water in a glass that doesn't notice the walls.

The researchers conclude: if precession really exists, the star's equilibrium is maintained by a delicate balance between internal forces pushing on the axis and the external braking torque from matter falling from the companion star. This opens a new window into the physics of superfluid interiors of neutron stars — a realm where quantum effects manifest on the scale of an entire celestial body.