Phase transition and spin clustering of neutron stars in X-ray binaries

The spins of X-ray neutron star accretors in low-mass binaries are found to cluster at about 300 Hz with the exception of a few higher frequency objects. We find that a postulated phase transition induced by the centrifugally driven dilution in the density profile of the star can produce a similar feature. It takes from 10(7) to 10(9) yr, depending on the mass accretion rate, to expel the high-density phase from the core. The corresponding growth in the moment of inertia retards spin-up during this epoch. Normal mass accretion-driven spin-up resumes at its completion. A phase change triggered by the changing spin and the accompanying evolution of the moment of inertia has its analog in rotating nuclei as was discovered in the 1970s.