Signal of Quark Deconfinement in Millisecond Pulsars and Reconfinement in Accreting X-ray Neutron Stars

Theoretically, the phase transition between the confined and deconfined phases of quarks can have a remarkable effect on the spin properties of millisecond pulsars and on the spin distribution of the population of x-ray neutron stars in lowmass binaries. In the latter class of stars, the effect has already been observed—a strong clustering in the population in a narrow band of spins. The observed clustering cannot presently be uniquely assigned to the phase transition as cause. However, there is another possible signal—not so far observed—in millisecond pulsars that we also discuss which would have the same origin, and whose discovery would tend to confirm the interpretation in terms of a phase transition in the stellar core.