Neutron Stars with Submillisecond Periods: A Population of High‐Mass Objects?

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Abstract
Rapidly spinning neutron stars, recycled in low-mass binaries, may have accreted a substantial amount of mass. The available relativistic measurements of neutron star masses, all clustering around 1.4 M☉, however, refer mostly to slowly rotating neutron stars that accreted a tiny amount of mass during evolution in a massive binary system. We develop a semianalytical model for studying the evolution of the spin period P of a magnetic neutron star as a function of the baryonic mass load Mac; evolution is followed down to submillisecond periods, and the magnetic field is allowed to decay significantly before the end of recycling. We use different equations of state and include rotational deformation effects and the presence of a strong gravitational field and of a magnetosphere. For the nonmagnetic case, comparison with numerical relativistic codes shows the accuracy of our description. The minimum accreted mass requested to spin up a magnetized 1.35 M☉ neutron star at a few milliseconds is ~0.05 M☉, while this value doubles for an unmagnetized neutron star. Below 1 ms, the request is for at least ~0.25 M☉. Only highly nonconservative scenarios for the binary evolution could prevent the transfer of such a mass to the compact object. Unless a physical mechanism limits the rotational period, there may exist a yet undetected population of massive submillisecond neutron stars. The discovery of a submillisecond neutron star would imply a lower limit for its mass of about 1.7 M☉.
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