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

Abstract

Rapidly spinning neutron stars, recycled in low-mass binaries, may have accreted asubstantial amount of mass. The available relativistic measurements ofneutron star masses, all clustering around 1.4 M_solar, however, refermostly to slowly rotating neutron stars that accreted a tiny amountof mass during evolution in a massive binary system. We develop asemianalytical model for studying the evolution of the spin period Pof a magnetic neutron star as a function of the baryonic mass loadM_ac; evolution is followed down to submillisecond periods, and themagnetic field is allowed to decay significantly before the end ofrecycling. We use different equations of state and include rotationaldeformation effects and the presence of a strong gravitational fieldand of a magnetosphere. For the nonmagnetic case, comparison withnumerical relativistic codes shows the accuracy of our description.The minimum accreted mass requested to spin up a magnetized 1.35M_solar neutron star at a few milliseconds is ~0.05 M_solar, whilethis value doubles for an unmagnetized neutron star. Below 1 ms, therequest is for at least ~0.25 M_solar. Only highly nonconservativescenarios for the binary evolution could prevent the transfer of such amass to the compact object. Unless a physical mechanism limits therotational period, there may exist a yet undetected population of massivesubmillisecond neutron stars. The discovery of a submillisecond neutronstar would imply a lower limit for its mass of about 1.7 M_solar.