Spin-Down of Radio Millisecond Pulsars at Genesis
Probing Pulsar Rotation Pulsars are strongly magnetized, rapidly rotating neutron stars. Those with periods of the order of milliseconds obtain their fast spin by accreting mass from a companion star in a binary system. Tauris (p. 561) combined numerical stellar evolution calculations with a model of how accretion torques act on a neutron star. During the late accretion phase of millisecond pulsars, the termination stage of mass transfer results in a loss of more than 50% of their rotational energy. This effect may explain the observed pulsar spin distributions. Numerical calculations show that processes responsible for spinning up millisecond pulsars may also lead them to slow down. Millisecond pulsars are old neutron stars that have been spun up to high rotational frequencies via accretion of mass from a binary companion star. An important issue for understanding the physics of the early spin evolution of millisecond pulsars is the impact of the expanding magnetosphere during the terminal stages of the mass-transfer process. Here, I report binary stellar evolution calculations that show that the braking torque acting on a neutron star, when the companion star decouples from its Roche lobe, is able to dissipate >50% of the rotational energy of the pulsar. This effect may explain the apparent difference in observed spin distributions between x-ray and radio millisecond pulsars and help account for the noticeable age discrepancy with their young white dwarf companions.