Formation of isolated millisecond pulsars in globular clusters

Abstract

After previous searches for radio pulsars in globular clusters^1,2, a radio pulsar with a period of 3.054 millisecond has now been discovered in NGC6626 (M28)³. The upper limit of 3×10^–17 to the observed period derivative indicates that the pulsar is not in a binary³. Millisecond pulsars are probably 'recycled' old pulsars which have been spun-up through accretion from a binary stellar companion^4,5. In a globular cluster such a binary is likely to have formed in a tidally dissipative collision between an old neutron star and a low-mass field star from the cluster. Eventually, a mass-transfer phase ensues and the accretion of only ∼0.1M_⊙ from the companion is sufficient to spin up the neutron star to millisecond periods. Later in the history of the binary, it can undergo a catastrophic collision with a passing field star from the cluster, the net result of which may be a neutron star with a massive disk orbiting it. After much of the material is expelled via accretion-generated radiation pressure, only an isolated millisecond radio pulsar remains. We also explore the possibility that accretion from a massive disk is able directly to spin up the neutron star; in this case, however, the predicted number of millisecond pulsars in globular clusters would be ten times the number of bright low-mass X-ray binaries. In the model where the neutron star is spun up in a binary the predicted number of millisecond pulsars is significantly smaller. If the millisecond pulsar in NGC6626 has a magnetic moment similar to that of the other three known millisecond pulsars, the epoch of spin-up must have terminated ≲8×10⁸ yr ago.