Low-mass X-ray binaries and millisecond pulsars in globular clusters

Abstract

Globular clusters harbor many millisecond pulsars and 10 bright low-mass X-ray binary systems. It has been generally believed that almost all millisecond pulsars are spun up by accretion from a companion star during a (progenitor) low-mass X-ray binary (LMXB) phase. However, most of the short-period millisecond pulsars are found in the apparently younger globular clusters, which have much lower core densities than those characteristic of the apparently older globulars containing the majority of the LMXBs. The vast majority of the globular LMXBs are X-ray burst sources, and among them there are no bright sources similar to those in the Galactic disk, such as Sco X-1, Cyg X-2, and GX 5-1. Furthermore, the radio pulse properties of the very short period millisecond pulsars in globular clusters differ from those of pulsars spun up in the Galactic disk, such as PSR 1937 + 21 and PSR 1957 + 20. Only the properties of the latter class are consistent with what is expected for the spin-up scenario. We argue that, in most globular cluster LMXBs, the neutron stars were formed spinning rapidly, and are likely to be the descendants, rather than the progenitors, of weakly magnetized millisecond pulsars.