Old Pulsars in the Low-density Globular

Millisecond pulsars are conventionally assumed to be spun up through the action of binary companions, although some subsequently lose their companions and appear as isolated pulsars. Such objects should therefore be more numerous in dense stellar systems. We report here the surprising discovery of two pulsars in low-density globular clusters: one is a single 10-ms pulsar (1639+36) in M13 (NGC6205), the other a 33-ms pulsar (1310+18) in a 256-d binary in M53 (NGC5024). Their ages, inferred from their luminosities and constraints on their period derivatives, seem to be 109 years, signi cantly greater than previously reported ages ( < 10 8 years) of cluster pulsars (Phinney and Kulkarni 1991). The implied birth rate is inconsistent with the conventional two-body tidal capture model (Verbunt 1988; Bailyn and Grindlay 1990), suggesting that an alternative mechanism such as tidal capture between primordial binaries and a reservoir of (hundreds of) primordial neutron stars may dominate the production of tidal binaries in such clusters (Phinney and Kulkarni 1991; Phinney and Sigurdsson 1991). The period derivative of PSR1639+36 is surprisingly small, and may be corrupted by acceleration due to the mean gravitational potential of the cluster (Phinney 1991).