Old pulsars in the low-density globular clusters M13 and M53

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-day binary in M53 (NGC5025). Their ages, inferred from their luminosities and constraints on their period derivatives, seem to be ∼10⁹ years, significantly greater than previously reported ages (≲10⁸ years) of cluster pulsars¹. The implied birth rate is inconsistent with the conventional two-body tidal capture model ^2,3, 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^1,4. The period derivative of PSR1639 + 36 is surprisingly small, and may be corrupted by acceleration due to the mean gravitational potential of the cluster⁵.