Two radio pulsars in the globular cluster NGC 6624

Abstract

We report the discovery and accurate timing measurements of two single radio pulsars in the globular cluster NGC 6624. PSR1820-30A is a 5-ms pulsar that is located close to the low-mass X-ray binary 4U 1820-30. It seems that the recently reported radio detection of 4U 1820-30 with the VLA is, in fact, that of the pulsar. The observed period and period-derivative of PSR 1820-30A make it disturbingly young, and also conflict with the accretion spin-up theory unless acceleration in the cluster potential is largely responsible for the observed period-derivative. This requires that the cluster core lies much closer to the pulsar than has been believed hitherto. Such an explanation may also explain the anomalous orbital period-derivative of 4U 1820-30. PSR 1820-30B has a pulse period of 378 ms, which is unusually long for a pulsar associated with a globular cluster, and also has an unusually large inferred magnetic field. If the neutron star is primordial, then this suggests that its magnetic field has decreased only slightly, if at all, since its formation.