Searching for radio pulsars in old open clusters from the Parkes archive

S. B. Zhang, J. J. Wei, X. Yang, S. Dai, J. S. Wang, L. Toomey, S. Q. Wang, G. Hobbs, X. F. Wu, L. Staveley-Smith

Published 2025-06-24Version 1

Motivated by the discovery of a pulsar in the direction of the old open cluster NGC 6791, we conducted a search for radio pulsars in archival Parkes observations targeting similar old open clusters. We reprocessed 224 observations totalling 75.02 hours from four clusters: Theia 1661, NGC 6259, Pismis 3, and Trumpler 20. Our analysis identified five known pulsars and three new rotating radio transient (RRAT) candidates. By comparing the measured dispersion measures (DMs) with the expected DM values for each cluster derived from YMW16 and NE2001 models, we conclude that most detected sources are likely background pulsars. However, RRAT J1749-25 in Theia 1661 and RRAT J1237-60 in Trumpler 20 have DMs reasonably close to their respective clusters, suggesting possible membership. The association between PSR J1750-2536 and Theia 1661 remains ambiguous due to its intermediate DM. These candidate cluster-associated neutron stars warrant follow-up with more sensitive telescopes such as MeerKAT or the SKA, potentially offering valuable insights into neutron star retention mechanisms and evolution in open cluster environments.