FAST Discovery of Eight Isolated Millisecond Pulsars in NGC 6517

Abstract

We present the discovery of eight isolated millisecond pulsars in globular cluster (GC) NGC 6517 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The spin periods of those pulsars (namely PSR J1801–0857K to R, or NGC 6517K to R) are all shorter than 10 ms. With these discoveries, NGC 6517 is currently the GC with the most known pulsars in the FAST sky. The largest difference in dispersion measure of the pulsars in NGC 6517 is 11.2 cm ^−3 pc, the second among all GCs. The fraction of isolated pulsars in this GC (16 of 17, 94%) is consistent with previous studies indicating an overabundance of isolated pulsars in the densest GCs, especially in those undergoing cluster core collapse. Considering the FAST GC pulsar discoveries, we modeled the GC pulsar population using the empirical Bayesian method described by Turk and Lorimer with the recent counts. Using this approach, we find that the expected number of potential pulsars in GCs seems to be correlated with the central escape velocity; hence, the GCs Liller 1, NGC 6441, M54 (NGC 6715), and ω -Cen (NGC 5139) are expected to host the largest numbers of pulsars.

Keywords

• Globular star clusters
• Millisecond pulsars