Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), Chinese astronomers have inspected two nearby galactic globular clusters, namely NGC 6517 and NGC 7078. The study resulted in the discovery of six new millisecond pulsars in these clusters, which are isolated and faint. The finding was detailed in a paper published April 9 on the arXiv pre-print server.
Search for the most rapidly spinning pulsars goes on
In general, pulsars are highly magnetized, rotating neutron stars emitting a beam of electromagnetic radiation and the most rapidly rotating ones (with rotation periods below 30 milliseconds) are known as millisecond pulsars (MSPs). It is assumed that MSPs are formed in binary systems when the initially more massive component turns into a neutron star that is then spun up due to accretion of matter from its companion.
A team of astronomers led by Yinfeng Dai of the Beijing Normal University analyzed archival FAST data to investigate NGC 6517 and NGC 7078 (also known as Messier 15, or M15)—the only two core-collapsed globular clusters (GCs) accessible to FAST. Core-collapsed clusters are particularly promising targets for GC pulsar searches, as observations and recent dynamical modeling suggest that they host large populations of isolated MSPs.
The observations were conducted as part of the FAST Globular Cluster Pulsar Survey (GC-FANS), which has so far discovered over 60 pulsars across 16 GCs.
“This work is only based on FAST archival data. A total of 23 archival FAST observations of NGC 6517, spanning from June 2019 to March 2024 (0.5–2.5 h per epoch), were used. The 19 observational data of M15 were conducted between September 2019 and February 2024 (0.5-4.5 h per epoch),” the researchers write in the paper.
Six new MSPs found. What are their properties?
Dai’s team identified four new MSPs in NGC 6517 and they received designations from NGC 6517S, NGC 6517T, NGC 6517U and NGC 6517V. These pulsars have spin periods ranging from 3.68 to 6.02 milliseconds, and dispersion measures between 182.45 and 182.85 pc/cm3.
The two MSPs detected in NGC 7078 were named M15M and M15N. M15M has a spin period of 4.83 milliseconds and dispersion measure of approximately 67.9 pc/cm3. When it comes to M15N, it has a spin period of 9.29 milliseconds and its dispersion measure was found to be 66.65 pc/cm3.
Timing analyses performed by the authors of the study indicate that all the newfound pulsars are isolated systems. They were missed by previous studies as their signals are too weak to be robustly recovered in standard single-epoch searches. This underscores the advantage of the stacking technique used by Dai’s team for detecting extremely faint isolated MSPs.
“These discoveries were enabled by stacking power spectra from multiple observations, a method that effectively boosts the signal-to-noise ratio of faint sources,” the scientists explain.
Summing up the results, the astronomers underline that their discovery increases the known pulsar populations in NGC 6517 and NGC 7078 by approximately 27% and 18%, respectively.
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Publication details
Yinfeng Dai et al, The Stack Search Tests on FAST Data: Discovery of Six Faint Isolated Millisecond Pulsars in NGC 6517 and NGC 7078 (M15), arXiv (2026). DOI: 10.48550/arxiv.2604.08268
Journal information:
arXiv
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