FAST uncovers six faint millisecond pulsars hiding in two core-collapsed globular clusters

A team of Chinese astronomers led by Yinfeng Dai, of Beijing Normal University, has reported the detection of six previously unnoticed isolated millisecond pulsars (MSPs) in the globular clusters NGC 6517 and NGC 7078 (better known as Messier 15). The discovery was made using archival data from the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China and was posted on the arXiv preprint server on April 9, 2026.

The new findings, laid out in the paper The Stack Search Tests on FAST Data: Discovery of Six Faint Isolated Millisecond Pulsars in NGC 6517 and NGC 7078 (M15) (arXiv:2604.08268), expand the known pulsar populations of these two ancient stellar systems by roughly 27 % and 18 %, respectively — a significant jump for clusters that have already been targeted by multiple surveys.

Why core-collapsed clusters are such tempting hunting grounds

Millisecond pulsars are rapidly rotating, highly magnetized neutron stars that emit narrow beams of electromagnetic radiation and complete a full revolution in less than 30 milliseconds. They are generally understood to form inside binary systems, where the more massive star evolves first into a neutron star and is later spun up by accretion from its companion — a process often described as «recycling».

NGC 6517 and M15 are the only two core-collapsed globular clusters within reach of FAST. Core-collapsed systems are especially appealing targets for pulsar searches because both observational evidence and recent dynamical modeling suggest they tend to host large populations of isolated MSPs, whose binary companions were stripped or disrupted by the dense stellar environment at the cluster core.

Archival data and a clever signal-stacking strategy

Rather than scheduling new time on the telescope, Dai and collaborators went back to the FAST archive. They processed 23 observations of NGC 6517 taken between June 2019 and March 2024, with exposures ranging from half an hour to two and a half hours per epoch, along with 19 observations of M15 carried out between September 2019 and February 2024, each lasting between roughly 30 minutes and 4.5 hours.

The breakthrough came from combining this material through a technique known as power-spectrum stacking. By summing the Fourier-transformed signals of many independent observations, the team pushed the effective signal-to-noise ratio well above what any single epoch can deliver. Faint, persistent periodic signals that would otherwise be buried in noise begin to emerge above the detection threshold. This work is part of the broader FAST Globular Cluster Pulsar Survey (GC-FANS), a programme that has already catalogued more than 60 pulsars across 16 globular clusters.

Meet the six new pulsars

In NGC 6517, four new MSPs were identified and labelled NGC 6517S, NGC 6517T, NGC 6517U and NGC 6517V. Their spin periods span a narrow range between 3.68 and 6.02 milliseconds, and their dispersion measures cluster tightly between 182.45 and 182.85 pc/cm³ — values fully consistent with membership in NGC 6517, whose line-of-sight electron column is well characterized.

In M15, two additional MSPs were uncovered: M15M, with a spin period of 4.83 ms and a dispersion measure of about 67.9 pc/cm³, and M15N, rotating every 9.29 ms and measured at 66.65 pc/cm³. The similar dispersion values reinforce the conclusion that both pulsars belong to the cluster rather than sitting along the same line of sight by chance.

Timing analyses carried out by the authors show no signs of orbital motion in any of the six sources: they appear to be genuinely isolated neutron stars, not members of binary systems. That is precisely the population core-collapsed clusters are expected to over-produce.

Why these faint objects slipped through before

All six pulsars had gone undetected in previous single-epoch searches of exactly the same data. Their radio signals are simply too weak for standard pipelines to flag with confidence from any individual observation. The stacking approach adopted by Dai and collaborators effectively accumulates signal across years of monitoring, turning marginal detections into robust ones.

The result is both a scientific and a methodological win. On the astrophysical side, the enlarged pulsar census in NGC 6517 and M15 provides new probes of cluster dynamics, interstellar turbulence and the intrinsic luminosity function of MSPs. On the methodological side, the study shows that large radio archives are far from exhausted: even after years of observation, well-studied clusters can still surrender new neutron stars to the right analysis technique.

Further timing follow-up will be needed to refine the rotational and astrometric parameters of the six new pulsars, and to test whether their long-term stability makes any of them useful additions to pulsar timing arrays — an increasingly important application in the search for nanohertz gravitational waves.

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