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The FAST Globular Cluster Pulsar Survey (GC FANS)

The FAST Globular Cluster Pulsar Survey (GC FANS) reports the discovery of 60 pulsars in 14 globular clusters, including record-wide binaries and a rare double neutron star candidate, while revealing that the majority of these clusters host millisecond pulsar populations resembling those in the Galactic disk due to lower stellar densities.

Original authors: Yujie Lian, Zhichen Pan, Haiyan Zhang, Shuo Cao, P. C. C. Freire, Lei Qian, Ralph P. Eatough, Lijing Shao, Scott M. Ransom, Duncan R. Lorimer, Dejiang Yin, Yinfeng Dai, Kuo Liu, Lin Wang, Yujie Wang

Published 2026-08-20

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CC BY 4.0

Original authors: Yujie Lian, Zhichen Pan, Haiyan Zhang, Shuo Cao, P. C. C. Freire, Lei Qian, Ralph P. Eatough, Lijing Shao, Scott M. Ransom, Duncan R. Lorimer, Dejiang Yin, Yinfeng Dai, Kuo Liu, Lin Wang, Yujie Wang, Zhongli Zhang, Zhonghua Feng, Baoda Li, Minghui Li, Tong Liu, Yaowei Li, Bo Peng, Yu Pan, Yuxiao Wu, Liyun Zhang, Xingnan Zhang, Peng Jiang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep within the ancient, crowded cores of globular clusters—spherical collections of hundreds of thousands of stars that have orbited the Milky Way for billions of years—lies a hidden population of cosmic lighthouses. These are pulsars, the incredibly dense, rapidly spinning remnants of massive stars that have exploded. While most stars in these clusters are old and quiet, the intense gravity of the cluster core forces stars to interact, often capturing one another into binary systems. In these tight dances, a dead star can siphon material from a living companion, spinning itself up to rotate hundreds of times per second. These "recycled" pulsars, known as millisecond pulsars, are the most precise clocks in the universe. Astronomers have long suspected that these clusters should be teeming with them, but for decades, our telescopes were simply not sensitive enough to see the faintest signals from the most distant or dimmest ones.

A team of astronomers has now changed that picture using the Five-hundred-meter Aperture Spherical radio Telescope, or FAST, located in China. This massive instrument, the largest single radio dish in the world, has conducted a deep, thorough search of forty-one globular clusters visible from its location. The result is a dramatic expansion of our knowledge: the survey, known as GC FANS, has discovered sixty new pulsars in fourteen different clusters. This single effort more than doubles the number of known pulsars in the northern sky and reveals that these ancient star clusters are far more diverse and active than previously thought, hosting everything from the fastest-spinning pulsars to the widest binary systems ever found in such dense environments.

The survey was designed to look for signals that previous telescopes missed. Because globular clusters are often very far away, the radio signals from their pulsars are incredibly faint. By pointing FAST at the centers of these clusters for hours at a time, the team could detect signals that were previously invisible. They found that while many of the new pulsars are indeed the expected millisecond variety, spinning in less than thirty-thousandths of a second, the population includes some truly bizarre outliers. In the cluster known as M71, which is surprisingly sparse compared to others, the team found a pulsar with a spin period of 101 milliseconds. This object is in a highly elliptical orbit with a massive companion, and the total mass of the system suggests it is a pair of two neutron stars. This is a rare find; most such pairs in globular clusters are thought to form through chaotic, random collisions between stars. However, the properties of this specific system suggest it formed naturally from a massive binary star system early in the cluster's history, much like similar pairs found in the quieter regions of our galaxy, rather than through a violent encounter.

The discovery in M71 also yielded two other remarkable systems, M71B and M71C. These are pulsars orbiting their companions in incredibly wide loops, taking nearly 466 and 378 days respectively to complete one revolution. These are the widest binary pulsar systems ever known in a globular cluster. In the dense environments where these clusters usually exist, such wide orbits should be easily broken apart by the gravitational tug of passing stars. The fact that they have survived suggests that M71 is a low-density cluster where such destructive encounters are rare, allowing these delicate, long-period systems to endure for billions of years. This finding challenges the simple idea that the number of pulsars in a cluster depends solely on how crowded the stars are; even in a sparse cluster, the right conditions can preserve exotic systems that would be destroyed elsewhere.

Beyond the exotic outliers, the survey confirmed a broader trend: the pulsars found in these clusters often resemble the millisecond pulsars found in the quiet disk of our galaxy, rather than the chaotic, disrupted systems seen in the densest clusters. This suggests that in many clusters, the binary stars evolve peacefully, recycling the pulsars without being torn apart by neighbors. The team also updated the precise timing measurements for several known pulsars, including some in the clusters M92 and NGC 6712. These measurements are crucial for testing the laws of gravity, as the precise ticking of these cosmic clocks can reveal subtle deviations from Einstein's theory of general relativity. While no such deviations have been found yet, the new data provides a much sharper tool for future tests.

The success of this survey highlights the power of sensitivity in astronomy. By being able to see fainter and further, FAST has revealed that the population of pulsars in globular clusters is far richer and more varied than our previous, less sensitive instruments could show. The discovery of these sixty new pulsars, including the widest binaries and a potential double neutron star system formed through natural evolution, forces astronomers to rethink how these systems form and survive. It suggests that even in the oldest and most crowded stellar environments, nature can produce a wide array of cosmic clocks, some of which have been ticking undisturbed for nearly the entire age of the universe. As the team continues to analyze the data and refine their observations, these pulsars will likely serve as even more powerful probes for understanding the dynamics of star clusters and the fundamental laws of physics.

Technical Summary: The FAST Globular Cluster Pulsar Survey (GC FANS)

Problem and Motivation
Globular clusters (GCs) possess high stellar densities that facilitate frequent dynamical interactions, leading to the formation of binary systems such as low-mass X-ray binaries (LMXBs) and their evolutionary descendants, millisecond pulsars (MSPs). While previous surveys using telescopes like Arecibo, GBT, and Parkes have identified hundreds of pulsars in GCs, sensitivity limitations have likely left the majority of these populations undetected, particularly in distant or low-density clusters. The Five-hundred-meter Aperture Spherical radio Telescope (FAST), with its superior sensitivity, offers the potential to conduct a deep, unbiased survey of the GC population within its sky coverage ( $-14^\circ < \delta < 65^\circ$ ), potentially doubling the known census of GC pulsars and revealing extreme systems that challenge current formation models.

Methodology
The GC FANS survey targets 41 GCs within 83 kpc of the Sun, selected from the 45 visible to FAST. The survey utilizes the 19-beam L-band receiver (1.05–1.45 GHz) with a system temperature of 24 K and an antenna gain of 16 K Jy $^{-1}$ . Observations began in May 2018, with a pilot survey of M92 conducted in 2017 using an ultra-wideband receiver.

Observation Strategy: Most targets were observed for the full predicted integration time at a zenith angle of $26.5^\circ$ to maximize sensitivity. While the central beam ( $\sim3'$ ) covers the cores of most clusters, the survey prioritized central pointing for computational efficiency, acknowledging that extended core radii in some clusters (e.g., Pal 5, NGC 6366) might result in some pulsars falling outside the beam.
Data Reduction: Data were processed using the PRESTO software package. The search strategy included:
- Fourier Domain: Acceleration searches (using accelsearch) with harmonic summation to detect isolated pulsars and binaries with mild orbital acceleration. For known binary systems, "jerk searches" were applied to detect highly accelerated binaries.
- Time Domain: The Fast Folding Algorithm (FFA) via Riptide-FFA was employed to detect slow pulsars ( $P > 50$ ms) and those with long orbital periods.
- Stack Search: Incoherent combination of power spectra from multiple observations was used to detect weak periodic signals in specific clusters (NGC 6517, M15).
Timing Analysis: For new binary detections, initial orbital parameters were fitted using fitorb.py. Phase-connected timing solutions were derived using TEMPO, employing the BTX model for "spider" pulsars (M92A, NGC 6712A, M71A, M71E) and the DD model for the eccentric system M71D.

Key Results
As of January 2025, GC FANS has discovered 60 pulsars in 14 clusters, more than doubling the number of known GC pulsars in the FAST sky (from 35 to 95).

Population Statistics: Of the 60 discoveries, 55 are MSPs ( $P < 30$ ms), and 35 are in binary systems. The spin periods range from 1.98 ms (M14A) to 3960.72 ms (M15L). The survey identified the first pulsars in M2, M10, M12, M14, M92, and NGC 6712.
Extreme Systems in M71: The low-density cluster M71 hosts a diverse population, including:
- M71B and M71C: The widest known GC binaries with orbital periods of $\sim466$ and $\sim378$ days, respectively. Their properties (low eccentricity, companion masses $\sim0.4 M_\odot$ ) resemble normal wide MSP-He WD systems in the Galactic disk.
- M71D: A 100.7 ms pulsar in a highly eccentric ( $e \approx 0.63$ ), 11-day orbit with a massive companion. The system has a total mass of $2.63 \pm 0.08 M_\odot$ . The authors argue this is likely a double neutron star (DNS) system formed via massive binary evolution early in the cluster's history, rather than through dynamical exchange encounters, which is the typical formation channel for candidate DNSs in GCs.
- M71E: A "black widow" pulsar with the shortest known orbital period for a pulsar binary ( $P_b \approx 53$ minutes).
Updated Timing Solutions: The paper presents updated timing solutions for M92A, NGC 6712A, M71A, and M71E, and new solutions for M71B, C, and D.
Acceleration Analysis: Analysis of spin period derivatives ( $\dot{P}$ ) reveals that for most pulsars in M92 and M71, the observed $\dot{P}$ is dominated by intrinsic spin-down. In contrast, NGC 6712A exhibits a negative $\dot{P}$ , which is consistent with acceleration toward the cluster center due to the cluster's gravitational potential.
Stellar Encounter Rates: The survey challenges the strict correlation between high stellar encounter rates ( $\Gamma$ ) and pulsar abundance. While high- $\Gamma$ clusters (e.g., NGC 6517, M15) host many isolated pulsars due to binary disruption, the survey found significant populations in low- $\Gamma$ clusters (e.g., M53, M71). This suggests that low-density environments may preserve wide binaries and coeval systems that would be disrupted in denser clusters.

Significance and Claims
The paper claims that GC FANS provides an unprecedented view of pulsar populations in low-density globular clusters, demonstrating that FAST's sensitivity allows for the detection of systems previously missed by less sensitive instruments.

Formation Channels: The discovery of M71D suggests that DNS systems can form via standard stellar evolution channels in GCs, distinct from the dynamical formation mechanisms usually invoked for such systems in dense clusters.
Binary Survival: The presence of wide binaries (M71B, C) in the low-density M71 supports the hypothesis that low stellar interaction rates allow these systems to survive for gigayear timescales, whereas they would likely be disrupted in denser environments.
Gravity Tests: The paper highlights M71B and M71C as potential targets for testing the Strong Equivalence Principle (SEP) and Local Lorentz Invariance (LLI) due to their long orbital periods and low eccentricities, though it notes that current timing precision is preliminary and future long-term observations are required for meaningful constraints.
Gamma-Ray Prospects: While the authors attempted to detect gamma-ray pulsations from several high-spin-down-energy candidates (including M92A) using Fermi-LAT data, no significant signals were found, likely due to beam geometry or flux thresholds.

The authors conclude that the survey underscores the necessity of deep, unbiased searches to fully characterize the diversity of pulsar populations in GCs, revealing that low-density clusters may harbor more exotic and numerous systems than previously predicted by encounter rate models alone.

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