Recently, researchers from the Shanghai Astronomical Observatory (SHAO),the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC),Guizhou University and Yunnan Observatory, have made a collection of progress on pulsar searching, precision timing, pulsar emission properties for pulsars in Globular Clusters (GCs). These work ulitized highly sensitivity data collected with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The results have been published in a total of four papers in《ASTROPHYSICAL JOURNAL》and《Research in Astronomy and Astrophysics》.

GCs are composed of a large number of stars bounded by gravity, with central densities reaching tens of thousands of times the stellar density around the Sun. Their dense stellar environment and frequent dynamic interactions provide favorable conditions for pulsar formation and evolution, making them efficient “factories” for millisecond pulsars (MSPs). Long-term monitoring of these MSPs allows for studies of their emission characteristics, measuring their orbital properties and masses, and understanding their binary evolution. Using a group of MSPs in a single GC as sensitive probes , we can also investigate the properties of the GC itself, including the distribution of its gravitational potential, magnetic fields, and interstellar medium.

Discovery of new pulsars and a detailed timing analysis of GC M2 pulsars

The research team have carried out a multiepoch search and detailed timing analysis of millisecond pulsars in the GC M2, using data collected with FAST in five years. This work has discovered two new binary millisecond pulsars in M2, designated M2F and M2G. The team also reported the first rotation measure at the distance and direction of this cluster. Additionally, the team reported the first phase-coherent timing solutions for the M2 pulsars, and measured their spin and orbital parameters with high precision. These include the advance of periastron for M2A and M2E, which correspond to a total system masse of 1.75 and 1.80 solar masses, respectively. Using archival data from the Hubble Space Telescope, the team have identified the white dwarf companion of the binary pulsar M2C. By combining optical and radio observations, the team have constructed the binary evolution track of this system and estimated the cooling age of the companion to be approximately 10 Myr. This makes it the youngest white dwarf in any known GC binary pulsars. Furthermore, the team have used period derivative measurements of M2 pulsars to constrain the gravitational potential of the cluster. This result strongly support the latest central stellar velocity dispersion measurement in M2.

Figure 1 Color-Mass Diagram of the whie-dwarf companion of M2C. Left panel: A combination of the F275W and F336W filters. Right panel: A combination of F275W and F438W filters. The red plus represents the measurement of the white dwarf companion, and the error bar corresponds to 1-sigma confidence interval. The colored curves are tracks of the white dwarf with different masses. Different cooling ages are represented by different symbols.

Searching for pulsars using FFA algotithm

The research team utilized FAST data and applied the Fast Folding Algorithm (FFA) to conduct a systematic search of 30 GCs (including 16 GCs with known pulsars and 14 GCs without known pulsars). They successfully detected 87 out of 93 known pulsars. The six undetected known pulsars included one faint isolated pulsar and four binary systems exhibiting significant orbital motion. The team discovered a new MSP, M13I (PSR J1641+3627I), in the GC M13. This system has an orbital period of 18.2 days, an orbital eccentricity of 0.06, and a companion mass of 0.54 solar masses consistent with a helium white dwarf model. A comparative study based on the search for M13I demonstrated that the FFA algorithm achieved a detection rate of 17.9%, which is 1.75 times higher than that of the traditional FFT method (10.3%). For faint signals, the FFA algorithm demonstrated a higher signal-to-noise ratio (SNR, as shown in Figure 1). These results indicate that FFA algorithm has significant potential for future searches of faint pulsars in GCs.

Figure 2, a plot of SNR comparison of M13I detections by FFA and FFT.

Scintillation-enhanced Discovery of Two MSPs in the GC M13

The research team utilized an improved FFT+jerk algorithm to search 84 timing observations of the GC M13 conducted by FAST from 2018 to 2025, leading to the discovery of two faint binary pulsars, M13G and M13H (PSR J1641+3627G/H). Both pulsars were detected during their scintillation-enhanced states, and exhibited relatively lower detection rates compared to other known pulsars in M13.The team successfully derived coherenttiming solution for M13G, confirming it as a typical black widow system with a spin period of 4.32 ms, an orbital period of 0.12 days, and a companion mass <0.01 solar masses ( see Figure 3). This system has the largest projected angular distance from the center of M13 among all known pulsars in this GC, located 1.8 arcminutes from the center. Its presence in a circular orbit supports the theoretical inverse relationship between orbital eccentricity and distance from the cluster center. M13H has a spin period of 11.21 ms. Due to its faint signal, it was detected in only two out of the 84 observations, and a coherent phase timing model has not yet been obtained.

Figure 3, The average pulse profile and timing residual of M13G. The left panel is the integrated pulse profile.The timing residual as a function of MJD (upper subplot) and orbital phase (lower subplot) are shown in the right panel.

A Study of Polarization Profiles of Pulsars in Globular Clusters

The research team conducted polarization measurements of 25 GC pulsars distributed across 8 GCs using FAST. The results show that their pulse width-period relation is consistent with that of Galactic pulsars. However, only 20% of the GC pulsars exhibit an S-shaped polarization position angle curve, which is significantly lower than the proportion observed in slow pulsars. The distribution of rotation measures (RM) confirms that clusters near the Galactic disk have higher RM values, consistent with the Galactic magnetic field model. The polarization characteristics of these 25 GC pulsars resemble those of Galactic pulsars (Figure 4), indicating that the GC environment has not significantly altered their radiation properties.

Figure4, histograms and cumulative distribution of polarization ratios for different pulsar categories. Normal pulsar data are from Wang et al. (2023).

Paper Links:

[1] https://iopscience.iop.org/article/10.3847/1538-4357/adf969

[2] https://iopscience.iop.org/article/10.3847/1538-4357/add6a5

[3] https://iopscience.iop.org/article/10.3847/1538-4357/adfa14

[4] https://iopscience.iop.org/article/10.1088/1674-4527/add566

Scientific Contacts:

[1] Kuo Liu, Shanghai Astronomical Observatory, Chinese Academy of Sciences, liukuo@shao.ac.cn

[2] Lin Wang, Shanghai Astronomical Observatory, Chinese Academy of Sciences, wanglin@shao.ac.cn