Prof. HOU Xian from Yunnan Observatories of the Chinese Academy of Sciences (CAS), and the international collaborators from the Fermi-LAT gamma-ray telescope, Five-hundred-meter Aperture Spherical radio Telescope (FAST) and Arecibo radio telescopes, conducted research on the gamma-ray emission of the globular cluster M5. The study was published online in The Astrophysical Journal. 

Globular clusters (GCs), the oldest and densest gravitationally bound star systems, have a higher abundance of low-mass X-ray binary (LMXB) and millisecond pulsar (MSP) systems compared to the Milky Way.

The Fermi-LAT gamma-ray telescope has detected gamma rays from the direction of about 40 GCs to date. The leading model proposes that the gamma-ray emissions of GCs are from MSPs within them. However, of the 305 GC radio pulsars, only three individual ones have shown gamma-ray pulsations. This is attributed to both the vast distances of GCs, where only the brightest pulsars can be detected in gamma-rays, and the challenge of obtaining precise ephemerides of GC pulsars to search for gamma-ray pulsations. Therefore, once such ephemerides become available, it is important to verify whether the pulsars in these GCs are very bright in gamma rays.

In this study, the researchers made use of precise ephemerides of seven MSPs detected in the GC from the observations using Arecibo and FAST to perform detailed gamma-ray analysis of M5 and to search for individual gamma-ray pulsars in it.

They found that the spectrum of the gamma-ray emission of M5 is consistent with that of MSPs, but no gamma-ray pulsations were detected from these seven MSPs. Thus, they proposed that M5's gamma-ray emission comes from the collective radiation of MSPs in M5, and the number of MSPs was estimated to be around 1-10. This is consistent with the number currently detected, suggesting that the sample of known MSPs in M5 is (nearly) complete.

Besides, the researchers proposed that the nondetection of individual gamma-ray MSPs in M5 is not surprising due to two aspects.

On the one hand, the upper limits of the spin-down power of the seven MSPs are comparable with those of the Milky Way gamma-ray MSPs, indicating that their true spin-down power may be significantly lower than the upper limits, making them too weak to be detected. On the other hand, the current three GC gamma-ray MSPs are all located in GCs with high binary encounter rates. High encounter rates will destroy the LMXB systems, interrupt the accretion process, and result in slower pulsars that are only partially recycled with stronger magnetic field than Galactic MSPs, making them brighter. In contrast, M5's low encounter rate will produce fully recycled, fast-rotating MSPs similar to those in the Milky Way, while the GC's larger distance makes them harder to be detected.

Although no individual gamma-ray MSPs have been detected in M5, the attempt to detect bright gamma-ray pulsars in low-encounter-rate GCs like M5 is a useful test. If a single bright gamma MSP is found in such GCs, its formation may not necessarily be related to the processes that occur almost exclusively in high-encounter-rate GCs, such as the disruption of LMXBs.

This study adds M5 to the list of well-studied GCs, and has great significance for verifying the formation mechanism of MSPs in GCs and constraining the GC gamma-ray emission models.