Globular clusters (GCs) are densely stellarclusters, with an evolutionary age generally exceeding ten billion years, making them one of the oldest celestial systems in the Universe. After billions of years of evolution, globular clusters have hosted a large population of pulsars. Understanding their properties can reveal the dynamical evolution of globular clusters, stellar population synthesis pathways within globular clusters.

In the Milky Way, there are 317 rapidly spinning pulsars found in 41 globular clusters. These pulsars, called millisecond pulsars, rotate incredibly fast, some in just tens of milliseconds. About 179 of them are in binary systems. Scientists believe that in these dense clusters, pulsars can capture companion stars and speed up their rotation through a process called "recycling." This creates a large population of millisecond pulsars.

However, there is still limited evidence for other evolutionary pathways of pulsars within globular clusters. To address this question, Dr. Dengke Zhou from the Zhejiang Lab, together with Dr. Pei Wang and Prof. Di Li from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) used a fast folding search scheme and systematically analyzed the publicly available observational data from FAST, the world's largest single-dish radio telescope. They discovered two long-period pulsars in the globular cluster M15: M15K with a spin period of approximately 1.9 seconds and M15L with a spin period of approximately 3.9 seconds. M15L broke the record for the longest period among pulsars in globular clusters, becoming the current longest-known pulsar in globular clusters.

The team found that both pulsars remain below the spin-up line on the period-period derivative diagram. This line represents the upper limit on the acceleration of pulsars through accretion. Additionally, they possess relatively strong magnetic fields compared to other pulsars in globular clusters, and pulsars may weaken their magnetic fields during accretion. These observations suggest that they may have been part of binary systems and underwent brief accretion processes. However, they likely encountered other celestial bodies and were disrupted from their binary systems, evolving into isolated long-period pulsars.

"This discovery reveals a new evolutionary pathway for pulsars in globular clusters. FAST is systematically changing our understanding of pulsars in globular clusters," said Dr. Di Li, the corresponding author of the paper and Chief Scientist of the FAST.

"This study highlights the great sensitivity of the FAST. We can no doubt expect further important detections, not only of globular-cluster pulsars, but also of other interesting pulsars in our Galaxy or even in other galaxies," said Richard Manchester, professor at Australia Telescope National Facility, CSIRO Space and Astronomy.

"If they are harbingers of more such discoveries, we may finally fill in the missing pieces of the pulsar population in globular clusters. This is crucial for understanding the formation process and dynamic interactions of pulsars in globular clusters," said Zhanwen Han, Professor from the Yunnan Observatories of the Chinese Academy of Sciences.

The research result was published online on April 18, 2024, in SCIENCE CHINA Physics, Mechanics& Astronomy. And it is available at https://link.springer.com/article/10.1007/s11433-023-2362-x