Intelligent computing accelerates search, China Sky Eye FAST finds the longest period pulsar in globular star clusters to date

We know that a globular star cluster is a celestial system bound by gravity. It is an ancient star cluster composed of tens of thousands to millions of stars. Most of them are spherical in appearance, but they may also be affected by the gravity of other celestial systems. The influence causes the shape to deviate from the sphere. The dynamic evolution process of globular star clusters and the synthesis path of star populations are hot research topics in the current literature circle.

After decades of evolution, the number of stars in globular clusters has continued to shrink into a few dense stars, and pulsars are one of them. By understanding the distribution and properties of pulsars in globular clusters, we can gain an in-depth understanding of the density distribution, mass distribution, and interactions with other celestial bodies inside globular clusters, and then obtain important information such as the dynamic evolution process of globular clusters and the synthesis paths of star populations. .

According to astronomical observations, it is found that pulsars will periodically emit electromagnetic wave signals outward, which is evidence of their existence in the vast universe. More than 3,000 pulsars have been discovered so far, which are mainly divided into two categories according to their rotation: normal pulsars and millisecond pulsars. There are currently more than 500 known millisecond pulsars, accounting for about 15% of known pulsars. The rotation period of a normal pulsar is about 0.1 seconds to a few seconds, while the rotation period of a millisecond pulsar is less than 30 milliseconds.

Millisecond pulsars have different formation histories from normal pulsars: normal pulsars are usually relatively young, less than a few million years old, while millisecond pulsars are relatively old, having formed through accretion in a close binary star system The mass acquires angular momentum so that the rotation period reaches the order of milliseconds. The current observational fact is that more than two-thirds of the known millisecond pulsars are in binary star systems. Globular clusters have higher star density and higher binary star formation rate. 317 pulsars have been discovered in 41 globular clusters. Most of these pulsars are millisecond pulsars with rotation periods in the range of tens of milliseconds. 179 of them are in the range of tens of milliseconds. Binary star system. Astronomical researchers are curious, is there another type of pulsar with a longer period that exists in globular clusters?

Finding long-period pulsars

After billions of years of evolution, pulsars should rotate slower and slower, that is, the rotation period becomes longer and longer. Why Are most of the millisecond pulsars we have searched for?

"In one hypothesis, because globular clusters are very dense, a pulsar can easily capture a companion star and accretion matter from the companion star. This matter is like a whip that whips a top, It will accelerate the rotation of this pulsar again," explained Dr. Zhou Dengke from the Astronomical Computing Research Center of Jiang Laboratory.

However, in theory, there are many possible ways for long-period pulsars to form. One situation is that when the accretion of two pulsars is interrupted by a third-party celestial body and the accretion is interrupted, it is possible to form a long-period pulsar. Another scenario is that white dwarfs form after the collapse of old stars, and the white dwarfs merge to form periodic pulsars.

So, what is the reason why we fail to find more long-period pulsars? This is because most long-period pulsars have low signal-to-noise ratios and are susceptible to red noise interference caused by long-term observations with astronomical telescopes. Therefore, the detection of long-period pulsars is extremely challenging.

In response to the difficult problems in long-period pulsar detection, in the past year or so of research work, Zhou Dengke, associate researcher Wang Pei of the National Astronomical Observatory of the Chinese Academy of Sciences, researcher Li Gu and others People used a new search method and successfully found long-period pulsars in globular star clusters.

"First, through simulation and quantitative analysis, we systematically evaluated the impact of red noise on the sensitivity of long-period pulsar searches. On this basis, we used the simulation results to carefully select appropriate parameters, thereby effectively removing Red noise interference in the data. Furthermore, the fast folding algorithm (FFA) was used to conduct a detailed and in-depth search of multiple globular star cluster survey public data observed by China Sky Eye.

Finally, the research team discovered two long-period pulsars with rotation periods of 1.9 seconds and 3.9 seconds in the globular cluster M15, named M15K and M15L respectively. M15L is also the pulsar with the longest rotation period among globular clusters discovered so far.

Figure 1 Two newly discovered long-period pulsars in the globular cluster M15. On the left is the pulse profile and phase-time waterfall diagram of the two pulsars, and on the right is a schematic diagram of the positions of the two pulsars in M15. Source: Zhou et al., 2024, Sci. China-Phys. Mech. Astron., 67, 269512.

Li He, corresponding author of the paper and chief scientist of FAST, said: "This discovery reveals a new evolution path for globular cluster pulsars. FAST is systematically changing our understanding of globular clusters. Understanding of cluster pulsars."

This breakthrough discovery was published as a cover article in the well-known academic journal "SCIENCE CHINA Physics, Mechanics & Astronomy" on April 18.

arXiv address: https://arxiv.org/pdf/2312.05868

The research team also further analyzed the physical properties of these two pulsars. It was found that their magnetic fields are also relatively strong. "Pulsars may weaken their magnetic fields during the accretion process, and their strong magnetic fields also further indicate that they only experienced a brief binary accretion process." Zhou Dengke, the first author of the paper, said.

Using this search plan, the research team continued to discover 13 long-period pulsars. These findings complete the missing link in the search for long-period pulsars in globular clusters, and are of great significance to understanding the classification of pulsars in globular clusters and the evolution of star populations.

Mining patterns from data

In this study, the team processed about 90 hours of China Sky Eye FAST observations from 2019 to 2022, totaling about 50TB data.

From these original observation data to the final identification of pulsars, we need to go through achromatic dispersion, parameter setting, interference elimination, red noise removal, period search, candidate screening, cross-validation, and timing analysis. Several steps, including a large amount of data processing work and consumption of computing resources.

^{# This article uses the flowchart in the ball -shaped star group in the ball -shaped star group.}

The first is the dispersion link. During the propagation process, the pulsar signal will be dispersed due to the influence of the interstellar medium, causing the high-frequency signal to reach the earth before the low-frequency signal. In order to superpose signals of different frequencies to obtain a pulse signal with a high signal-to-noise ratio, data processing first needs to Carry out dispersion work.

^{The long-period pulsar discovered in M15. On the left is the long-period pulsar M15K, with a period of about 1.928 seconds, and DM of about 66.5 pc·cm^−3; on the right is the long-period pulsar M15L, with a period of about 3.961 seconds , DM is about 66.1 pc·cm^−3. In each subfigure, from top to bottom are the frequency vs. phase diagram with/without achromatic dispersion, the pulse curve and the time phase diagram.}

"The astronomical computing team of Zhijiang Laboratory has optimized the achromatic software, which has increased the data processing efficiency several times." Zhou Dengke said.

De-dispersion is only the first step. What is more time-consuming and labor-intensive is the subsequent candidate screening process.

After searching through parameter estimation and folding algorithms, a large number of candidate body result maps will be obtained. Researchers must rely on the naked eye to identify whether the result maps conform to the signal characteristics of pulsars.

^{# Series systematically use the fast folding algorithm to conduct cyclical search for the ball -shaped star group data listed in Table 1, and finally found two long cycle pulse stars M15K and M15L.}

"One hour of observation may produce tens of thousands of candidate images. It is very difficult to distinguish very weak signals from so many images. Zhou Dengke said. Using the AI ​​visual model method independently developed by the Astronomical Computing Research Center, candidate information can be efficiently screened, and the number of candidates that require manual intervention can be reduced by three orders of magnitude.

"Nowadays, the amount of data in the astronomical field is huge and processing is very time-consuming. Using intelligent computing technologies such as AI algorithms to assist in processing these data can free us from heavy data analysis and devote more energy to Understanding the physical images behind the data can greatly improve scientific research efficiency," said Zhou Dengke.

##                                       Zhou Dengke

Intelligent computing has become an indispensable tool in Zhou Dengke's scientific research work. In the next step of research, he plans to correct the binary Doppler effect and then search for long-period pulsars in globular clusters to improve the search Completeness, and by training the AI ​​model to identify Doppler effect phase shift images, the calculation speed is improved and the entire search process is accelerated.

"The job of astronomers is to try to discover or verify the basic laws of nature from the analysis of astronomical observation data. In an era without modern computing tools, astronomers represented by Kepler analyzed a large number of astronomical observations The laws of planetary motion are summarized in the data. We can now use large-scale computing clusters and intelligent computing methods to greatly improve scientific research efficiency. For example, this new discovery can help understand the multi-path evolution history of star populations. The parameters of multiple pulsars can be used to constrain the gravitational potential and mass of objects in the center of related globular clusters. We hope to use large-scale computing clusters and intelligent computing technology to discover more basic natural laws from the data," said Zhou Dengke. .

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