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Why do millisecond pulsars often appear in globular clusters? What is their mysterious connection?
Millisecond pulsars (MSPs) are pulsars with periods less than ten milliseconds, and one of the main concerns is why they are often found in globular clusters. The dense environments of these clusters may be related to the formation process of millisecond pulsars, posing an interesting and important astronomical problem.
Millisecond pulsars are generally thought to have evolved from low-mass X-ray binary systems.
Traditional theory holds that millisecond pulsars are old neutron stars that have absorbed matter and accelerated their speed. In such a binary system, the outer layers of the companion star may flow into the neutron star's accretion disk, which may speed up the pulsar's rotation to hundreds of times per second, which is exactly what we observe. Characteristics of millisecond pulsars.
However, with the advancement of observation technology, astronomers have discovered that a single evolutionary model cannot explain all millisecond pulsars. Especially for some young millisecond pulsars that have relatively high magnetic field strengths, such as PSR B1937+21, in these cases the researchers proposed at least two different formation processes. The specific mechanisms of these processes remain a mystery.
There are currently about 130 millisecond pulsars known to be located in globular clusters.
The study found that the environments of these globular clusters are particularly dense, which means that the chances of pulsars capturing companions or interacting with other stars are greatly increased. Take Terzan 5 as an example, which contains 37 millisecond pulsars. Another famous star cluster, 47 Tucanae, also has 22 pulsars discovered. These prolific pulsars provide astronomers with valuable opportunities for further research.
Pulsar rotation speed limit
First discovered in 1982, the millisecond pulsar PSR B1937+21 rotates at a rate of about 641 times per second, making it the second-fastest pulsar to date. PSR J1748-2446ad was discovered in 2004 and rotates 716 times per second, making it the fastest pulsar known.
Current models predict that a pulsar will collapse when it spins faster than about 1,500 times per second.
These phenomena not only triggered in-depth studies on the structure and evolution of neutron stars, but also made us rethink the relationship between rotation speed and gravitational waves. Studies have shown that pulsars that rotate faster than 1,000 times per second will lose energy due to gravitational radiation, and various observational projects currently underway are expected to shed further light on this prospect.
Gravitational wave detection using pulsars
Gravitational waves are an important prediction of Einstein's general theory of relativity, arising from the large-scale motion of matter and fluctuations in the early universe. Rapidly spinning pulsars have unique clock properties that make them ideal candidates for studying gravitational waves. It is said that by monitoring the signals emitted by pulsars, scientists can detect the fluctuations in space-time caused by gravitational waves.
This idea can be traced back to the late 1970s and has continued to develop over time.
With the emergence of digital data capture systems and the use of new radio telescopes, various calibration and analysis techniques have become increasingly mature, and the sensitivity of pulsars as gravitational wave detectors has been improved many times. Each data release from the NANOGrav project, which began in 2013, has shown more accurate limits on the gravitational wave background. Especially in 2023, the newly released data showed the first evidence of the gravitational wave background, once again bringing refreshing discoveries to the astronomical community.
The special characteristics of millisecond pulsars make them a window to understand the universe. They can not only detect gravitational waves, but also provide invaluable data for the study of stellar evolution, exoplanets, gravitational fields, etc. The initial discovery of planets around pulsars led humans to think more about the possibility of life in the universe. And as our knowledge of them deepens, will there be other unknown mysteries of the universe waiting for our exploration?
