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From stars to pulsars: What makes the formation of millisecond pulsars so amazing?
Millisecond pulsars (MSPs) are pulsars with a rotation period of less than about 10 milliseconds. These objects have been detected in the electromagnetic spectrum in radio, X-rays and gamma rays. Scientists believe that the formation of millisecond pulsars is related to the mass transfer of companion stars. It is generally believed that they are old, rapidly rotating neutron stars that are "recycled" or accelerated by accreting material from the companion star in a close binary star system. Therefore, millisecond pulsars are sometimes called recycling pulsars.
The origin of millisecond pulsars is related to low-mass X-ray binary star systems, in which X-rays are emitted by the accretion disk of neutron stars.
According to current research, the formation process of millisecond pulsars may have experienced at least two different evolution mechanisms, which may explain why some young millisecond pulsars have relatively high magnetic fields, such as PSR B1937+21. According to research by Bülent Kiziltan and S. E. Thorsett (UCSC), this new finding implies that traditional evolutionary models are insufficient to explain the development of all millisecond pulsars. Notably, many millisecond pulsars are concentrated in globular clusters, which is consistent with the rotational acceleration hypothesis of their formation, as the high stellar density of these clusters increases the pulsar's chance of capturing a large companion star.
Currently, about 130 millisecond pulsars are known to be located in globular clusters, including 37 in Terzan 5, 22 in 47 Tucanae, and 8 each in M28 and M15.
The limit of pulsar rotation speed
In 1982, PSR B1937+21 became the first millisecond pulsar discovered. Its rotation speed is about 641 times per second, and it is still the second fastest known millisecond pulsar. The fastest known spinning pulsar in 2023, PSR J1748-2446ad spins at a rate of 716 times per second. Existing models of neutron star structure and evolution predict that if pulsars reach spin speeds of about 1,500 rpm or higher, they may break apart, and at speeds above 1,000 rpm, they will lose energy faster than they absorb. The acceleration rate of the integration process.
In 2007, data from the Rossi X-ray Timing Detector and the INTEGRAL spacecraft discovered a neutron star named XTE J1739-285 with a rotation rate of 1122 Hz. However, this result is not statistically significant.
Affected by gravitational radiation, the rotation rate of millisecond pulsars may slow down. One of the X-ray pulsars, IGR J00291+5934, with a rotation speed of 599 revolutions per second, may become a strong candidate for detecting gravitational waves in the future.
Gravity wave detection using pulsars
Gravity waves are an important prediction of Einstein's general theory of relativity, originating from the movement of a large amount of matter and the fluctuations in the early universe. A pulsar is a rapidly rotating, highly magnetic neutron star that usually forms after a supernova explosion. Due to their stability, pulsars can be used for the detection of gravitational waves, an idea first proposed by Sazhin and Detweiler in the 1970s.
Pulsars are considered reference clocks that send timing signals on one end, while the observer is on Earth.
When gravitational waves pass through, they will cause disturbances in the local space-time metric, thereby affecting the observed rotation frequency of the pulsar. In 1983, Hellings and Downs extended this concept by proposing that a comprehensive network of pulsars could detect a time-varying gravitational wave background. In the early 1980s, with the discovery of the first millisecond pulsars, Foster and Backer refined the sensitivity of gravitational wave detection.
In the following years, with the advancement of digital data acquisition systems and the discovery of more new millisecond pulsars, the sensitivity of gravitational wave detection continued to increase. In June 2023, NANOGrav released 15 years of data and provided the first evidence of a gravity wave background. The Hellings-Downs curves of these observations are a clear indication of the source of gravity waves.
The formation of millisecond pulsars and their exploration of the universe not only reveal the evolution of the universe, but also force us to think about how many unknown processes are waiting for us to explore and understand in this vast sea of stars?
