FAST observation pulsar diagram provided by the author
Pulsar rotation diagram
Recently, the Chinese Academy of Sciences National Observatory's 500-meter spherical telescope (FAST) has made new breakthroughs. Through cooperation with NASA's Fermi Gamma-ray satellite, FAST discovered a millisecond pulsar and obtained international certification. This is the first astronomical discovery of Sino-American scientific installations on the ground and in space, radio and high-energy bands. It is another important result of FAST's subsequent discovery of pulsars.
A millisecond pulsar is a neutron star that spins hundreds of circles per second. This millisecond pulsar, numbered J0318+0253, is about 4,000 light years away from Earth and has a rotation period of 5.19 milliseconds. It is used to study the internal structure of neutron stars, to detect low-frequency gravitational waves, and It is of great significance to understand the formation and evolution of the universe structure.
Not periodic signals from aliens
With regard to pulsars, many people have heard stories about it being discovered.
In 1967, only 24 year-old graduate student Joelyn Bell of the University of Cambridge in the United Kingdom discovered that a star in the direction of the fox sends an electromagnetic pulse signal to the earth every 1.37 seconds. Its signal cycle is extremely stable, and scientists even excitedly It is believed that the evidence for the existence of extraterrestrials was found: Periodically emitted electromagnetic waves are signals sent to the earth by extraterrestrial 'little green men'.
However, the eagerness of extraterrestrial civilizations also requires rational scientific verification. After a period of research and analysis, scientists confirmed that periodic electromagnetic signals come from a new class of celestial bodies and named it pulsars.
The pulsar is actually a fast rotating neutron star. A part of the star will have a violent supernova explosion at the end of its life, leaving a dense spherical object in the center - the neutron star. This mass is equivalent to that of the Sun, with a radius of only 10 kilometers. The left and right celestial bodies carry the residual energy of the stellar explosion and rapidly rotate and radiate energy along the direction of the magnetic axis in the form of electromagnetic beams. The neutron star at this time is like a fast-rotating lighthouse in the ocean. It emits An electromagnetic wave is like a rapidly rotating light beam on a lighthouse. If the Earth happens to be in the 'sweeping' range of the electromagnetic beam, scientists may be able to observe periodic electromagnetic pulses from space, ie to find a pulsar.
The discovery of pulsars identified the existence of a neutron star and its relationship with supernova explosions. It also provided a new method for estimating the distance of celestial bodies in the Milky Way. Together with quasars, cosmic microwave background radiation, and interstellar molecules, it was called the 20th century. The four major discoveries of astronomy in the 1960s.
Rapidly rotating 'small'
Since being first discovered in 1967, astronomers have discovered more than 2,700 pulsars. Their rotation cycles are mostly in the tens of milliseconds to several seconds. In contrast, millisecond pulsars rotate much faster, in milliseconds. The time will be able to rotate one week. The pulsar with the fastest rotation has been found to rotate 716 revolutions per second. The period is only 1.39 milliseconds.
Astronomers do not agree on why millisecond pulsars turn so fast. The mainstream view is that they were originally long-pulse pulsars, accelerating in the process of accreting the companion material. This acceleration process It is usually millions of years, so millisecond pulsars are usually old pulsars.
It is not easy to discover millisecond pulsars. The first millisecond pulsar was discovered in 1982. To date, more than 300 such bizarre pulsars have been discovered, accounting for about 10% of the total number of pulsars. Its discovery not only depends on telescopes. Discrete data records also require data analysis methods such as high-performance computing clusters and artificial intelligence. In addition, the galactic distribution of millisecond pulsars is also one of the causes of observations. Normal pulsars are mainly concentrated in the galactic plane, and millisecond pulsars are The distribution of the galaxy space is more diffuse, so looking for it can be described as collecting money, and it takes more time and hardships.
During the preparation of the FAST search for the millisecond pulsar, the Chinese Academy of Sciences Ao Li team reprocessed the long-term monitoring data of the Australian Parkes telescope. In 2016, two new millisecond pulsars were discovered in the 47 globular cluster of the constellation Aquarius, which became the Chinese Academy of Sciences. New radio pulsar discovered by the team for the first time.
Some millisecond pulsars may radiate electromagnetic waves in multiple frequency bands, including gamma rays and radio waves. The FAST team found this convenience and used it. On February 27, FAST complied with NASA Fermi's large-field telescope. The location of the gamma ray point source to be authenticated 3FGL J0318.1+0252 was provided for one-hour tracking observation. Through careful data analysis, a new radio millisecond pulsar J0318+0253 was discovered.
It is worth mentioning that, compared with the gamma radiation of these millisecond pulsars, its radio radiation is much weaker, which puts a very high sensitivity requirement on the radio telescope. The millisecond pulsars discovered so far are One of the weakest high-energy millisecond pulsars discovered. The 305-meter international large-scale radio telescope of Arecibo in the United States used to perform multiple radio pulsar searches on the same point source in the past. None of them succeeded.
Exploring the window formed by the structure of the universe
Pulsars are closely related to gravitational waves. For the first time, humans have confirmed the existence of gravitational waves, which is the rotation cycle of a binary system containing the pulsar PSR1913+16. Its slower and slower rotation coincides with Einstein's general theory of relativity. The predicted gravitational wave effect.
The discovery and observation of millisecond pulsars provide new possibilities for astronomers to study gravitational waves.
The pulsar is the most stable clock in the universe, its rotation is extremely stable, and its rotation period is generally about one billion years. It is comparable to the precision of the cesium clock. Scientists envisage forming pulsars with dozens of well-timed millisecond pulsars. Chronograph array to detect gravitational waves. When a gravitational wave event occurs, the gravitational wave background is superimposed on the pulsar astrometric time observation data and appears as an additional 'noise', that is, the observed pulsar cycle occurs. Long or short changes. According to this change, the location of gravitational waves, waveforms, periods, and the quality of gravitational wave sources can be obtained by combining the pulsars and their positional relationship with the Earth.
Unlike surface exploration programs such as LIGO and space exploration programs such as LISA, the pulsar chronograph can detect longer-period gravitational waves with periods ranging from several years to more than a dozen years. Such gravitational waves are called low frequencies (nano Hertz) Gravitational waves, which originate from the superposition of supermassive black holes with a mass of 100 to 10 billion solar masses in the center of the galaxy. The pulsar chronometry array is currently the only method capable of directly detecting low-frequency gravitational waves. Because of supermassive black holes Convergence plays a leading role in the formation and evolution of the universe structure. The detection of low-frequency gravitational wave sources is equivalent to directly opening the gravitational wave window to explore the structure of the universe.
Currently, astronomers make use of several of the largest radio telescopes in the world to form three pulsar time arrays, namely EPTA in Europe, PPTA in Australia, and NANOGRAV in the United States. These three projects have also jointly established international pulsars. Time-of-flight array (IPTA), whose sensitivity has approached the 'discovery' of gravitational waves.
Pulsar search is the basis for conducting gravitational wave detection. Because low-frequency gravitational wave detection has severe requirements for the stability of the millisecond pulsar's cycle and the space environment in which it is located, the current number of millisecond pulsars that can be used by IPTA is 50. The FAST multi-scientific objectives that are being planned by the FAST project team to survey the sky will find a large number of millisecond pulsars, which will greatly increase the sensitivity of pulsar arrays to detect gravitational waves.
Source: Science and Technology Daily
