Neutron stars are a type of neutron star (from a pulsating radio source) described in detail in the previous article, which is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation from its magnetic poles. This radiation can only be observed when the emitted beam is pointed at the Earth (similar to how a lighthouse can only be seen if the light is pointed in the direction of the observer), and causes the emitted pulses to appear. Neutron stars are very dense and have a short and regular rotation period. For a single pulsar, this results in very precise pulse intervals, ranging from milliseconds to seconds. Pulsars are one of the candidates for ultra-high-energy cosmic ray sources.
Chandra's X-ray telescope and WISE infrared telescope observations of the pulsar PSR B1509 58.
The cycles of pulsars make them a very useful tool for astronomers. Observations of pulsars in binary neutron star systems are used to indirectly confirm the presence of gravitational radiation. In 1992, the first extrasolar planet was discovered around the pulsar PSR B1257+12. In 1983, certain types of pulsars were detected, when their timing accuracy surpassed that of atomic clocks.
Schematic diagram of the "lighthouse" effect of pulsars, where pulsed signals can only be received in a specific direction.
The events that lead to pulsar formation begin when the core of a massive star is compressed and collapses into a neutron star during a supernova explosion. The neutron star retains most of its angular momentum, and since its radius is only a fraction of that of its predecessor star (and therefore its moment of inertia decreases dramatically), it forms at a very high rotational velocity. The radiation beam is emitted along the magnetic axis of the pulsar, which rotates with the rotation of the neutron star. The magnetic axis of a pulsar determines the direction of the electromagnetic beam, and the magnetic axis is not necessarily the same as its axis of rotation. This misalignment causes the neutron star to see a beam of light every time it rotates, resulting in the "pulsed" nature of its appearance.
Schematic diagram of a pulsar. The sphere in the middle represents the neutron star, the curve represents the magnetic field lines, the protruding cone represents the emission beam, and the green line represents the axis of rotation of the star.
In a rotationally driven pulsar, the beam is the result of the rotational energy of the neutron star, which generates an electric field through the movement of a very strong magnetic field, which causes the acceleration of protons and electrons on the surface of the star and produces an electromagnetic beam emitted from the poles of the magnetic field. Nicer's observations of PSR J0030+0451 indicate that both beams originate from a hot spot located at the South Pole and that there may be more than two such hot spots on the star. This rotation slows down over time as electromagnetic power is emitted. When the rotation period of a pulsar slows down sufficiently, the radio pulsar mechanism is thought to shut down (the so-called "death line"). This shutdown appears to have occurred about 10 million to 100 million years later, meaning that about 99% of all neutron stars born in the universe in 13.6 billion years no longer pulsate.
Optical X-ray composite image of the Crab Nebula, synchrotron emission in the nebula surrounding the pulsar, powered by the magnetic field and particles of the central pulsar.
Despite the widespread belief that pulsars are rapidly rotating neutron stars, Werner Becker of the Max Planck Institute for Extraterrestrial Physics stated in 2006 that "even after nearly four decades of research, the theory of how pulsars emit radiation is still in its infancy." ”
The discovery of pulsars allowed astronomers to study something that had never been observed before: neutron stars. Such an object is the only place where the behavior of matter at the nuclear density can be observed (although not directly). In addition, millisecond pulsars can also test general relativity under strong gravitational field conditions. Here are some specific application scenarios:
1.Star Chart: The Pulsar map is included in the metal plates of the two Pioneers and the Voyager gold record. They show the Sun's position relative to the 14 pulsars, which are identified by the unique timing of their electromagnetic pulses, so our position in space and time can be calculated by potential extraterrestrial intelligence. Since pulsars emit very regular pulses of radio waves, their radio transmissions do not require daily correction. In addition, pulsar positioning can be used to independently create a spacecraft navigation system or in combination with satellite navigation.
The relative position of the Sun to the center of the Milky Way, as well as the 14 pulsars and their periods, are shown on the metal sheet of the Vanguard probe.
2.Pulsar Navigation:X-ray pulsar-based navigation and timing (XN**) or pulsar navigation for short is a navigation technique that uses periodic X-ray signals emitted by pulsars to determine the position of an aircraft, such as a deep-space spacecraft. Vehicles using XN** compare the received X-ray signal with a database of known pulsar frequencies and locations. Similar to GPS, this comparison allows the vehicle to accurately calculate its position (5 km). The advantage of using X-ray signals over radio waves is that X-ray telescopes can be made smaller and lighter. Experimental demonstrations have been reported in 2018.
3.Precision Clock:In general, the regularity of pulsar emissions is not comparable to the stability of atomic clocks. They can still be used as external references. For example, J0437 4715 has a period of 0005757451936712637 s with an error of 17×10−17 s。This stability allows millisecond pulsars to be used to establish ephemeris time or to construct pulsar clocks. Timing noise is the name given to rotational irregularities observed in all pulsars. This timing noise can be observed by random drift of pulse frequency or phase. It is unclear whether timing noise is related to pulsar glitches. According to a study published in 2023, the timing noise observed in pulsars is thought to be caused by background gravitational waves, or it can also be caused by random fluctuations in the pulsar's internal (associated with the presence of superfluids or turbulence) and external (due to magnetospheric activity).
4.Interstellar Medium Detector:Pulsar radiation travels through the interstellar medium (ISM) before reaching Earth. Free electrons in the warm (8000 K) ionizing composition of the ISM and H II regions affect radiation in two main ways. The resulting changes in pulsar radiation provide important probes for the ISM itself.
In addition, density inhomogeneity in ISM causes scattering of radio waves emitted by pulsars. The resulting scintillation of radio waves – the same effect as the stellar scintillation in visible light caused by changes in density in the Earth's atmosphere – can be used to reconstruct information about changes at the small and medium scales in the ISM. Due to the high velocity of many pulsars (up to hundreds of kilometers per second), a single pulsar scans the ISM quickly, causing the scintillation pattern to change over a timescale of a few minutes. The exact cause of these density inhomogeneities remains an open question, with possible explanations including turbulence and current sheets.
5.Space-Time Probe:Pulsars orbiting in curved space-time around the supermassive black hole SGR A* at the center of the Milky Way can act as gravitational detectors in a strong field state. [60] The arrival time of a pulse is affected by special and general relativity Doppler shifts, as well as the complex path of radio waves through strongly curved space-time around the black hole. In order to measure the effects of general relativity with existing instruments, pulsars with orbital periods of less than about 10 years need to be discovered[60] The orbital distance of such pulsars is sgr a* 001 pc or less. The search is ongoing;Currently, five pulsars are known to be located within 100 PCs of SGR A*.
6.Space-Time Probe:Pulsars orbiting in curved space-time around the supermassive black hole SGR A* (Cygnus A) at the center of the Milky Way can act as gravitational detectors in a strong field state. The arrival time of a pulse is affected by the special and general relativity Doppler shifts, as well as the complex path of radio waves through strongly curved space-time around the black hole. In order to measure the effects of general relativity with existing instruments, pulsars with orbital periods of less than about 10 years need to be discoveredThe orbital distance of such pulsars is sgr a* 001 pc or less. The search is ongoing;Currently, five pulsars are known to be located within 100 PCs of SGR A*.
7.Gravitational Wave Detector:There are currently three international organizations that use pulsars to find gravitational waves. In Europe, there is the European Pulsar Timing Array (EPTA);Australia has the Parkes Pulsar Timing Array (PPTA);Canada and the United States have the North American Nanohertz Gravitational Wave Observatory (NANOGR**), which together form the International Pulsar Timing Array (IPTA). Pulses from millisecond pulsars (MSPs) are used as galactic clock systems. Interference from clocks can be measured on Earth. Perturbations from passing gravitational waves will have specific characteristics throughout the pulsar group and can therefore be detected.
Gamma-ray pulsars detected by the Fermi Gamma-ray Space Telescope.
Jocelyn Bell initially observed signals from the first pulsar, a newly commissioned radio telescope she helped build, while analyzing data recorded on August 6, 1967. Initially, her supervisor and telescope developer, Antony Hewish, dismissed it as radio interference, and the fact that the signal always appeared in the same declination and right ascension, quickly ruled out the possibility of terrestrial **. On November 28, 1967, Bell and Heush used a fast bar chart recorder to parse the signal into a series of pulses, each 1337 seconds evenly spaced once. Celestial bodies of this nature have never been observed before. On December 21, Bell discovered a second pulsar, disproving speculation that these pulsars could be signals emitted to Earth by extraterrestrial intelligence.
Diagram of Jocelyn Bell's first identification of pulsar evidence, on display at the University of Cambridge Library.
Joseph Hooton Taylor, Jr., 1974and Russell Hulse discovered a pulsar for the first time in a binary star system, PSR B1913+16. The pulsar orbits another neutron star with an orbital period of only eight hours. According to Einstein's theory of general relativity**, this system should emit strong gravitational radiation, causing the orbit to shrink as it loses orbital energy. Observations of pulsars quickly confirmed this, providing the first evidence of the existence of gravitational waves. As of 2010, observations of this pulsar continue to conform to the general theory of relativity. In 1993, Taylor and Hulse were awarded the Nobel Prize in Physics for their discovery of the pulsar.
The first radio pulsar, CP 1919 (now known as PSR B1919+21), was discovered in 1967 with a pulse period of 1337 seconds with a pulse width of 004 seconds.
The orbit of the first double pulsar, PSR 1913+16, is decaying due to the emission of gravitational radiation at the precise rate of general relativity**.
The brightest radio pulsar, the sail pulsar.
The first millisecond pulsar, PSR B1937+21
The brightest millisecond pulsar, PSR J0437 4715
The first X-ray pulsar, CEN X-3
The first accretion millisecond X-ray pulsar, SAX J18084−3658
The first pulsar with a planet, PSR B1257+12
The first pulsar to be observed under the influence of an asteroid: PSR J0738 4042
The first binary pulsar binary star system, PSR J0737 3039
The shortest period pulsar is PSR J1748 2446AD with a period of about 00014 seconds or about 14 ms (716 times per second).
The longest period neutron star pulsar, PSR J0901-4046, has a period of 759 seconds.
The pulsar with the longest period is 1182 seconds, and one of only two known white dwarf pulsars, AR Scorpii.
The first white dwarf pulsar, AE Aquarii.
The most stable pulsar with the period, PSR J0437 4715
The first millisecond pulsar with 2 sidereal masses, PSR J0337+1715
PSR J1841 0500, stop pulsing for 580 days. One of only two pulsars known to have stopped pulsing for more than a few minutes.
PSR B1931+24, with one cycle. It pulses for about a week and stops pulsing for about a month. [66] One of only two pulsars known to have stopped pulsing for more than a few minutes.
swift j0243.6+6124 strongest magnetic pulsar, 16×10^13 g.
PSR J0952-0607 heaviest pulsar 235±0.17 m☉。
PSR J1903+0327, one piece is about 2A pulsar of 15 milliseconds, found in a highly eccentric binary star system with a sun-like star.
PSR J2007+2722 is a 40The 8 Hz solitary pulsar was the first pulsar discovered by volunteers based on data collected in February 2007 and analyzed by the Distributed Computing Project einstein@home.
PSR J1311 3430, the first millisecond pulsar discovered by gamma-ray pulsations, is also part of a binary star system with the shortest orbital period.