
A pulsar (''pulsating star, on the model of
quasar
A quasar ( ) is an extremely Luminosity, luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by accretion onto a supermassive black hole with a mass rangi ...
'') is a highly magnetized rotating
neutron star
A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
that emits beams of
electromagnetic radiation
In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...
out of its
magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Earth (similar to the way a
lighthouse
A lighthouse is a tower, building, or other type of physical structure designed to emit light from a system of lamps and lens (optics), lenses and to serve as a beacon for navigational aid for maritime pilots at sea or on inland waterways.
Ligh ...
can be seen only when the light is pointed in the direction of an observer), and is responsible for the pulsed appearance of emission. Neutron stars are very
dense
Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be use ...
and have short, regular rotational
periods. This produces a very precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar. Pulsars are one of the candidates for the source of
ultra-high-energy cosmic ray
In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray with an energy greater than 1 EeV (1018 electronvolts, approximately 0.16 joules), far beyond both the rest mass and energies typical of other cosmic ray part ...
s (see also
centrifugal mechanism of acceleration).
Pulsars’ highly regular pulses make them very useful tools for astronomers. For example, observations of a pulsar in a
binary neutron star system were used to indirectly confirm the existence of
gravitational radiation
Gravitational waves are oscillations of the gravitational field that travel through space at the speed of light; they are generated by the relative motion of gravitating masses. They were proposed by Oliver Heaviside in 1893 and then later by ...
. The first
extrasolar planets were discovered in 1992 around a pulsar, specifically
PSR B1257+12. In 1983,
certain types of pulsars were detected that, at that time, exceeded the accuracy of
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s in
keeping time.
History of observation
Discovery
Signals from the
first discovered pulsar were initially observed by
Jocelyn Bell while analyzing data recorded on August 6, 1967, from a
newly commissioned radio telescope that she helped build. Initially dismissed as
radio interference by her supervisor and developer of the telescope,
Antony Hewish
Antony Hewish (11 May 1924 – 13 September 2021) was a British radio astronomer who won the Nobel Prize for Physics in 1974 (together with fellow radio-astronomer Martin Ryle) for his role in the discovery of pulsars. He was also awarded the ...
,
the fact that the signals always appeared at the same
declination
In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or ...
and
right ascension
Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...
soon ruled out a terrestrial source.
[ On November 28, 1967, Bell and Hewish using a fast strip ]chart recorder
A chart recorder is an electromechanical device that records an electrical or mechanical input trend onto a piece of paper (the chart). Chart recorders may record several inputs using different color pens and may record onto strip charts or circu ...
resolved the signals as a series of pulses, evenly spaced every 1.337 seconds. No astronomical object of this nature had ever been observed before. On December 21, Bell discovered a second pulsar, quashing speculation that these might be signals beamed at earth from an extraterrestrial intelligence
Extraterrestrial intelligence (ETI) refers to hypothetical intelligent extraterrestrial life. No such life has ever been verifiably observed to exist. The question of whether other inhabited worlds might exist has been debated since ancient histo ...
.
When observations with another telescope confirmed the emission, it eliminated any sort of instrumental effects. At this point, Bell said of herself and Hewish that "we did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem—if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly?"[ (after-dinner speech with the title of ''Petit Four'' given at the Eighth Texas Symposium on Relativistic Astrophysics; first published in ''Annals of the New York Academy of Science'', vol. 302, pp. 685–689, Dec. 1977).
] Even so, they nicknamed the signal ''LGM-1'', for " little green men" (a playful name for intelligent beings of extraterrestrial origin).
It was not until a second pulsating source was discovered in a different part of the sky that the "LGM hypothesis" was entirely abandoned. Their pulsar was later dubbed CP 1919, and is now known by a number of designators including PSR B1919+21 and PSR J1921+2153. Although CP 1919 emits in radio wavelengths, pulsars have subsequently been found to emit in visible light, X-ray
An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
, and gamma ray
A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...
wavelengths.
The word "pulsar" first appeared in print in 1968:
The existence of neutron stars was first proposed by Walter Baade and Fritz Zwicky in 1934, when they argued that a small, dense star consisting primarily of neutrons would result from a supernova
A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...
. Based on the idea of magnetic flux conservation from magnetic main sequence stars, Lodewijk Woltjer proposed in 1964 that such neutron stars might contain magnetic fields as large as 1014 to 1016 gauss
Johann Carl Friedrich Gauss (; ; ; 30 April 177723 February 1855) was a German mathematician, astronomer, Geodesy, geodesist, and physicist, who contributed to many fields in mathematics and science. He was director of the Göttingen Observat ...
(=1010 to 1012 tesla). In 1967, shortly before the discovery of pulsars, Franco Pacini suggested that a rotating neutron star with a magnetic field would emit radiation, and even noted that such energy could be pumped into a supernova remnant
A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...
around a neutron star, such as the Crab Nebula
The Crab Nebula (catalogue designations M1, NGC 1952, Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus (constellation), Taurus. The common name comes from a drawing that somewhat resembled a crab with arm ...
. After the discovery of the first pulsar, Thomas Gold
Thomas Gold (May 22, 1920 – June 22, 2004) was an Austrian-born astrophysicist, who also held British and American citizenship. He was a professor of astronomy at Cornell University, a member of the U.S. National Academy of Sciences, and a Fe ...
independently suggested a rotating neutron star model similar to that of Pacini, and explicitly argued that this model could explain the pulsed radiation observed by Bell Burnell and Hewish.
In 1968, Richard V. E. Lovelace with collaborators discovered period ms of the Crab Nebula pulsar using Arecibo Observatory
The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
.
The discovery of the Crab pulsar
The Crab Pulsar (PSR B0531+21 or Baade's Star) is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054. provided confirmation of the rotating neutron star model of pulsars. The Crab pulsar 33-millisecond
A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units equal to one thousandth (0.001 or 10−3 or 1/1000) of a second or 1000 microseconds.
A millisecond is to one second, as one second i ...
pulse period was too short to be consistent with other proposed models for pulsar emission. Moreover, the Crab pulsar is so named because it is located at the center of the Crab Nebula, consistent with the 1933 prediction of Baade and Zwicky.[Lyne & Graham-Smith, pp. 1–7 (1998).]
In 1974, Antony Hewish and Martin Ryle, who had developed revolutionary radio telescope
A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...
s, became the first astronomers to be awarded the Nobel Prize in Physics
The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...
, with the Royal Swedish Academy of Sciences
The Royal Swedish Academy of Sciences () is one of the Swedish Royal Academies, royal academies of Sweden. Founded on 2 June 1739, it is an independent, non-governmental scientific organization that takes special responsibility for promoting nat ...
noting that Hewish played a "decisive role in the discovery of pulsars". Considerable controversy is associated with the fact that Hewish was awarded the prize while Bell, who made the initial discovery while she was his PhD student, was not. Bell claims no bitterness upon this point, supporting the decision of the Nobel prize committee.
Milestones
In 1974, Joseph Hooton Taylor, Jr. and Russell Hulse discovered for the first time a pulsar in a binary system
A binary system is a system of two astronomical bodies of the same kind that are comparable in size. Definitions vary, but typically require the center of mass to be located outside of either object. (See animated examples.)
The most common ki ...
of stars, PSR B1913+16. This pulsar orbits another neutron star with an orbital period of just eight hours. Einstein
Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...
's theory of general relativity
General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...
predicts that this system should emit strong gravitational radiation
Gravitational waves are oscillations of the gravitational field that travel through space at the speed of light; they are generated by the relative motion of gravitating masses. They were proposed by Oliver Heaviside in 1893 and then later by ...
, causing the orbit to continually contract as it loses orbital energy
In the gravitational two-body problem, the specific orbital energy \varepsilon (or specific ''vis-viva'' energy) of two orbiting bodies is the constant quotient of their mechanical energy (the sum of their mutual potential energy, \varepsilon_p ...
. Observations of the pulsar soon confirmed this prediction, providing the first ever evidence of the existence of gravitational waves. As of 2010, observations of this pulsar continue to agree with general relativity. In 1993, the Nobel Prize in Physics was awarded to Taylor and Hulse for the discovery of this pulsar.
In 1982, Don Backer
Donald Charles Backer (November 9, 1943 – July 25, 2010) was an American astrophysicist who primarily worked in radio astronomy. Backer made important contributions to the understanding and study of pulsars (including the discovery of the first ...
led a group that discovered PSR B1937+21
PSR may refer to:
Organizations
* Pacific School of Religion, Berkeley, California, US
* Palestinian Center for Policy and Survey Research
* Payment Systems Regulator in the United Kingdom
* Physicians for Social Responsibility, US
Political par ...
, a pulsar with a rotation period of just about 1.6 milliseconds (38,500 rpm
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines.
One revolution per minute is equivalent to hertz.
Standards
ISO 80000-3:2019 def ...
). Observations soon revealed that its magnetic field was much weaker than ordinary pulsars, while further discoveries cemented the idea that a new class of object, the "millisecond pulsar
A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio pulsar, radio, X-ray pulsar, X-ray, and gamma ray portions of the electromagnetic spectrum. The leadi ...
s" (MSPs) had been found. MSPs are believed to be the end product of X-ray binaries. Owing to their extraordinarily rapid and stable rotation, MSPs can be used by astronomers
An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, moons, comets and galaxies – in either observ ...
as clocks rivaling the stability of the best atomic clocks
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
on Earth. Factors affecting the arrival time of pulses at Earth by more than a few hundred nanosecond
A nanosecond (ns) is a unit of time in the International System of Units (SI) equal to one billionth of a second, that is, of a second, or seconds.
The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit (e ...
s can be easily detected and used to make precise measurements. Physical parameters accessible through pulsar timing include the 3D position of the pulsar, its proper motion
Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...
, the electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
content of the interstellar medium
The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...
along the propagation path, the orbital parameters of any binary companion, the pulsar rotation period and its evolution with time. (These are computed from the raw timing data by Tempo
In musical terminology, tempo (Italian for 'time'; plural 'tempos', or from the Italian plural), measured in beats per minute, is the speed or pace of a given musical composition, composition, and is often also an indication of the composition ...
, a computer program specialized for this task.) After these factors have been taken into account, deviations between the observed arrival times and predictions made using these parameters can be found and attributed to one of three possibilities: intrinsic variations in the spin period of the pulsar, errors in the realization of Terrestrial Time
Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth.
For example, the Astronomical Almanac uses ...
against which arrival times were measured, or the presence of background gravitational waves. Scientists are currently attempting to resolve these possibilities by comparing the deviations seen between several different pulsars, forming what is known as a pulsar timing array
A pulsar timing array (PTA) is a set of galactic pulsars that is monitored and analyzed to search for correlated signatures in the pulse arrival times on Earth. As such, they are galactic-sized detectors. Although there are many applications for p ...
. The goal of these efforts is to develop a pulsar-based time standard
A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of cu ...
precise enough to make the first ever direct detection of gravitational waves. In 2006, a team of astronomers at LANL proposed a model to predict the likely date of pulsar glitches with observational data from the Rossi X-ray Timing Explorer. They used observations of the pulsar PSR J0537−6910, that is known to be a quasi-periodic glitching pulsar. However, no general scheme for glitch forecast is known to date.
In 1992, Aleksander Wolszczan discovered the first extrasolar planet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first detect ...
s around PSR B1257+12. This discovery presented important evidence concerning the widespread existence of planets outside the Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
, although it is very unlikely that any life form
The following outline is provided as an overview of and topical guide to life forms:
A life form (also spelled life-form or lifeform) is an entity that is living, such as plants (flora), animals (fauna), and fungi ( funga). It is estimated tha ...
could survive in the environment of intense radiation near a pulsar.
Pulsar-like white dwarfs
White dwarf
A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...
s can also act as pulsars. Because the moment of inertia
The moment of inertia, otherwise known as the mass moment of inertia, angular/rotational mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is defined relatively to a rotational axis. It is the ratio between ...
of a white dwarf is much higher than that of a neutron star, the white-dwarf pulsars rotate once every several minutes, far slower than neutron-star pulsars.
By 2024, three pulsar-like white dwarfs have been identified.
* In 1998, Nazar Ikhsanov showed that a white dwarf in the binary system AE Aquarii acts like a radio pulsar. The confirmation of the pulsar-like properties of the white dwarf in AE Aquarii was provided in 2008 by a discovery of X-ray pulsations, which showed that this white dwarf acts not only as a radio pulsar, but also as an X-ray pulsar
X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity. The X-ray periods range from as little as a fraction of a second to as much as several m ...
.
* In 2016, a white dwarf in the binary system AR Scorpii was identified as a pulsar (it is often mistakenly called the first discovered pulsar-like white dwarf). The system displays strong pulsations from ultraviolet to radio wavelengths, powered by the spin-down of the strongly magnetized white dwarf.
* In 2023, it was suggested that the white dwarf eRASSU J191213.9−441044 acts as a pulsar both in radio and X-rays.
There is an alternative tentative explanation of the pulsar-like properties of these white dwarfs. In 2019, the properties of pulsars have been explained using a numerical magnetohydrodynamic model explaining was developed at Cornell University
Cornell University is a Private university, private Ivy League research university based in Ithaca, New York, United States. The university was co-founded by American philanthropist Ezra Cornell and historian and educator Andrew Dickson W ...
. According to this model, AE Aqr is an intermediate polar-type star, where the magnetic field is relatively weak and an accretion disc
An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...
may form around the white dwarf. The star is in the propeller regime, and many of its observational properties are determined by the disc-magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
interaction. A similar model for eRASSU J191213.9−441044 is supported by the results of its observations at ultraviolet wave lengths, which showed that its magnetic field strength does not exceed 50 MG.
Nomenclature
Initially pulsars were named with letters of the discovering observatory followed by their right ascension
Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the equinox (celestial coordinates), March equinox to the (hour circle of the) point in questio ...
(e.g. CP 1919). As more pulsars were discovered, the letter code became unwieldy, and so the convention then arose of using the letters PSR (Pulsating Source of Radio) followed by the pulsar's right ascension and degrees of declination
In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or ...
(e.g. PSR 0531+21) and sometimes declination to a tenth of a degree (e.g. PSR 1913+16.7). Pulsars appearing very close together sometimes have letters appended (e.g. PSR 0021−72C and PSR 0021−72D).
The modern convention prefixes the older numbers with a B (e.g. PSR B1919+21), with the B meaning the coordinates are for the 1950.0 epoch. All new pulsars have a J indicating 2000.0 coordinates and also have declination including minutes (e.g. PSR J1921+2153). Pulsars that were discovered before 1993 tend to retain their B names rather than use their J names (e.g. PSR J1921+2153 is more commonly known as PSR B1919+21). Recently discovered pulsars only have a J name (e.g. PSR J0437−4715). All pulsars have a J name that provides more precise coordinates of its location in the sky.
Formation, mechanism, turn off
The events leading to the formation of a pulsar begin when the core of a massive star is compressed during a supernova
A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...
, which collapses into a neutron star. The neutron star retains most of its angular momentum
Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...
, and since it has only a tiny fraction of its progenitor's radius, it is formed with very high rotation speed. A beam of radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes:
* ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
is emitted along the magnetic axis of the pulsar, which spins along with the rotation of the neutron star. The magnetic axis of the pulsar determines the direction of the electromagnetic beam, with the magnetic axis not necessarily being the same as its rotational axis. This misalignment causes the beam to be seen once for every rotation of the neutron star, which leads to the "pulsed" nature of its appearance.
In rotation-powered pulsars, the beam is the result of the rotational energy
Rotational energy or angular kinetic energy is kinetic energy due to the rotation of an object and is part of its total kinetic energy. Looking at rotational energy separately around an object's axis of rotation, the following dependence on the ob ...
of the neutron star, which generates an electrical field and very strong magnetic field, resulting in the acceleration of protons and electrons on the star surface and the creation of an electromagnetic beam emanating from the poles of the magnetic field. Observations by NICER of PSR J0030+0451 indicate that both beams originate from hotspots located on the south pole and that there may be more than two such hotspots on that star. This rotation slows down over time as electromagnetic
In physics, electromagnetism is an interaction that occurs between particles with electric charge via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...
power is emitted. When a pulsar's spin period slows down sufficiently, the radio pulsar mechanism is believed to turn off (the so-called "death line"). This turn-off seems to take place after about 10–100 million years, which means of all the neutron stars born in the 13.6-billion-year age of the universe, around 99% no longer pulsate.
Though the general picture of pulsars as rapidly rotating neutron stars is widely accepted, Werner Becker of the Max Planck Institute for Extraterrestrial Physics said in 2006, "The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work."
Categories
Three distinct classes of pulsars are currently known to astronomer
An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
s, according to the source of the power of the electromagnetic radiation:
* rotation-powered pulsars, where the loss of rotational energy of the star provides the power,
* accretion-powered pulsars (accounting for most but not all X-ray pulsar
X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity. The X-ray periods range from as little as a fraction of a second to as much as several m ...
s), where the gravitational potential energy
Gravitational energy or gravitational potential energy is the potential energy an object with mass has due to the gravitational potential of its position in a gravitational field. Mathematically, it is the minimum Work (physics), mechanical work t ...
of accreted matter is the power source (producing X-rays that are observable from the Earth),
* magnetar
A magnetar is a type of neutron star with an extremely powerful magnetic field (~109 to 1011 T, ~1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.Ward; Br ...
s, where the decay of an extremely strong magnetic field
A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
provides the electromagnetic power.
Although all three classes of objects are neutron stars, their observable behavior and the underlying physics are quite different. There are, however, some connections. For example, X-ray pulsar
X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity. The X-ray periods range from as little as a fraction of a second to as much as several m ...
s are probably old rotationally-powered pulsars that have already lost most of their energy, and have only become visible again after their binary companions had expanded and begun transferring matter on to the neutron star.
The process of accretion can, in turn, transfer enough angular momentum
Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...
to the neutron star to "recycle" it as a rotation-powered millisecond pulsar
A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio pulsar, radio, X-ray pulsar, X-ray, and gamma ray portions of the electromagnetic spectrum. The leadi ...
. As this matter lands on the neutron star, it is thought to "bury" the magnetic field of the neutron star (although the details are unclear), leaving millisecond pulsars with magnetic fields 1000–10,000 times weaker than average pulsars. This low magnetic field is less effective at slowing the pulsar's rotation, so millisecond pulsars live for billions of years, making them the oldest known pulsars. Millisecond pulsars are seen in globular clusters, which stopped forming neutron stars billions of years ago.
Of interest to the study of the state of the matter in a neutron star are the '' glitches'' observed in the rotation velocity of the neutron star. This velocity decreases slowly but steadily, except for an occasional sudden variation known as "glitch". One model put forward to explain these glitches is that they are the result of " starquakes" that adjust the crust of the neutron star. Models where the glitch is due to a decoupling of the possibly superconducting
Superconductivity is a set of physical properties observed in superconductors: materials where electrical resistance vanishes and magnetic fields are expelled from the material. Unlike an ordinary metallic conductor, whose resistance decreases g ...
interior of the star have also been advanced. In both cases, the star's moment of inertia
The moment of inertia, otherwise known as the mass moment of inertia, angular/rotational mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is defined relatively to a rotational axis. It is the ratio between ...
changes, but its angular momentum
Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...
does not, resulting in a change in rotation rate.
Disrupted recycled pulsar
When two massive stars are born close together from the same cloud of gas, they can form a binary system and orbit each other from birth. If those two stars are at least a few times as massive as the Sun, their lives will both end in supernova explosions. The more massive star explodes first, leaving behind a neutron star. If the explosion does not kick the second star away, the binary system survives. The neutron star can now be visible as a radio pulsar, and it slowly loses energy and spins down. Later, the second star can swell up, allowing the neutron star to suck up its matter. The matter falling onto the neutron star spins it up and reduces its magnetic field.
This is called "recycling" because it returns the neutron star to a quickly-spinning state. Finally, the second star also explodes in a supernova, producing another neutron star. If this second explosion also fails to disrupt the binary, a double neutron star (neutron star binary) is formed. Otherwise, the spun-up neutron star is left with no companion and becomes a "disrupted recycled pulsar", spinning between a few and 50 times per second.
Applications
The discovery of pulsars allowed astronomers to study an object never observed before, the neutron star
A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...
. This kind of object is the only place where the behavior of matter at nuclear
Nuclear may refer to:
Physics
Relating to the nucleus of the atom:
*Nuclear engineering
*Nuclear physics
*Nuclear power
*Nuclear reactor
*Nuclear weapon
*Nuclear medicine
*Radiation therapy
*Nuclear warfare
Mathematics
* Nuclear space
*Nuclear ...
density can be observed (though not directly). Also, millisecond pulsars have allowed a test of general relativity
General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...
in conditions of an intense gravitational field.
Maps
Pulsar maps have been included on the two ''Pioneer'' plaques as well as the ''Voyager'' Golden Record. They show the position of the Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
, relative to 14 pulsars, which are identified by the unique timing of their electromagnetic pulses, so that Earth's position both in space and time can be calculated by potential extraterrestrial intelligence. Because pulsars are emitting very regular pulses of radio waves, their radio transmissions do not require daily corrections. Moreover, pulsar positioning could create a spacecraft navigation system independently, or be used in conjunction with satellite navigation.
Pulsar navigation
''X-ray pulsar-based navigation and timing (XNAV)'' or simply ''pulsar navigation'' is a navigation technique whereby the periodic X-ray
An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
signals emitted from pulsars
A pulsar (''pulsating star, on the model of quasar'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointin ...
are used to determine the location of a vehicle, such as a spacecraft in deep space. A vehicle using XNAV would compare received X-ray signals with a database of known pulsar frequencies and locations. Similar to GPS, this comparison would allow the vehicle to calculate its position accurately (±5 km). The advantage of using X-ray signals over radio waves
Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths ...
is that X-ray telescopes
An X-ray telescope (XRT) is a telescope that is designed to observe remote objects in the X-ray spectrum. X-rays are absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, ...
can be made smaller and lighter. Experimental demonstrations have been reported in 2018.
Precise clocks
Generally, the regularity of pulsar emission does not rival the stability of atomic clocks
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
. They can still be used as external reference. For example, J0437−4715 has a period of s with an error of .
This stability allows millisecond pulsars to be used in establishing ephemeris time
or in building pulsar clocks.
''Timing noise'' is the name for rotational irregularities observed in all pulsars. This timing noise is observable as random wandering in the pulse frequency or phase. It is unknown whether timing noise is related to pulsar glitches. According to a study published in 2023, the timing noise observed in pulsars is believed to be caused by background gravitational wave
Gravitational waves are oscillations of the gravitational field that Wave propagation, travel through space at the speed of light; they are generated by the relative motion of gravity, gravitating masses. They were proposed by Oliver Heaviside i ...
s. Alternatively, it may be caused by stochastic fluctuations in both the internal (related to the presence of superfluids or turbulence) and external (due to magnetospheric activity) torques in a pulsar.
Probes of the interstellar medium
The radiation from pulsars passes through the interstellar medium
The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...
(ISM) before reaching Earth. Free electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s in the warm (8000 K), ionized component of the ISM and H II region
An H II region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light year ...
s affect the radiation in two primary ways. The resulting changes to the pulsar's radiation provide an important probe of the ISM itself.
Because of the dispersive nature of the interstellar plasma, lower-frequency radio waves travel through the medium slower than higher-frequency radio waves. The resulting delay in the arrival of pulses at a range of frequencies is directly measurable as the '' dispersion measure'' of the pulse. The dispersion measure is the total column density of free electrons between the observer and the pulsar:
:
where is the distance from the pulsar to the observer, and is the electron density of the ISM. The dispersion measure is used to construct models of the free electron distribution in the Milky Way
The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
.
Additionally, density inhomogeneities in the ISM cause scattering
In physics, scattering is a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiat ...
of the radio waves from the pulsar. The resulting scintillation of the radio waves—the same effect as the twinkling of a star in visible light
Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm ...
due to density variations in the Earth's atmosphere—can be used to reconstruct information about the small scale variations in the ISM. Due to the high velocity (up to several hundred km/s) of many pulsars, a single pulsar scans the ISM rapidly, which results in changing scintillation patterns over timescales of a few minutes. The exact cause of these density inhomogeneities remains an open question, with possible explanations ranging from turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...
to current sheets.
Probes of space-time
Pulsars orbiting within the curved space-time
In physics, spacetime, also called the space-time continuum, is a mathematical model that fuses the three-dimensional space, three dimensions of space and the one dimension of time into a single four-dimensional continuum (measurement), continu ...
around Sgr A*, the supermassive black hole
A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (). Black holes are a class of astronomical ...
at the center of the Milky Way, could serve as probes of gravity in the strong-field regime. Arrival times of the pulses would be affected by special- and general-relativistic Doppler shift
The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described t ...
s and by the complicated paths that the radio waves would travel through the strongly curved space-time around the black hole. In order for the effects of general relativity to be measurable with current instruments, pulsars with orbital periods less than about 10 years would need to be discovered;[ such pulsars would orbit at distances inside 0.01 pc from Sgr A*. Searches are currently underway; at present, five pulsars are known to lie within 100 pc from Sgr A*.
]
Gravitational wave detectors
There are four consortia around the world which use pulsars to search for gravitational waves
Gravitational waves are oscillations of the gravitational field that travel through space at the speed of light; they are generated by the relative motion of gravitating masses. They were proposed by Oliver Heaviside in 1893 and then later by H ...
: the European Pulsar Timing Array (EPTA) in Europe, the Parkes Pulsar Timing Array
Parkes Observatory is a radio astronomy observatory, located north of the town of Parkes, New South Wales, Australia. It hosts Murriyang, the 64 m CSIRO Parkes Radio Telescope also known as "The Dish", along with two smaller radio telescopes. T ...
(PPTA) in Australia, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) in Canada and the US, and the Indian Pulsar Timing Array (InPTA) in India. Together, the consortia form the International Pulsar Timing Array
The International Pulsar Timing Array (IPTA) is a multi-institutional, multi-telescope collaboration comprising the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the Parkes Pulsar ...
(IPTA). The pulses from Millisecond Pulsars (MSPs) are used as a system of galactic clocks. Disturbances in the clocks will be measurable at Earth. A disturbance from a passing gravitational wave will have a particular signature across the ensemble of pulsars, and will be thus detected.
Significant pulsars
The pulsars listed here were either the first discovered of its type, or represent an extreme of some type among the known pulsar population, such as having the shortest measured period.
* The first radio pulsar "CP 1919" (now known as PSR B1919+21), with a pulse period of 1.337 seconds and a pulse width of 0.04-second, was discovered in 1967.[Hewish, A., Bell, S. J., et al.]
Observation of a Rapidly Pulsating Radio Source
". ''Nature'', Volume 217, 1968 (pp. 709–713).
* The first binary pulsar, PSR 1913+16, whose orbit is decaying due to the emission of gravitational radiation
Gravitational waves are oscillations of the gravitational field that travel through space at the speed of light; they are generated by the relative motion of gravitating masses. They were proposed by Oliver Heaviside in 1893 and then later by ...
at the exact rate predicted by general relativity
General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...
.
* The brightest radio pulsar, the Vela Pulsar.
* The first millisecond pulsar
A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio pulsar, radio, X-ray pulsar, X-ray, and gamma ray portions of the electromagnetic spectrum. The leadi ...
, PSR B1937+21
PSR may refer to:
Organizations
* Pacific School of Religion, Berkeley, California, US
* Palestinian Center for Policy and Survey Research
* Payment Systems Regulator in the United Kingdom
* Physicians for Social Responsibility, US
Political par ...
* The brightest millisecond pulsar
A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio pulsar, radio, X-ray pulsar, X-ray, and gamma ray portions of the electromagnetic spectrum. The leadi ...
, PSR J0437−4715
* The first X-ray pulsar
X-ray pulsars or accretion-powered pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity. The X-ray periods range from as little as a fraction of a second to as much as several m ...
, Cen X-3
* The first accreting millisecond X-ray pulsar, SAX J1808.4−3658
The first accreting millisecond pulsar discovered in 1998 by the Italian-Dutch BeppoSAX satellite, SAX J1808.4−3658 revealed X-ray pulsations at the 401 Hz neutron star spin frequency when it was observed during a subsequent outburst in 19 ...
* The first pulsar with planets, PSR B1257+12
* The first pulsar observed to have been affected by asteroids
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
: PSR J0738−4042
PSR J0738−4042 is the first pulsar observed to have been affected by asteroids.
It was originally discovered in 1968. In 2013 scientists at the University of Oxford, Hartebeesthoek Observatory and CSIRO announced that they had observed ...
* The first double pulsar binary system, PSR J0737−3039
* The shortest period pulsar, PSR J1748−2446ad
, -
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, - style="vertical-align: top;"
, Cosmic distance ladder, Distance
, 18,000 Light-year, Ly
PSR J1748−2446ad is the fastest-spinning pulsar known, at 716 Hertz, Hz (time ...
, with a period of ~0.0014 seconds or ~1.4 milliseconds (716 times a second).
* The longest period neutron star pulsar, PSR J0901-4046, with a period of 75.9 seconds.
* The longest period pulsar, at 118.2 seconds, as well as one of the only known two white dwarf pulsars, AR Scorpii.
* The first white dwarf pulsar AE Aquarii.
* The pulsar with the most stable period, PSR J0437−4715
* The first millisecond pulsar
A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio pulsar, radio, X-ray pulsar, X-ray, and gamma ray portions of the electromagnetic spectrum. The leadi ...
with 2 stellar mass companions, PSR J0337+1715
* PSR J1841−0500, stopped pulsing for 580 days. One of only two pulsars known to have stopped pulsing for more than a few minutes.
* PSR B1931+24, has a cycle. It pulses for about a week and stops pulsing for about a month. One of only two pulsars known to have stopped pulsing for more than a few minutes.
* Swift J0243.6+6124 most magnetic pulsar with .
* PSR J0952-0607 heaviest pulsar with .
* PSR J1903+0327, a ~2.15 ms pulsar discovered to be in a highly eccentric binary star
A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...
system with a Sun-like star.
* PSR J2007+2722
PSR J2007+2722 is a 40.8-hertz isolated pulsar in the Vulpecula constellation, 5.3 kpc (17,000 ly) distant in the plane of the Galaxy, and is most likely a disrupted recycled pulsar (DRP).
J2007+2722 was found on data taken by ...
, a 40.8-hertz 'recycled' isolated pulsar was the first pulsar found by volunteers on data taken in February 2007 and analyzed by distributed computing
Distributed computing is a field of computer science that studies distributed systems, defined as computer systems whose inter-communicating components are located on different networked computers.
The components of a distributed system commu ...
project Einstein@Home
Einstein@Home is a volunteer computing project that searches for signals from spinning neutron stars in data from gravitational-wave detectors, from large radio telescopes, and from a gamma-ray telescope. Neutron stars are detected by their puls ...
.
* PSR J1311–3430, the first millisecond pulsar
A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio pulsar, radio, X-ray pulsar, X-ray, and gamma ray portions of the electromagnetic spectrum. The leadi ...
discovered via gamma-ray pulsations and part of a binary system with the shortest orbital period.
Gallery
Image:Crab Lucky video2.gif, Video – Crab Pulsar
The Crab Pulsar (PSR B0531+21 or Baade's Star) is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054. – bright pulse and interpulse.
Image:Vela Pulsar jet seen by Chandra Observatory.ogv, Video – Vela pulsar – X-ray light.
Image:Artist’s impression video of the exotic binary star system AR Scorpii (video).webm, Video – Artist's impression of AR Scorpii.
See also
References
Further reading
*
*
*
*
*
External links
*
A Pulsar Discovery: First Optical Pulsar
". ''Moments of Discovery'', American Institute of Physics, 2007 (Includes audio and teachers guides).
Discovery of Pulsars
Interview with Jocelyn Bell Burnell. Jodcast, June 2007
Low Quality Version
.
Audio: Cain/Gay – Astronomy Cast. Pulsars – Nov 2009
* Australia National Telescope Facility
Pulsar Catalogue
* Johnston, William Robert.
. Johnston Archive, 22 March 2005.
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