PIA18848-PSRB1509-58-ChandraXRay-WiseIR-20141023

A pulsar (from ''pulsating radio source'') is a highly magnetized rotating

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

that emits beams of

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...

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 lenses and to serve as a beacon for navigational aid, for maritime pilots at sea or on inland waterways. Lighthouses m ...

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 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 pa ...

s. (See also

centrifugal mechanism of acceleration Centrifugal acceleration of astroparticles to relativistic energies might take place in rotating astrophysical objects (see also Fermi acceleration). It is strongly believed that active galactic nuclei and pulsars have rotating magnetospheres, th ...

.) The periods of pulsars make them very useful tools for astronomers. Observations of a pulsar in a binary neutron star system were used to indirectly confirm the existence of

gravitational radiation Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...

. The first

extrasolar planets An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, in ...

were discovered around a pulsar,

PSR B1257+12 PSR B1257+12, previously designated PSR 1257+12, alternatively designated PSR J1300+1240, is a millisecond pulsar located 2,300 light-years from the Sun in the constellation of Virgo, rotating at about 161 times per second (faster tha ...

in 1992. 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 betwe ...

s in

keeping time ''Keeping Time'' is the second studio album by American actor, singer and songwriter Paul Jabara. The album, which was released on Casablanca Records in 1978, includes Jabara's song "Trapped in a Stairway" from the movie '' Thank God It's Frida ...

History of observation

Discovery

Signals from the first discovered pulsar were initially observed by

Jocelyn Bell Dame Susan Jocelyn Bell Burnell (; Bell; born 15 July 1943) is an astrophysicist from Northern Ireland who, as a postgraduate student, discovered the first radio pulsars in 1967. The discovery eventually earned the Nobel Prize in Physics in ...

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,

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 th ...

, developer of the telescope, 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. Declination's angle is measured north or south of th ...

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 March equinox to the ( hour circle of the) point in question above the earth. When pair ...

soon ruled out a terrestrial source. On November 28, 1967, Bell and Hewish using a fast recorder resolved the signals as a series of pulses, evenly spaced every 1.33 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 (often abbreviated ETI) refers to hypothetical intelligent extraterrestrial life. The question of whether other inhabited worlds might exist has been debated since ancient times. The modern form of the concept emerged ...

. 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 Little green men is the stereotypical portrayal of extraterrestrials as little humanoid creatures with green skin and sometimes with antennae on their heads. The term is also sometimes used to describe gremlins, mythical creatures known for cau ...

" (a playful name for intelligent beings of extraterrestrial origin). Chart Showing Radio Signal of First Identified Pulsar

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 PSR B1919+21 is a pulsar with a period of 1.3373 seconds and a pulse width of 0.04 seconds. Discovered by Jocelyn Bell Burnell on 28 November 1967, it is the first discovered radio pulsar. The power and regularity of the signals were ...

, 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, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

, and

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

wavelengths. The word "pulsar" first appeared in print in 1968: Chandra-crab

The existence of neutron stars was first proposed by

Walter Baade Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was a German astronomer who worked in the United States from 1931 to 1959. Biography The son of a teacher, Baade finished school in 1912. He then studied maths, physics and astr ...

and

Fritz Zwicky Fritz Zwicky (; ; February 14, 1898 – February 8, 1974) was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical an ...

in 1934, when they argued that a small, dense star consisting primarily of neutrons would result from a

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...

. Based on the idea of magnetic flux conservation from magnetic main sequence stars,

Lodewijk Woltjer Lodewijk Woltjer (26 April 1930 – 25 August 2019) was an astronomer, and the son of astronomer Jan Woltjer. He studied at the University of Leiden under Jan Oort earning a PhD in astronomy in 1957 with a thesis on the Crab Nebula. This was foll ...

proposed in 1964 that such neutron stars might contain magnetic fields as large as 10¹⁴ to 10¹⁶

gauss Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes refer ...

es (=10¹⁰ to 10¹² teslas). 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. The common name comes from William Parsons, 3rd Earl of Rosse, who observed the object in 1842 u ...

. After the discovery of the first pulsar,

Thomas Gold Thomas Gold (May 22, 1920 – June 22, 2004) was an Austrian-born American astrophysicist, a professor of astronomy at Cornell University, a member of the U.S. National Academy of Sciences, and a Fellow of the Royal Society (London). Gold was ...

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

P \approx 33

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 ...

.''Crab nebula pulsar NP 0532''
1969, J. M. Comella, H. D. Craft, R. V. E. Lovelace, J. M. Sutton, G. L. Tyler Nature 221 (5179), 453–454.''Digital Search Methods for Pulsars''
1969, R. V. E. Lovelace, J. M. Sutton, E. E. Salpeter Nature 222 (5190), 231–233. The discovery of the

Crab pulsar The Crab Pulsar (PSR B0531+21) 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.''On the discovery of the period of the Crab Nebula pulsar''
R. V. E. Lovelace and G. L. Tyler 2012, The Observatory, 132, 186. The Crab pulsar 33-

millisecond A millisecond (from '' milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds. A unit of 10 milliseconds may be calle ...

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 Sir Martin Ryle (27 September 1918 – 14 October 1984) was an English radio astronomer who developed revolutionary radio telescope systems (see e.g. aperture synthesis) and used them for accurate location and imaging of weak radio sourc ...

, who had developed revolutionary

radio telescope A radio telescope is a specialized 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 radio frequency ...

s, became the first astronomers to be awarded the

Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...

, with the

Royal Swedish Academy of Sciences The Royal Swedish Academy of Sciences ( sv, Kungliga Vetenskapsakademien) is one of the royal academies of Sweden. Founded on 2 June 1739, it is an independent, non-governmental scientific organization that takes special responsibility for prom ...

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 which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that th ...

PSR B1913+16 PSR may refer to: Organizations * Pacific School of Religion, Berkeley, California, US * Palestinian Center for Policy and Survey Research * Physicians for Social Responsibility, US ;Political parties: * Revolutionary Socialist Party (Portugal) ( ...

. This pulsar orbits another neutron star with an orbital period of just eight hours.

Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born Theoretical physics, theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for d ...

's theory of

general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...

predicts that this system should emit strong

, causing the orbit to continually contract as it loses orbital energy. 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 led a group which discovered PSR B1937+21, a pulsar with a rotation period of just 1.6 milliseconds (38,500 rpm). 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 pulsars" (MSPs) had been found. MSPs are believed to be the end product of

X-ray binaries X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', which ...

. 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 their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either obse ...

as clocks rivaling the stability of the best atomic clocks 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 10 seconds. The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit ( ...

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 the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more distan ...

, the

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...

content of the

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

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, 'time'; plural ''tempos'', or ''tempi'' from the Italian plural) is the speed or pace of a given piece. In classical music, tempo is typically indicated with an instruction at the start of a piece (ofte ...

, 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. 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 o ...

precise enough to make the first ever direct detection of gravitational waves. In June 2006, the astronomer John Middleditch and his team at

LANL Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development laboratories of the United States Department of Energy (DOE), located a short distance northwest of Santa Fe, New Mexico, ...

announced the first prediction of pulsar glitches with observational data from the

Rossi X-ray Timing Explorer The Rossi X-ray Timing Explorer (RXTE) was a NASA satellite that observed the time variation of astronomical X-ray sources, named after physicist Bruno Rossi. The RXTE had three instruments — an All Sky Monitor, the High-Energy X-ray Timing ...

. They used observations of the pulsar PSR J0537−6910. In 1992,

Aleksander Wolszczan Aleksander Wolszczan (born 29 April 1946) is a Polish astronomer. He is the co-discoverer of the first confirmed extrasolar planets and pulsar planets. Early life and education Wolszczan was born on 29 April 1946 in Szczecinek located in pre ...

discovered the first

extrasolar planet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

s around

. This discovery presented important evidence concerning the widespread existence of planets outside the

Solar System The Solar System Capitalization 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 "Solar ...

, although it is very unlikely that any life form could survive in the environment of intense radiation near a pulsar. In 2016, AR Scorpii was identified as the first pulsar in which the compact object is a white dwarf instead of a neutron star. Because its moment of inertia is much higher than that of a neutron star, the white dwarf in this system rotates once every 1.97 minutes, far slower than neutron-star pulsars. The system displays strong pulsations from ultraviolet to radio wavelengths, powered by the spin-down of the strongly magnetized white dwarf.

Nomenclature

Initially pulsars were named with letters of the discovering observatory followed by their

(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

(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

, which collapses into a neutron star. The neutron star retains most of its

angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...

, and since it has only a tiny fraction of its progenitor's radius (and therefore its

moment of inertia The moment of inertia, otherwise known as the mass moment of inertia, angular mass, second moment of mass, or most accurately, rotational inertia, of a rigid body is a quantity that determines the torque needed for a desired angular accele ...

is sharply reduced), 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 through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

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 from the movement of the 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 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. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...

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 The Max Planck Institute for Extraterrestrial Physics is a Max Planck Institute, located in Garching, near Munich, Germany. In 1991 the Max Planck Institute for Physics and Astrophysics split up into the Max Planck Institute for Extraterrestrial P ...

said in 2006, "The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work."

Applications

The discovery of pulsars allowed astronomers to study an object never observed before, the

. 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

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 center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, relative to 14 pulsars, which are identified by the unique timing of their electromagnetic pulses, so that our position both in space and in time can be calculated by potential extraterrestrial intelligences. Because pulsars are emitting very regular pulses of radio waves, its 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

signals emitted from

pulsars A pulsar (from ''pulsating radio source'') 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 pointing toward E ...

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 are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (s ...

is that X-ray telescopes 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. They can still be used as external reference. For example, J0437−4715 has a period of 0.005757451936712637 s with an error of . This stability allows millisecond pulsars to be used in establishing ephemeris time or in building

pulsar clock A pulsar clock is a clock which depends on counting radio pulses emitted by pulsars. Pulsar clock in Gdańsk The first pulsar clock in the world was installed in St Catherine's Church, Gdańsk, Poland, in 2011. It was the first clock to coun ...

s. ''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

Probes of the interstellar medium

The radiation from pulsars passes through the

(ISM) before reaching Earth. Free

s in the warm (8000 K), ionized component of the ISM and

H II region An H II region or HII 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 ...

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 In optics, and by analogy other branches of physics dealing with wave propagation, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency; sometimes the term chromatic dispersion is used for specificity to o ...

'' of the pulsar. The dispersion measure is the total

column density The area density (also known as areal density, surface density, superficial density, areic density, mass thickness, column density, or density thickness) of a two-dimensional object is calculated as the mass per unit area. The SI derived unit is ...

of free electrons between the observer and the pulsar: :

\mathrm = \int_0^D n_e(s) \,ds,

where

D

is the distance from the pulsar to the observer, and

n_e

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 is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

. Additionally, density inhomogeneities in the ISM cause

scattering Scattering is a term used in physics to describe 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 ...

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 or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...

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 a 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 is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differen ...

around

Sgr A* Sagittarius A* ( ), abbreviated Sgr A* ( ), is the supermassive black hole at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, vi ...

, 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 ob ...

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 or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who ...

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 3 consortia around the world which use pulsars to search for

gravitational waves Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside i ...

. In Europe, there is the European Pulsar Timing Array (EPTA); there is 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. ...

(PPTA) in Australia; and there is the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) in Canada and the US. Together, the consortia form the International Pulsar Timing Array (IPTA). The pulses from

Millisecond Pulsars A millisecond (from ''milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds. A unit of 10 milliseconds may be called ...

(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 PSR B1919+21 is a pulsar with a period of 1.3373 seconds and a pulse width of 0.04 seconds. Discovered by Jocelyn Bell Burnell on 28 November 1967, it is the first discovered radio pulsar. The power and regularity of the signals wer ...

), 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 A binary pulsar is a pulsar with a binary companion, often a white dwarf or neutron star. (In at least one case, the double pulsar PSR J0737-3039, the companion neutron star is another pulsar as well.) Binary pulsars are one of the few objects ...

, PSR 1913+16, whose orbit is decaying at the exact rate predicted due to the emission of

* The brightest radio pulsar, the

Vela Pulsar The Vela Pulsar (PSR J0835-4510 or PSR B0833-45) is a radio, optical, X-ray- and gamma-emitting pulsar associated with the Vela Supernova Remnant in the constellation of Vela. Its parent Type II supernova exploded approximately 11,000� ...

. * The first millisecond pulsar, PSR B1937+21 * The brightest millisecond pulsar, PSR J0437−4715 * The first X-ray pulsar,

Cen X-3 Centaurus X-3 (4U 1118-60) is an X-ray pulsar with a period of 4.84 seconds. It was the first X-ray pulsar to be discovered, and the third X-ray source to be discovered in the constellation Centaurus. The system consists of a neutron star ...

* The first accreting millisecond X-ray pulsar, SAX J1808.4−3658 * The first pulsar with planets,

* The first pulsar observed to have been affected by

asteroids An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

: PSR J0738−4042 * The first double pulsar binary system, PSR J0737−3039 * The shortest period pulsar, PSR J1748−2446ad, 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 the only known example of a white dwarf pulsar, AR Scorpii. * The pulsar with the most stable period, PSR J0437−4715 * The first millisecond pulsar 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

1.6\times 10^

G. * 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 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 using a telescope as separate stars, in ...

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 A distributed system is a system whose components are located on different networked computers, which communicate and coordinate their actions by passing messages to one another from any system. Distributed computing is a field of computer sci ...

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 pul ...

. * PSR J1311–3430, the first millisecond pulsar 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) 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.Vela pulsar The Vela Pulsar (PSR J0835-4510 or PSR B0833-45) is a radio, optical, X-ray- and gamma-emitting pulsar associated with the Vela Supernova Remnant in the constellation of Vela. Its parent Type II supernova exploded approximately 11,000� ...

– 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.

Notes

References and further reading

* * * * *

External links

*
Pinning Down a Pulsar’s Age
. ''Science News''. *

. ''Astronomy Now''.
Animation of a Pulsar
Einstein.com, 17 January 2008. *
The Discovery of Pulsars
. BBC, 23 December 2002. *
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. * Staff Writers.

. ''Space Daily'', 7 June 2006. * Staff Writers.

. ''Space Daily'', 27 July 2006. * Than, Ker.

. Space.com, 27 July 2006.
"New Kind of Pulsar Discovered"
. Cosmos Online. *
"Astronomers discover the first white dwarf pulsar in history"
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