PSR B1509−58 –X-rays fromChandra are gold;infrared fromWISE in red, green and blue/max.Animation of a rotating pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission zones.Illustration of the "lighthouse" effect produced by a pulsar
Apulsar (pulsating star, on the model ofquasar)[1] is a highly magnetized rotatingneutron star that emits beams ofelectromagnetic radiation out of itsmagnetic poles.[2] This radiation can be observed only when a beam of emission is pointing toward Earth (similar to the way alighthouse 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 verydense and have short, regular rotationalperiods. 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 ofultra-high-energy cosmic rays (see alsocentrifugal mechanism of acceleration).
Signals from thefirst discovered pulsar were initially observed byJocelyn Bell while analyzing data recorded on August 6, 1967, from anewly commissioned radio telescope that she helped build. Initially dismissed asradio interference by her supervisor and developer of the telescope,Antony Hewish,[4][5] the fact that the signals always appeared at the samedeclination andright ascension soon ruled out a terrestrial source.[6] On November 28, 1967, Bell and Hewish using a fast stripchart recorder resolved the signals as a series of pulses, evenly spaced every 1.337 seconds.[7] 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 anextraterrestrial intelligence.[8][9][10][11]
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?"[12] Even so, they nicknamed the signalLGM-1, for "little green men" (a playful name for intelligentbeings 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.[13] Their pulsar was later dubbedCP 1919, and is now known by a number of designators including PSR B1919+21 and PSR J1921+2153. Although CP 1919 emits inradio wavelengths, pulsars have subsequently been found to emit in visible light,X-ray, andgamma ray wavelengths.[14]
The wordpulsar first appeared in print in 1968:
An entirely novel kind of star came to light on Aug. 6 last year and was referred to, by astronomers, as LGM (Little Green Men). Now it is thought to be a novel type between a white dwarf and a neutron [star]. The name Pulsar is likely to be given to it. Dr. A. Hewish told me yesterday: '... I am sure that today every radio telescope is looking at the Pulsars.'[15]
The existence of neutron stars was first proposed byWalter Baade andFritz Zwicky in 1934, when they argued that a small, dense star consisting primarily of neutrons would result from asupernova.[16] 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 1016gauss (=1010 to 1012tesla).[17] 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 asupernova remnant around a neutron star, such as theCrab Nebula.[18] After the discovery of the first pulsar,Thomas Gold 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.[19] In 1968,Richard V. E. Lovelace with collaborators discovered period ms of theCrab Nebula Pulsar usingArecibo Observatory.[20][21]The discovery of theCrab Pulsar provided confirmation of the rotating neutron star model of pulsars.[22] The Crab Pulsar 33-millisecond 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.[23]In 1974, Antony Hewish andMartin Ryle, who had developed revolutionaryradio telescopes, became the first astronomers to be awarded theNobel Prize in Physics, with theRoyal Swedish Academy of Sciences noting that Hewish played a "decisive role in the discovery of pulsars".[24] 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.[25]
In 1974,Joseph Hooton Taylor, Jr. andRussell Hulse discovered for the first time a pulsar in abinary system of stars,PSR B1913+16. This pulsar orbits another neutron star with an orbital period of just eight hours.Einstein's theory ofgeneral relativity predicts that this system should emit stronggravitational radiation, causing the orbit to continually contract as it losesorbital 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.[26] In 1993, the Nobel Prize in Physics was awarded to Taylor and Hulse for the discovery of this pulsar.[27]
In 1982,Don Backer led a group that discoveredPSR B1937+21, a pulsar with a rotation period of just about 1.6 milliseconds (38,500rpm).[28] 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 ofX-ray binaries. Owing to their extraordinarily rapid and stable rotation, MSPs can be used byastronomers as clocks rivaling the stability of the bestatomic clocks on Earth. Factors affecting the arrival time of pulses at Earth by more than a few hundrednanoseconds can be easily detected and used to make precise measurements. Physical parameters accessible through pulsar timing include the 3D position of the pulsar, itsproper motion, theelectron content of theinterstellar medium 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 byTempo, 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 ofTerrestrial Time 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 apulsar timing array. The goal of these efforts is to develop a pulsar-basedtime standard precise enough to make the first ever direct detection of gravitational waves. In 2006, a team of astronomers atLANL proposed a model to predict the likely date ofpulsar glitches with observational data from theRossi X-ray Timing Explorer. They used observations of the pulsarPSR J0537−6910, that is known to be a quasi-periodic glitching pulsar.[29] However, no general scheme for glitch forecast is known to date.[29]
Artist's impression of the planets orbitingPSR B1257+12. The one in the foreground is planetC.
In 1992,Aleksander Wolszczan discovered the firstextrasolar planets aroundPSR B1257+12. This discovery presented important evidence concerning the widespread existence of planets outside theSolar System, although it is very unlikely that anylife form could survive in the environment of intense radiation near a pulsar.
White dwarfs can also act as pulsars. However, as themoment of inertia 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 2025, three pulsar-like white dwarfs have been identified.
In 1998, Nazar Ikhsanov showed that a white dwarf in the binary systemAE Aquarii acts like a radio pulsar.[30] 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,[31] which showed that this white dwarf acts not only as a radio pulsar, but also as anX-ray pulsar.
In 2016, a white dwarf in the binary systemAR Scorpii was identified as a pulsar[32][33] (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.[32]
In 2023, it was suggested that the white dwarf eRASSU J191213.9−441044 acts as a pulsar both in radio and X-rays.[34][35]
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 atCornell University.[36] According to this model, AE Aqr is anintermediate polar-type star, where the magnetic field is relatively weak and anaccretion disc 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 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.[37]
Initially pulsars were named with letters of the discovering observatory followed by theirright ascension (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 ofdeclination (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.[38]
Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines, the protruding cones represent the emission beams and the green line represents the axis on which the star rotates.
The events leading to the formation of a pulsar begin when the core of a massive star is compressed during asupernova, which collapses into a neutron star. The neutron star retains most of itsangular momentum, and since it has only a tiny fraction of its progenitor's radius, it is formed with very high rotation speed. A beam ofradiation 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.
Animation of the increased spin of a pulsar as it collapses. It begins with (1) The rotating progenitor (2) The collapse and speedup and (3) The final fast spinning pulsar remnant. The animation demonstrates theconservation of momentum as the star spins faster as it collapses. Theangular speed () and radius () relative to the progenitor are shown throughout the process. This does not capture the entire collapse scale as the final star would be too small to see compared to its progenitor.
In rotation-powered pulsars, the beam is the result of therotational energy 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.[39][40] Observations byNICER ofPSR 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.[41][42] This rotation slows down over time aselectromagnetic 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.[43]
Though the general picture of pulsars as rapidly rotating neutron stars is widely accepted, Werner Becker of theMax 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."[44]
magnetars, where the decay of an extremely strongmagnetic field 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 pulsars are probably old rotationally-powered pulsars that have already lost most of their energy, and have only become visible again after theirbinary companions had expanded and begun transferring matter on to the neutron star.
The process of accretion can, in turn, transfer enoughangular momentum to the neutron star to "recycle" it as a rotation-poweredmillisecond pulsar. 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.[43]
Of interest to the study of the state of the matter in a neutron star are theglitches observed in the rotation velocity of the neutron star.[29] 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 possiblysuperconducting interior of the star have also been advanced. In both cases, the star'smoment of inertia changes, but itsangular momentum does not, resulting in a change in rotation rate.[29]
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.[45]
The discovery of pulsars allowed astronomers to study an object never observed before, theneutron star. This kind of object is the only place where the behavior of matter atnuclear density can be observed (though not directly). Also, millisecond pulsars have allowed a test ofgeneral relativity in conditions of an intense gravitational field.
Relative position of theSun to the center of theMilky Way Galaxy and 14 pulsars with their periods denoted, shown on aPioneer plaque
Pulsar maps have been included on the twoPioneer plaques as well as theVoyager Golden Record. They show the position of theSun, 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 potentialextraterrestrial intelligence.[46] 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.[47][48]
X-ray pulsar-based navigation and timing (XNAV) or simplypulsar navigation is a navigation technique whereby the periodicX-ray signals emitted frompulsars 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 toGPS, this comparison would allow the vehicle to calculate its position accurately (±5 km). The advantage of using X-ray signals overradio waves is thatX-ray telescopes can be made smaller and lighter.[49][50][51] Experimental demonstrations have been reported in 2018.[52]
Generally, the regularity of pulsar emission does not rival the stability ofatomic clocks.[53] They can still be used as external reference.[54] For example, J0437−4715 has a period of0.005757451936712637 s with an error of1.7×10−17 s.This stability allows millisecond pulsars to be used in establishingephemeris time[55]or in buildingpulsar clocks.[56]
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.[57] It is unknown whether timing noise is related to pulsarglitches. According to a study published in 2023,[58] the timing noise observed in pulsars is believed to be caused by backgroundgravitational waves. 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.[59]
The radiation from pulsars passes through theinterstellar medium (ISM) before reaching Earth. Freeelectrons in the warm (8000 K), ionized component of the ISM andH II regions affect the radiation in two primary ways. The resulting changes to the pulsar's radiation provide an important probe of the ISM itself.[60]
Because of thedispersive nature of the interstellarplasma, 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 thedispersion measure of the pulse. The dispersion measure is the totalcolumn 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 theMilky Way.[61]
Additionally, density inhomogeneities in the ISM causescattering of the radio waves from the pulsar. The resultingscintillation of the radio waves—the same effect as the twinkling of a star invisible light due to density variations in the Earth's atmosphere—can be used to reconstruct information about the small scale variations in the ISM.[62] 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.[63] The exact cause of these density inhomogeneities remains an open question, with possible explanations ranging fromturbulence tocurrent sheets.[64]
Pulsars orbiting within the curvedspace-time aroundSgr A*, thesupermassive black hole at the center of the Milky Way, could serve as probes of gravity in the strong-field regime.[65] Arrival times of the pulses would be affected byspecial- andgeneral-relativisticDoppler shifts 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 relativityto be measurable with current instruments, pulsars with orbital periods less than about 10 years would need to be discovered;[65] 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*.[66]
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 asPSR B1919+21), with a pulse period of 1.337 seconds and a pulse width of 0.04-second, was discovered in 1967.[6]
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.[71] One of only two pulsars known to have stopped pulsing for more than a few minutes.
Swift J0243.6+6124 most magnetic pulsar with1.6×1013G.[72][73]
^Longair, Malcolm S. (1992).High energy astrophysics (2nd ed.). Cambridge New York Port Chester [etc.]: Cambridge university press. p. 99.ISBN978-0-521-38374-5.
^S. Jocelyn Bell Burnell (1977)."Little Green Men, White Dwarfs or Pulsars?". Cosmic Search Magazine. Retrieved2008-01-30. (after-dinner speech with the title ofPetit Four given at the Eighth Texas Symposium on Relativistic Astrophysics; first published inAnnals of the New York Academy of Science, vol. 302, pp. 685–689, Dec. 1977).
^abcdAntonelli, Marco; Montoli, Alessandro; Pizzochero, Pierre (November 2022), "Insights into the Physics of Neutron Star Interiors from Pulsar Glitches",Astrophysics in the XXI Century with Compact Stars, pp. 219–281,arXiv:2301.12769,doi:10.1142/9789811220944_0007,ISBN978-981-12-2093-7
^Champion, David J.; Ransom, S. M.; Lazarus, P.; Camilo, F.; Bassa, C.; Kaspi, V. M.; Nice, D. J.; Freire, P. C. C.; Stairs, I. H.; Van Leeuwen, J.; Stappers, B. W.; Cordes, J. M.; Hessels, J. W. T.; Lorimer, D. R.; Arzoumanian, Z.; Backer, D. C.; Bhat, N. D. R.; Chatterjee, S.; Cognard, I.; Deneva, J. S.; Faucher-Giguere, C.-A.; Gaensler, B. M.; Han, J.; Jenet, F. A.; Kasian, L.; Kondratiev, V. I.; Kramer, M.; Lazio, J.; McLaughlin, M. A.; et al. (2008). "An Eccentric Binary Millisecond Pulsar in the Galactic Plane".Science.320 (5881):1309–1312.arXiv:0805.2396.Bibcode:2008Sci...320.1309C.doi:10.1126/science.1157580.PMID18483399.S2CID6070830.
^Pletsch, H. J.; Guillemot; Fehrmann, H.; Allen, B.; Kramer, M.; Aulbert, C.; Ackermann, M.; Ajello, M.; De Angelis, A.; Atwood, W. B.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Bechtol, K.; Bellazzini, R.; Borgland, A. W.; Bottacini, E.; Brandt, T. J.; Bregeon, J.; Brigida, M.; Bruel, P.; Buehler, R.; Buson, S.; Caliandro, G. A.; Cameron, R. A.;Caraveo, P. A.; Casandjian, J. M.; Cecchi, C.; et al. (2012). "Binary millisecond pulsar discovery via gamma-ray pulsations".Science.338 (6112):1314–1317.arXiv:1211.1385.Bibcode:2012Sci...338.1314P.doi:10.1126/science.1229054.PMID23112297.S2CID206544680.
