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[[File:Vela Pulsar jet seen by Chandra Observatory.ogv|thumb|right|258px|Video of the [[Vela pulsar]] in X-ray light]]
A '''pulsar''' ([[portmanteau]] of ''pulsating star'') is a highly magnetized, electromagnetic source that emits a beam of [[electromagnetic radiation]]. In 1968 it was thought this radiation can only be observed when the beam of emission is pointing toward the Earth, much the way a lighthouse can only be seen when the light is pointed in the direction of an observer, and is responsible for the pulsed appearance of emission. Neutron stars are theoretically very [[density|dense]], and have short, regular rotational [[Period (physics)|period]]s. This produces a very precise interval between pulses that range from roughly milliseconds to seconds for an individual pulsar.
The precise periods of pulsars makes them useful tools. Observations of a pulsar in a binary neutron star system were used to indirectly confirm the existence of [[gravitational radiation]]. The first [[extrasolar planet]]s were discovered around a pulsar, [[PSR B1257+12]]. Certain types of pulsars rival [[atomic clock]]s in their accuracy in keeping time.
{{bquote|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 {{sic}}. 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."<ref>''Daily Telegraph,'' 21/3, 5 March 1968.</ref>}}
The suggestion that pulsars were rotating neutron stars was put forth independently by [[Thomas Gold]] and [[Franco Pacini]] in 1968, and was soon challenged by the discovery of a pulsar with a very short (33-[[millisecond]]) pulse period in the [[Crab nebula]]. Necessitating a rotation speed of 1980[[revolutions per minute]]!
In 1974, Antony Hewish became the first astronomer to be awarded the [[Nobel Prize in physics]]. Considerable controversy is associated with the fact that Professor Hewish was awarded the prize while Bell, who made the initial discovery while she was his Ph.D student, was not. Bell claims no bitterness upon this point, supporting the decision of the Nobel prize committee.<ref>Burnell, S. Jocelyn Bell. Little Green Men, White Dwarfs, or Pulsars? Annals of the New York Academy of Science, vol. 302, pages 685–689, Dec., 1977 [http://www.bigear.org/vol1no1/burnell.htm]</ref>
|publisher=IOP Publishing |url=http://iopscience.iop.org/0004-637X/722/2/1030/pdf/0004-637X_722_2_1030.pdf|arxiv = 1011.0718 |bibcode = 2010ApJ...722.1030W |doi = 10.1088/0004-637X/722/2/1030 }}</ref> In 1993, the Nobel Prize in Physics was awarded to Taylor and Hulse for the discovery of this pulsar.<ref>{{cite web | title = Nobel Prize in Physics 1993 | url = http://nobelprize.org/nobel_prizes/physics/laureates/1993/ | accessdate = 2010-01-07}}</ref>
In 1982, [[Don Backer]] led a group which discovered [[PSR B1937+21]], a pulsar with a rotation period of just 1.6 milliseconds, suggesting a speed of 38,500rpm.<ref>{{cite journal | author = D. Backer et al. | title = A millisecond pulsar | journal = Nature | volume = 300 | issue = 5893 | pages = 315–318 | year = 1982 | doi = 10.1038/300615a0|bibcode = 1982Natur.300..615B }}</ref> 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]]s" (MSPs) had been found. MSPs are believed to be the end product of [[X-ray binary|X-ray binaries]]. Owing to their extraordinarily rapid and stable rotation, MSPs can be used by [[astronomers]] as clocks rivaling the stability of the best [[atomic clocks]] on Earth. Factors affecting the arrival time of pulses at the Earth by more than a few hundred [[nanosecond]]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]], the [[electron]] content of the [[interstellar 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 by [[Tempo (astronomy)|Tempo]], 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]] 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]] precise enough to make the first ever direct detection of gravitational waves.
In June 2006, the astronomer [[John Middleditch]] and his team at [[LANL]] announced the first prediction of [[Glitch (astronomy)|pulsar glitches]] with observational data from the [[Rossi X-ray Timing Explorer]]. They used observations of the pulsar [[PSR J0537-6910]].
==Formation==
[[Image:Pulsar schematic.svg|thumb|right|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 theoretical events leading to the formation of a pulsar begin when the core of a massive star is compressed during a [[supernova]], which collapses into a neutron star. The neutron star retains most of its [[angular momentum]], and since it has only a tiny fraction of its progenitor's radius (and therefore its [[moment of inertia]] is sharply reduced), it is formed with very high rotation speed. A beam of [[radiation]] 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. The beam originates from the [[rotational energy]] 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.<ref>{{cite web | url = http://www3.amherst.edu/~gsgreenstein/progs/animations/pulsar_beacon/ | title = Pulsar Beacon Animation | accessdate = 2010-04-03}}</ref><ref>{{cite web | url = http://imagine.gsfc.nasa.gov/docs/science/know_l2/pulsars.html | title = Pulsars | accessdate = 2010-04-03}}</ref> This rotation slows down over time as [[Electromagnetic radiation|electromagnetic]] 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 in the 13.6 billion year age of the universe, around 99% no longer pulsate.<ref name="cv.nrao.edu">http://www.cv.nrao.edu/course/astr534/Pulsars.html</ref> The longest known pulsar period is 8.51 seconds.<ref>{{cite journal | doi = 10.1038/23650 | last1 = Young | first1 = M. D. | last2 = Manchester | first2 = R. N. | last3 = Johnston | first3 = S. | title= A Radio Pulsar with an 8.5-Second Period that Challenges Emission Models | journal = Nature | volume = 400 | year = 1999 | issue = 6747 | accessdate = 2010-04-03 | pages = 848–849|bibcode = 1999Natur.400..848Y }}</ref>
Though this very general picture of pulsars is mostly 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."<ref>{{cite web | url = http://www.esa.int/esaCP/SEMB6IBUQPE_index_0.html |title= Old Pulsars Still Have New Tricks to Teach Us |publisher=[[European Space Agency|ESA]] |date=26 July 2006 |accessdate=30 April 2013|work=Staff }}</ref> An alternative theory suggests "magnetospheric disk-field-aligned-current transmission line system as the origin of the observed radiation".<ref>{{cite web | url = http://adsabs.harvard.edu/abs/1995Ap%26SS.227..229H |title= Radiation Properties of Pulsar Magnetospheres: Observation, Theory, and Experiment |publisher=The SAO/NASA Astrophysics Data System }}</ref>
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