Pulsars are highly magnetized rotating neutron stars which emit a beam of detectable electromagnetic radiation in the form of radio waves. Their observed periods range from 1.5 ms to 8.5 s. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name. Because neutron stars are very dense objects, the rotation period and thus the interval between observed pulses are very regular. For some pulsars, the regularity of pulsation is as precise as an atomic clock. Pulsars are known to have planets orbiting them, as in the case of PSR B1257+12. Werner Becker of the Max-Planck-Institut für extraterrestrische Physik said in 2006,
“ The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work."
Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission beams. (*)
The first pulsar was observed in July 1967 by Jocelyn Bell Burnell and Antony Hewish. Initially baffled as to the seemingly unnatural regularity of its emissions, they dubbed their discovery LGM-1, for "little green men" (a comical name for intelligent beings of extraterrestrial origin). Their pulsar was later dubbed CP 1919, and is now known by a number of designators including PSR 1919+21, PSR B1919+21 and PSR J1921+2153.
According to Martin Rees, the hypothesis that pulsars were beacons from extraterrestrial civilizations was never taken very seriously. However, astrophysicist Peter A. Sturrock writes that the possibility of an extraterrestrial origin was "seriously considered ... They debated this possibility and decided that, if this proved to be correct, they could not make an announcement without checking with higher authorities. There was even some discussion about whether it might be in the best interests of mankind to destroy the evidence and forget it!" (Sturrock, 154)
CP 1919 emits in radio wavelengths, but pulsars have subsequently been found to emit in the X-ray and/or gamma ray wavelengths.
The word pulsar is a contraction of "pulsating star", and 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 [sic]. The name Pulsar (Pulsating Star) 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.'" ”
The suggestion that pulsars were rotating neutron stars was put forth independently by Thomas Gold and Franco Pacini in 1968, and was soon proven beyond doubt by the discovery of a pulsar with a very short 33-millisecond pulse period in the Crab nebula.
In 1974, Antony Hewish was awarded the Nobel Prize in physics, becoming the first astronomer to do so. 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.
In 1974, Joseph Taylor and Russell Hulse discovered for the first time a pulsar in a binary system, PSR B1913+16. This pulsar orbits another neutron star with an orbital period of just eight hours. Einstein's theory of general relativity predicts that this system should emit strong gravitational radiation, 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 2004, 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, a pulsar with a rotation period of just 1.6 milliseconds was discovered, by Shri Kulkarni and Don Backer. 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. Owing to their extraordinarily rapid and stable rotation, MSPs can be used by astronomers as clocks rivalling 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 nanoseconds can be easily detected and used to make precise measurements. Physical parameters accessible through pulsar timing include the three-dimensional 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, 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 amongst several different pulsars, forming what is known as a Pulsar Timing Array. With luck, these efforts may lead to a time scale a factor of ten or more better than currently available, and the first ever direct detection of gravitational waves.
The first ever detected extrasolar planets were found orbiting a millisecond pulsar in 1990, by Aleksander Wolszczan. This discovery presented important evidence concerning the widespread existence of planets outside the solar system, although it is very unlikely that any life form could survive in the environment of intense radiation near a pulsar.
Three distinct classes of pulsars are currently known to astronomers, according to the source of energy that powers the radiation:
* Rotation-powered pulsars, where the loss of rotational energy of the star powers the radiation
* Accretion-powered pulsars (accounting for most but not all X-ray pulsars), where the gravitational potential energy of accreted matter is the energy source (producing X-rays that are observable from Earth), and
* Magnetars, where the decay of an extremely strong magnetic field powers the radiation.
Although all three classes of objects are neutron stars, their observable behaviour and the underlying physics are quite different. There are, however, connections. For example, X-ray pulsars are probably old rotation-powered pulsars that have already lost most of their energy, and have only become visible again after their binary companions expanded and began transferring matter on to the neutron star. The process of accretion can in turn transfer enough angular momentum to the neutron star to "recycle" it as a rotation-powered millisecond pulsar.
The Vela Pulsar and its surrounding pulsar wind nebula. (*)
Initially pulsars were named with letters of the discovering observatory followed by their right ascension (e.g. CP 1919). As more pulsars were discovered, the letter code became unwieldy and so the convention was then superseded by the letters PSR (Pulsating Source of Radio) followed by the pulsar's right ascension and degrees of declination (e.g. PSR 0531+21) and sometimes declination to a tenth of a degree (e.g. PSR 1913+167). Pulsars that are very close together sometimes have letters appended (e.g. PSR 0021-72C and PSR 0021-72D).
The modern convention is to prefix 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.
In June 2006, astronomer John Middleditch and his team at LANL announced the first prediction of glitches with observational data from the Rossi X-ray Timing Explorer. They used observations of the pulsar PSR J0537-6910.
The study of pulsars has resulted in many applications in physics and astronomy. Striking examples include the confirmation of the existence of gravitational radiation as predicted by general relativity and the first detection of an extrasolar planetary system.
As probes of the interstellar medium
The radiation from pulsars passes through the interstellar medium (ISM) before reaching Earth. Free electrons in the warm (8000 K), ionized component of the ISM and H 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.
Due to the dispersive nature of the interstellar plasma, lower-frequency radio waves travel through the medium faster 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 pulsar. The dispersion measure is the total column density of free electrons between the observer and the pulsar,
where D is the distance from the pulsar to the observer and ne is the electron density of the ISM. The dispersion measure is used to construct models of the free electron distribution in the Milky Way Galaxy.
Additionally, turbulence in the interstellar gas causes density inhomogeneities in the ISM which cause scattering 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 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 (up to several hundred km/sec) velocity of many pulsars, a single pulsar scans the ISM rapidly, which results in changing scintillation patterns over timescales of a few minutes.
* The first radio pulsar, CP 1919 (now known as PSR 1919+21), with a pulse period of 1.337 seconds and a pulse width of 0.04 second, was discovered in 1967 (Nature 217:709-713, 1968). A drawing of this pulsar's radio waves was used as the cover of British rock band Joy Division's debut album, Unknown Pleasures.
* The first binary pulsar, PSR 1913+16, whose orbit is decaying at the exact rate predicted due to the emission of gravitational radiation by general relativity
* The first millisecond pulsar, PSR B1937+21
* The brightest millisecond pulsar, PSR J0437-4715
* The first X-ray pulsar, Cen X-3
* The first accreting millisecond X-ray pulsar, SAX J1808.4-3658
* The first extrasolar planets to be discovered orbit the pulsar PSR B1257+12
* The first double pulsar binary system, PSR J0737−3039
* The magnetar SGR 1806-20 produced the largest burst of energy in the Galaxy ever experimentally recorded on 27 December 2004
* PSR B1931+24 "... appears as a normal pulsar for about a week and then 'switches off' for about one month before emitting pulses again. [..] this pulsar slows down more rapidly when the pulsar is on than when it is off. [.. the] braking mechanism must be related to the radio emission and the processes creating it and the additional slow-down can be explained by a wind of particles leaving the pulsar's magnetosphere and carrying away rotational energy. 
* PSR J1748-2446ad, at 716 Hz, the pulsar with the highest rotation speed.
* PSR J0108-1431, the closest known pulsar to the Earth. It lies in the direction of the constellation Cetus, at a distance of about 85 parsecs (or 280 light years). Nevertheless, it was not discovered until 1993 due to its extremely low luminosity. It was discovered by the Danish astronomer Thomas Tauris in collaboration with a team of Australian and European astronomers using the Parkes 64-meter radio telescope. The pulsar is 1000 times weaker than an average radio pulsar and thus this pulsar may represent the tip of an iceberg of a population of more than half a million such dim pulsars crowding our Milky Way.
* Neutron star
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