Rotation-powered pulsar
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Crab_Pulsar.jpg
A rotation-powered pulsar is a rapidly rotating neutron star, whose electromagnetic radiation is observed in regularly spaced intervals, or pulses. It differs from other types of pulsars in that the source of power for the production of radiation is the loss of rotational energy.
As the first type of pulsars to be discovered, rotation-powered pulsars were originally known simply as pulsars, a term coined by a Daily Telegraph journalist as a contraction of "pulsating star". Although it was soon learned that the pulses were related to rotation rather than to physical expansion and contraction, as in true pulsating variable stars, the term stuck. After the discovery of accretion-powered x-ray pulsars, rotation-powered pulsars were known as radio pulsars. Since there are now rotation-powered pulsars known that emit x-rays but not radio waves, the term "rotation-powered pulsar" is preferred.
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History
The first known pulsar was discovered by Jocelyn Bell and Antony Hewish in 1967 while they were using a radio array to study the scintillation of quasars. They found a very regular signal, consisting of pulses of radiation at a rate of one in every few seconds. Terrestrial origin of the signal was ruled out because the time it took the object to reappear was a sidereal day instead of a solar day.
The original name given to the object was "LGM-1", short for "Little Green Men", a comical name for intelligent beings of extraterrestrial origin. Although this choice of naming is indicative of the mystery surrounding the origin of the signals, according to Martin Rees, the hypothesis that they were beacons from extraterrestrial civilisations were never taken very seriously. 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, the first astronomer to do so (astronomer Martin Ryle also received the prize this year). Considerable controversy is associated with the fact that Professor Hewish was awarded the prize while Bell, who made the initial discovery while she was a PhD student, was not.
Also in 1974, Joseph Taylor and Russell Hulse discovered for the first time a pulsar in a binary system. 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 proof 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, "millisecond pulsars" (MSPs) had been found. MSPs are believed to be the end product of X-ray binaries. Owing to their extrordinarily 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. Once 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.
Theory
There is general agreement that what we observe as a pulse is what happens when a beam of radiation points in our direction, once for every rotation of the neutron star. The origin of the beam is related to the misalignment of the rotation axis and the axis of the magnetic field of the star. The beam is emitted from the poles of the neutron star's magnetic field, which may be offset from the rotational poles by a wide angle. The source of energy of the beam is the rotational energy of the neutron star, which slows down over time as the energy is emitted.
Millisecond pulsars are thought to have been spun up to high rotational speed by infalling matter pulled off of a companion star.
Of interest to the study of the state of the matter in a neutron stars are the glitches observed in the rotation velocity of the neutron star. This velocity is decreasing slowly but steadily, except by sudden variations. These were for a time believed to be "starquakes" due to the adjustment of the crust of the neutron star. Models where the glitch is due to a decoupling of the possibly superconducting interior of the star have also been advanced.
In 2003 observations of the Crab nebula pulsar's signal revealed "sub-pulses" within the main signal with durations of only nanoseconds. It is thought that these nanosecond pulses are emitted by regions on the pulsar's surface 60cm in diameter or smaller, making them the smallest structures outside the solar system to be measured.
Importance
As mentioned above, the discovery of pulsars allowed astronomers to study an object never observed before, the neutron star. This kind of object is the only place where the behaviour of matter at nuclear density can be observed (though not directly). Also, millisecond pulsars have allowed one test of general relativity in conditions of an intense gravitational field.
Significant pulsars
- 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. 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 B1913+16, confirming general relativity and proving the existence of gravitational waves
- The first millisecond pulsar, PSR B1937+21
- The first pulsar with planets, PSR B1257+12
- The first double pulsar binary system, PSR J0737−3039
- The longest period pulsar, PSR J2144−3933
See Also
cs:Pulsar da:pulsar de:Pulsar et:Pulsar fr:Pulsar it:Pulsar ja:パルサー nl:Pulsar pl:Pulsar fi:Pulsari sv:Pulsar zh:中子星