Pulsars are rotating neutron stars which emit detectable electromagnetic radiation in the form of radio waves. Pulsar radiation intensity varies with a regular period, believed to correspond to the rotation period of the star. Pulsars also exhibit a so-called lighthouse effect, which occurs when the light and other radiation from a pulsar are only seen at specific intervals and not all of the time. Werner Becker of the Max-Planck-Institut für extraterrestrische Physik recently said,
The theory of how pulsars emit their
radiation is still in its infancy, even after nearly forty years of work."
History and discovery of pulsars.
The first pulsar was discovered in 1967, by Jocelyn Bell Burnell and Antony Hewish of the University of Cambridge, UK. While 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.
Initially baffled as to the unnaturally regular nature of its emissions, the pair 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. 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 civilizations were never taken very seriously. However, astrophysicist Peter A. Sturrock writes that "when the first regular radio signals from pulsars were discovered, the Cambridge scientists seriously considered that they might have come from an extraterrestrial civilization. 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. Hewis... 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, the first astronomer to do so (astronomer Martin Ryle also received the prize in 1974 for his observations and inventions, in particular of the Aperture synthesis technique). 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.
Subsequent history of pulsars.
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 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, 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. 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.
Three distinct classes of pulsars are currently known to astronomers, according to the source of energy that 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.
Glitch prediction of pulsars.
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.
Application of pulsars.
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 extra-solar planetary system.
Sources of material about pulsars.