A cosmic source of rapid and regular bursts of radio waves. The time between successive radio pulses ranges between a few thousandths of a second for the so-called millisecond pulsars to over 4 s for the slowest pulsars. Pulsars are collapsed neutron stars, having a mass similar to the Sun, but a diameter of only 10 km/6 mi or so. Over 1000 are known.
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
Discovery
The first pulsar was discovered in 1967, by Jocelyn Bell Burnell and Antony Hewish of the University of Cambridge, UK. Their pulsar was later dubbed CP 1919, and is now known by a number of designators including PSR 1919+21. 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. (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 name Pulsar (Pulsating Star) is likely to be given to it.
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.
Subsequent History
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. 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. 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.
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.
Pulsar classes
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. 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. They used observations of the pulsar PSR J0537-6910.Application
The study of pulsars has resulted in many applications in physics and astronomy.
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