Pulsar

A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Earth (similar to the way a lighthouse can be seen only when the light is pointed in the direction of an observer), and is responsible for the pulsed appearance of emission. Neutron stars are very dense and have short, regular rotational periods. This produces a very precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar. Pulsars are one of the candidates for the source of ultra-high-energy cosmic rays. (See also centrifugal mechanism of acceleration.)

The periods of pulsars make them very useful tools for astronomers. Observations of a pulsar in a binary neutron star system were used to indirectly confirm the existence of gravitational radiation. The first extrasolar planets were discovered around a pulsar, PSR B1257+12 in 1992. In 1983, certain types of pulsars were detected that, at that time, exceeded the accuracy of atomic clocks in keeping time.

History of observation

Discovery

Signals from the first discovered pulsar were initially observed by Jocelyn Bell while analyzing data recorded on August 6, 1967 from a newly commissioned radio telescope that she helped build. Initially dismissed as radio interference by her supervisor, Antony Hewish, developer of the telescope, the fact that the signals always appeared at the same declination and right ascension soon ruled out a terrestrial source. On November 28, 1967, Bell and Hewish using a fast recorder resolved the signals as a series of pulses, evenly spaced every 1.33 seconds. No astronomical object of this nature had ever been observed before. On December 21, Bell discovered a second pulsar, quashing speculation that these might be signals beamed at earth from an extraterrestrial intelligence.

When observations with another telescope confirmed the emission, it eliminated any sort of instrumental effects. At this point, Bell said of herself and Hewish that "we did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem—if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly?" (after-dinner speech with the title of ''Petit Four'' given at the Eighth Texas Symposium on Relativistic Astrophysics; first published in ''Annals of the New York Academy of Science'', vol. 302, pp. 685–689, Dec. 1977).
Even so, they nicknamed the signal ''LGM-1'', for "little green men" (a playful name for intelligent beings of extraterrestrial origin).

It was not until a second pulsating source was discovered in a different part of the sky that the "LGM hypothesis" was entirely abandoned. Their pulsar was later dubbed CP 1919, and is now known by a number of designators including PSR B1919+21 and PSR J1921+2153. Although CP 1919 emits in radio wavelengths, pulsars have subsequently been found to emit in visible light, X-ray, and gamma ray wavelengths.

The word "pulsar" first appeared in print in 1968:

The existence of neutron stars was first proposed by Walter Baade and Fritz Zwicky in 1934, when they argued that a small, dense star consisting primarily of neutrons would result from a supernova. Based on the idea of magnetic flux conservation from magnetic main sequence stars, Lodewijk Woltjer proposed in 1964 that such neutron stars might contain magnetic fields as large as 10¹⁴ to 10¹⁶ gausses (=10¹⁰ to 10¹²  teslas). In 1967, shortly before the discovery of pulsars, Franco Pacini suggested that a rotating neutron star with a magnetic field would emit radiation, and even noted that such energy could be pumped into a supernova remnant around a neutron star, such as the Crab Nebula. After the discovery of the first pulsar, Thomas Gold independently suggested a rotating neutron star model similar to that of Pacini, and explicitly argued that this model could explain the pulsed radiation observed by Bell Burnell and Hewish.
In 1968, Richard V. E. Lovelace with collaborators discovered period $P \approx 33$ ms of the Crab Nebula pulsar using Arecibo Observatory.''Crab nebula pulsar NP 0532''
1969, J. M. Comella, H. D. Craft, R. V. E. Lovelace, J. M. Sutton, G. L. Tyler Nature 221 (5179), 453–454.''Digital Search Methods for Pulsars''
1969, R. V. E. Lovelace, J. M. Sutton, E. E. Salpeter Nature 222 (5190), 231–233.
The discovery of the Crab pulsar

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