PSR B1937+21 is a pulsar located in the constellation Vulpecula a few degrees in the sky away from the first discovered pulsar, PSR B1919+21. The name PSR B1937+21 is derived from the word "pulsar" and the declination and right ascension at which it is located, with the "B" indicating that the coordinates are for the 1950.0 epoch. PSR B1937+21 was discovered in 1982 by Don Backer, Shri Kulkarni, Carl Heiles, Michael Davis, and Miller Goss. It is the first discovered millisecond pulsar, with a rotational period of 1.5578 milliseconds, meaning it completes 641.92 rotations per second. This period was far shorter than astronomers considered pulsars capable of reaching, and led to the suggestion that pulsars can be spun-up by accreting mass from a companion. The rotation of PSR B1937+21, along with other millisecond pulsars discovered later, are very stable in their rotation. They are capable of keeping time as well as atomic clocks. PSR B1937+21 is unusual in that it is one of few pulsars which occasionally emits particularly strong pulses. The flux density of the giant pulses emitted by PSR B1937+21 are the brightest radio emission ever observed. These properties of PSR B1937+21, and its unexpected discovery, are credited with helping revitalize research on pulsars.
Background
The first pulsar was discovered in 1967 by Jocelyn Bell and her PhD supervisor Antony Hewish using the Interplanetary Scintillation Array. Shortly after the discovery of pulsars, Franco Pacini and Thomas Gold independently suggested that pulsars are highly magnetized rotating neutron stars, which form as a result of a supernova at the end of the life of stars more massive than about 10 times the mass of the Sun. The radiation emitted by pulsars is caused by interaction of the plasma surrounding the neutron star with its rapidly rotating magnetic field. This interaction leads to emission "in the pattern of a rotating beacon," as emission escapes along the magnetic poles of the neutron star. The "rotating beacon" property of pulsars arises from the misalignment of their magnetic poles with their rotational poles.
Discovery In the late 1970s, the radio source 4C21.53 captured the attention of radio astronomers, "because of its anomalously high level of interplanetary scintillation." As interplanetary scintillation is associated with compact radio sources, the interplanetary scintillation observations suggested that 4C21.53 might be a supernova remnant, but a pulsar survey carried out at Arecibo Observatory in 1974 by Russell Hulse and Joseph Taylor in the region did not discover a pulsar associated with 4C21.53. With the lack of success in finding a pulsar in the region, other explanations for the scintillation were explored, including suggestion of entirely new classes of objects. After realizing in 1982 that previous searches for a pulsar in the region of 4C21.53 were not sensitive to periods short enough to produce the observed scintillation, Don Backer initiated a search in the area that would be sensitive to a wide range of pulse periods and dispersion measures, including very short periods. The initial search plan was to sample at a rate of 500 Hz, which would have been insufficiently fast to detect a pulsar spinning at 642 Hz. To simplify the search apparatus, Backer's then student, Shri Kulkarni, sampled as quickly as was possible, and time averaged the signal over a period of 0.4 milliseconds, thus effectively sampling at 2500 Hz. As a result, Backer et al. determined in November 1982 that the source was a pulsar rotating every 1.5578 milliseconds, a rate far beyond anything that astronomers studying pulsars had expected.
Characteristics
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