Pulsars (from PULSAting stARs) were discovered, albeit accidentally, by Jocelyn Bell at Cambridge in 1967. The apparently sporadic bursts of radio emission appeared during the course of a survey to investigate the effects of interplanetary scintillation of radio sources. Working as a graduate student in a team lead by Anthony Hewish, Bell soon realized that the emission always occurred at the same position in the celestial sphere indicating that the source was not of terrestrial origin. Subsequent observations with greater time resolution showed the emission to be a train of pulses with a precise repetition period of 1337 ms (hear this pulsar with are Arecibo Observatory's 305-m dish!). Soon afterwards, the Cambridge team announced the discovery of 3 more pulsars found from subsequent inspection of the remaining survey data.
At first, pulsars were thought to be white dwarf stars. In 1967, neutron stars were a purely theoretical concept, developed by Landau, Oppenheimer, Volkoff and others in the 1930s. Walter Baade and Fritz Zwicky suggested in 1934 that neutron stars are the remnants of massive stars that went supernova, and should therefore be found near supernova rennants. In 1967, Pacini suggested that a highly magnetized, fast-spinning neutron star was the energy source of the Crab Nebula.
In 1968, at the Arecibo Observatory, Comella et al. (1969) measured the spin period of the pulsar at the center of the Crab Nebula: 33 ms. No white dwarf star can vibrate or rotate that fast. This showed immediately that pulsars are neutron stars with a strongly anisotropic emission of electromagnetic radiation, mostly thought to emanate from the object's magnetic poles. If these are misaligned with the geographic poles, then the rotation of the object causes distant observers to detect apparent pulses of radiation. As had been suggested by Thomas Gold of Cornell University, the pulsar is seen to lose rotational energy due to emission of electro-magnetic radiation.
Neutron stars actually exist! Not only that, they are the remnants of massive stars thant exploded a long time ago. The Crab pulsar is the youngest known: its supernova explosion was witnessed by Chinese astronomers in 1054 AD. As predicted by Pacini, the observed energy loss of the Crab pulsar is similar to the observed emission of the Crab Nebula, the remnant of the supernova explosion. The connection between pulsars and rotating neutron stars is now universally accepted.
In over 40 years since their discovery, pulsars have proved to be exciting objects to study and, presently about 1700 are known. Most of these are ``normal'' in the sense that their pulse periods are of order one second and, with few exceptions, are observed to increase secularly at the rate of about one complete period in 1,000,000,000,000,000! This is naturally explained as the gradual spin-down of the neutron star as it radiates energy at the expense of its rotational kinetic energy. A small fraction of the observed sample are the so-called ``millisecond pulsars'' which have much shorter periods (< 20 ms) and rates of slowdown of typically only one period in 10,000,000,000,000,000,000, proving to be extremely accurate clocks. In addition, some pulsars are known to be members of binary systems in which the companion is another neutron star, a white dwarf and even a main sequence star.
From the early days researchers at Arecibo Observatory joined the effort to search for new pulsars and to understand the physical processes responsible for this remarkable phenomenon. As dicussed above, their measurement of the periodicity of the Crab pulsar proved that neutron stars exist in the Universe and that they are the end point of stellar evolution for massive stars. A few other landmark contributions by Arecibo pulsar astronomers are summarized here.
During a systematic search of the galactic plane with the Arecibo telescope in July 1974, Russell A. Hulse and Joseph H. Taylor discovered an extraordinary of 59 ms pulsar PSR B1913+16. It soon became clear that the timing properties of this source could only be understood if the pulsar were in a highly eccentric 7.75 hour orbit around another neutron star. The intense gravitational field generated by the neutron stars results in several special and general relativistic effects unobtainable in a terrestrial physics laboratory, or even in Solar System experiments. Perhaps the most important effect is the emission of gravitational radiation from the system at the expense of its gravitational binding energy. According to Einstein's general theory of relativity, the separation of the stars should decrease by 3 mm each orbit due to this process.
From regular observations, the orbital decay was measured by Taylor and collaborators within 6 years of the discovery and is in agreement with general relativity to better than a percent after nearly 20 years of observations. The discovery of this fascinating binary system and the subsequent measurements provide the only experimental evidence to date for the existence of gravitational radiation. In recognition of this achievement, Hulse and Taylor were awarded the 1993 Nobel Prize in Physics. For more about the importance of this discovery, see the Press release from the Swedish Academy and Cornell's Astronomy Course web page. Click here for more information on PSR 1913+16.
The discovery of the first `millisecond pulsar' PSR 1937+21 was made at Arecibo by Don Backer, Shri Kulkarni collaborators in 1982. This remarkable neutron star with a spin period of just 1.5578 ms rotates about its axis almost 642 times per second. This value introduced fundamental constraints to equations of state for cold matter at supra-nuclear densities. The discovery revitalized pulsar astronomy since it demonstrated that a potentially large population of similar objects exists which had been missed by pulsar searches conducted during the 1970s which had sampling rates > 20 ms. These objects have extraordinary rotational stability, which opened up new avenues of research: high precision timing and solar system studies, high precision tests of GR and the nature of space-time itself, stellar evolution, tests of nuclear physics and search for long-period gravitational waves.
Although subsequent pulsar surveys with better sensitivity to short period objects have found a large number of similar objects, for 24 years PSR B1937+21 stayed as the most rapidly rotating neutron star known. The next shortest period object, PSR B1957+20 was discovered at Arecibo in 1988 by Andy Fruchter and collaborators. This 1.6074 ms pulsar turns out to be in a near circular 9 hr orbit around a low-mass companion star. The pulsar is eclipsed by its companion for about 50 minutes per orbit. For a few minutes before an eclipse becomes complete, and for more than 20 minutes after the signal reappears, the pulses are delayed by as much as several hundred microseconds- presumably as a result of propagation through plasma surrounding the companion. Detailed studies of this system suggest that the companion is being gradually ablated by the relativistic `wind' of the pulsar. This system may be a progenitor for single millisecond pulsar like PSR 1937+21.
The Arecibo discoveries continued in 1992 with the announcement of
extra-solar planetary system known. This orbits around
the millisecond pulsar PSR B1257+12, and was discovered by Alex
Wolszczan and Dale Frail.
Timing observations of this system have so far revealed the presence of
at least three Earth-mass bodies in orbit around the neutron star.