What is a pulsar?

Discovery

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.

Pulsar Research at Arecibo

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.