Pulsars and neutron stars/History of pulsar discoveries

Before the first pulsar discovery
Around 1910, a white dwarf star, Sirius B, was shown to have a mass similar to our Sun. From its known distance and luminosity it was clear that the star must be incredibly and, as it seemed at the time impossibly, dense. In 1931, Chandrasekhar showed that there is a limiting mass that no white dwarf can exceed - around 1.4 solar masses.

Rutherford first suggested that the nucleus of an atom consisted of positive protons and neutrally charge particles. In 1932 Chadwick discovered the neutron and only a few years later Baade & Zwicky (1934) and Chandrasekhar (1935) showed that the contraction of a 1.4 solar mass object would only be halted by the internal pressure exerted by a degenerate neutron gas. They termed the resulting stable star a neutron star and suggested that such stars would be formed during supernovae. At the time, neutron stars were thought to be too faint to be detectable. Pacini (1967) and Gold (1968) described how the neutron star would have a radius of approximately 10km and a surface magnetic field strength of ~1012Gauss. It was noted that if the star were spinning then electromagnetic waves would be emitted because of the strong magnetic fields.

During April 1967 Shklovsky reported on a study of Scorpius XR-1:

It was during this year that the first pulsar was also detected.

History of pulsar discoveries
The first pulsar was discovered during the year 1967 by Jocelyn Bell Burnell and published by Hewish et al. (1967). The entire abstract of their paper is as follows:

There are two intriguing twists to this first discovery. In the late 1950s, a visitor (with no professional astronomy training) viewed the Crab Nebula using an optical telescope at the University of Chicago. She noted to a local astronomer that she could see a source flashing, but her observation was disregarded simply as ``scintillation". Perhaps she actually was observing optical pulses from the Crab pulsar. The second twist was brought up during a conference in Montreal in 2007.  During that meeting Charles Schisler described how he had been a US Air Force sergeant at a remote Alaskan outpost.  At the conference he produced his notes from 1967 which showed that his military radar equipment had detected the pulsar in the Crab Nebula.  However, it was not until much later that he was to learn what it was that he had seen.

The Nobel Prize in physics in 1974 was presented to Anthony Hewish "for his decisive role in the discovery of pulsars".

By the end of 1968 a total of 21 pulsars were known. These included the Crab pulsar (Comella et al. 1969) and the Vela pulsar (Large, Vaughan & Mills 1968). The discoveries kept coming. More than 100 pulsars were known by the end of 1973. Most of these were discovered in large surveys carried out by the Molonglo, Jodrell Bank and GreenBank telescopes. During 1974 and 1975, the Arecibo telescope was used to discover 40 new pulsars. This sample included the first binary pulsar, PSR B1913+16 (J1915+1606) (Hulse & Taylor 1975). This pulsar was shown to have an orbital period of 0.3 days and observations of the pulsar led to tests of the general theory of relativity and to the confirmation of the existence of gravitational waves. The Nobel Prize in Physics in 1993 was awarded to Hulse & Taylor. for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation

The pulsar discoveries kept coming. Prior to 1982 a total of 326 pulsars were known. The sample included three binary pulsars and PSRs J0525-6607 and J2301+5852, which are pulsars that are undetectable with a radio telescope (now known as Anomalous X-ray Pulsars; AXPs). At this date the shortest period pulsar was the Crab pulsar that was known to be young and has a pulse period of 33ms. During the 1970s the radio source 4C21.53 had been a puzzle. Interplanetary scintillation implied that it was a compact radio source and likely to be a pulsar, but Arecibo surveys of the region could not discover any such pulsar. Backer et al. realised that the source still could be a pulsar, but with a pulse rate faster than that which would have been detected in the earlier surveys. With more Arecibo observations, which were more sensitive to fast pulsars, they were successful and, in November 1982, reported the first millisecond pulsar with a pulse period of 1.6ms. This pulse rate was much faster than any previously known pulsar. By the end of the 1980s more than 450 pulsars were known with seven having a period of less than 10ms.



In 1987 the first pulsar, B1821-24A (J1824-2452A) in a globular cluster (M28) was discovered, soon followed (in 1988 and 1989) by four more globular cluster pulsars (in M4, M5 and NGC6440). In 1983 the first extra-Galactic pulsar, PSR B0529-66 (J0529-6652) was discovered in the large Magellanic Cloud.

The 1990s led to the discovery of a further 300 pulsars. These pulsars include PSR B1237+12 (J1300+1240) by Wolszczan et al., which led to the discovery of the first extra-Solar planets, the discovery of 12 pulsars in the globular cluster 47 Tucanae and a total of 41 binary pulsars including PSR J0437-4715, the brightest millisecond pulsar  (Johnston et al. 1993). Near the end of the 90s the installation of a multibeam receiver system at the Parkes Observatory led to the Parkes Multibeam Pulsar Survey, which discovered more than 800 pulsars. The multibeam receiver was also used in the Parkes High Latitude survey that found, in 2003, PSR J0737-3039A, the most relativistic pulsar to date and the first pulsar to be discovered orbiting another detectable pulsar (PSR J0737-3039B).

A radio pulsar with an 8-second period was reported by Young, Manchester & Johnston (2000). The first double pulsar system was reported by Lyne et al. 2004). Ransom et al. (2005) reported 21 millisecond pulsars in the globular cluster Terzan 5. This cluster also led to the discovery of the fastest spinning pulsar to date (Hessels et al. 2006).  Ransom et al. (2014) reported on the discovery of a millisecond pulsar in a triple system.

Pulsar surveys have also led to unexpected discoveries. For instance, a reprocessing of archival data taken in 2001 by the Parkes radio telescope led to the detection of a bright (30-Jy) dispersed radio burst (Lorimer et al.). Such bursts are now known as Fast Radio Bursts (FRBs) and their origin is still unknown.

Most pulsar searches are based on finding the pulsed emission. However, searches for pulsars have also been made using radio continuum images. For instance, De Breuck searched for high redshift radio galaxies by looking at ultra-steep spectrum radio sources and noted that the steepest sources are excellent Galactic pulsar candidates). Such searches were also discussed in terms of pulsars by Avramenko, Ilyasov & Potapov (2000).

Of course, not all pulsar surveys have been successful and some methods are currently impractical with existing telescopes. For instance, Mollerach & Roulet (1997) considered the possibility of finding pulsars in microlensing surveys. Multiple attempts have also been made to find pulsars very close to the Galactic centre (see, e.g., ) or in particular supernova remnants (such as SNR1987A; see Shigeyama et al. 1987 and Curtis, Kennel, Fowler 1987). Edwards, van Straten & Bailes (2001) unsuccessfully searched for submillisecond pulsars in globular clusters. Bhat et al. (2011) failed to find any pulsars or transient sources in M33. Population studies of pulsars suggest over 105 potentially detectable pulsars in our galaxy alone. With the new generation of telescopes we will also be able to find pulsars in distant galaxies. There are plenty more pulsars to find!