R. N. Manchester
, PASA, 18 (1), in press.
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Introduction
Since the discovery of the first pulsar by Jocelyn Bell and Tony Hewish in 1967 (Hewish et al. 1968), many observatories throughout the world have undertaken searches for these fascinating objects. Prior to the commencement of the Parkes multibeam survey in mid-1997, these searches had resulted in the discovery of about 750 pulsars. All but a few of these lie within our Galaxy - the only known extra-galactic pulsars are associated with the Magellanic Clouds - and all but a few have been discovered at radio wavelengths. Most are relatively close on a Galactic scale - their median distance from the Sun is only about 3.5 kpc. This is not because pulsars are clustered about the Sun. It is purely a result of the rather low radio luminosity of pulsars, which makes them difficult to detect at large distances. It is now almost universally accepted that pulsars are neutron stars, with the basic periodicity defined by rotation of the star. The pulse period is very predictable, but it is not constant. Despite various short-term fluctuations observed to a greater or lesser extent in most pulsars, in the long term the intrinsic period of all pulsars increases with time. Pulsars are powered by the kinetic energy of rotation. They steadily lose energy, mainly in the form of a high-energy wind of charged particles and magnetic-dipole radiation, that is, electromagnetic waves at the neutron-star rotation frequency. Pulsars may be divided into two main groups, based on the pulse period and the rate at which it increases. The first group, often called `normal' pulsars, typically have periods of between 0.05 and 5 sec, and characteristic ages, defined by 
, where P is the pulsar period and 
is its secular rate of change, in the range 103 to 107 years. The other group are the `millisecond' pulsars (MSPs), most of which have periods of between 1.5 and 25 ms. A key property of MSPs is their great age, typically between 108 and 1010 years. Another key property is that most MSPs are members of a binary system, in an orbit with another star. These properties suggest that MSPs are in fact `recycled' neutron stars, spun up by accretion from a binary companion. For an extensive review of MSP formation mechanisms and their relation to X-ray binary systems, see Bhattacharya & van den Heuvel (1991). Not long after the discovery of the first MSP by Backer et al. (1982) it was realised that the cores of globular clusters were a favourable environment for the formation of MSPs (Hamilton, Helfand & Becker 1985) . Several groups began searches toward globular clusters and this effort was rewarded by the discovery of the 3-ms pulsar PSR B1821-24 in the core of M28 by Lyne et al. (1987). Over the next few years, more than 30 MSPs were discovered in globular clusters. Of these, M15, with 8 pulsars (Wolszczan et al. 1989, Anderson 1992), and 47 Tucanae with 11 (Manchester et al. 1991, Robinson et al. 1995) stand out as the most prolific. With 750 pulsars already known, why bother to find more? There are many good reasons. Even though 750 sounds like a large number, when you divide them into luminosity, distance and/or age bins, the number in some bins is not all that large. In particular, low-luminosity pulsars dominate the Galactic birthrate (e.g., Lyne et al. 1998) and yet we have a rather small sample of them, leading to large statistical uncertainties in birthrate calculations. Similarly, we know of very few pulsars at distances comparable to the Galactic Centre, so estimates of the Galactic population of pulsars are very uncertain except in the Solar neighbourhood. An increased sample of pulsars also is of great value to timing investigations and studies of the emission process. Young pulsars are known to suffer glitches, that is, sudden increases in spin rate, and various other forms of period irregularities. These phenomena are believed to result from transitions in the superfluid interior of the neutron star and are one of the few ways that we have of investigating the physics of ultra-dense matter (Alpar, Cheng & Pines 1989). However, the number of known glitching pulsars is relatively modest (Wang et al. 2000) and there is great variety in glitch properties. Similarly, there is a wide variety of pulse emission properties, with special groups of pulsars such as those with interpulses or wide profiles, high polarisation, drifting sub-pulses or null periods. An increased sample is very useful for studies of properties such as these. Pulsars are also excellent probes of the interstellar medium (ISM). They are pulsed, allowing measurement of the dispersive delay due to free electrons in the ISM, and hence the column density of electrons along the path, commonly expressed as a dispersion measure (DM) in the units cm-3 pc. Given a model for the interstellar free-electron distribution (e.g. Taylor & Cordes 1993) pulsar distances can be estimated from their DM. Compared to most celestial radio sources, pulsars have strong linear polarisation, and hence measurement of Faraday rotation is relatively easy. Pulsars have the unique advantage that the DM along the path is also known, so the mean line-of-sight magnetic field strength can be directly estimated, leading to models for the Galactic magnetic field (Han, Manchester and Qiao 1999). Pulsars also have the almost unique property that they are of very small angular size, making possible observation of the full range of effects due to scattering by small-scale fluctuations in the interstellar electron density (Rickett 1990). Pulsars are also excellent probes of the interstellar neutral gas (Frail et al. 1994). Most of these investigations are limited by the spatial density of known pulsars in the Galaxy, and so increasing the sample is of great value. One of the most fascinating things about pulsars is the fact that pulsar surveys keep turning up new and sometimes totally unexpected classes of object. Even the discovery of the first pulsar itself was serendipitious. Outstanding examples are the discovery of the Vela and Crab pulsars (Large, Vaughan & Mills 1968, Staelin & Reifenstein 1968), the first binary pulsar, PSR B1913+16 (Hulse & Taylor 1974), the first millisecond pulsar (Backer et al. 1982), the first globular cluster pulsar (Lyne et al. 1987), the first eclipsing binary pulsar (Fruchter, Stinebring & Taylor 1988) and the first pulsar with a high-mass non-degenerate companion (Johnston et al. 1992b). These objects offer valuable and sometimes profound insight into physics and astrophysics (e.g. Taylor et al. 1992). Much of the motivation for continued searches comes from the expectation of finding the unexpected.
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