Following the above discussions on demography and evolution, it
is instructive to briefly summarize the rationale behind the
major searches being carried out at the present time.
The oldest radio pulsars form a virialised population of stars
oscillating in the Galactic gravitational potential. The scale
height for such a population is at least 500 pc, about 10
times that of the massive stars which populate the Galactic
plane. Since the typical ages of millisecond pulsars are several
Gyr or more, we expect, from our vantage point in the Galaxy, to
be in the middle of an essentially isotropic population of nearby
sources. All-sky searches for millisecond pulsars at high
Galactic latitudes have been very effective in probing this
population. Much of the initial interest and excitement in this
area was started at Arecibo when Wolszczan discovered two classic
recycled pulsars at high latitudes: the neutron star binary
B1534+12 [270] and the planets pulsar B1257+12 [273]. Surveys carried out at Arecibo, Parkes, Jodrell Bank and Green
Bank by others in the 1990s found many other millisecond and
recycled pulsars in this way. Camilo has written several
excellent reviews of these surveys [41,
44,
45]. See also Tables
2,
3
and
4
in the appendix.
Young pulsars are most likely to be found near to their place of
birth, close to the Galactic plane. This is the target region of
one of the Parkes multibeam surveys and has already resulted in
the discovery of around 600 new pulsars [48,
159], almost half the number currently known! Such a large haul
inevitably results in a number of interesting individual objects
such as: PSR J1141-6545, a young pulsar in a relativistic 4-hr
orbit around a white dwarf [116]; PSR J1740-3052, a young pulsar orbiting an
star (probably a giant [159]); several intermediate-mass binary pulsars [49] and a likely double neutron star system, PSR J1811-1736 [147].
In order to probe more deeply into the population of millisecond
and recycled pulsars than possible at high Galactic latitudes,
Edwards et al. have recently completed a survey of
intermediate latitudes with the Parkes multibeam system [74,
75]. The results of this survey are extremely exciting - 58 new
pulsars including 8 relatively distant recycled objects. Two of
the new recycled pulsars from this survey recently announced by
Edwards & Bailes [75] are mildly relativistic neutron star-white dwarf binaries. An
analysis of the full results from this survey should
significantly improve our knowledge on the Galaxy-wide population
and birth-rate of millisecond pulsars.
Globular clusters have long been known to be breeding grounds for
millisecond and binary pulsars. The main reason for this is the
high stellar density in globular clusters relative to most of the
rest of the Galaxy. As a result, low-mass X-ray binaries are
almost 10 times more abundant in clusters than in the Galactic
disk. In addition, exchange interactions between binary and
multiple systems in the cluster can result in the formation of
exotic binary systems. Since a single globular cluster usually
fits well within a single telescope beam, deep targeted searches
can be made. Once the DM of a pulsar is known in a globular
cluster, the DM parameter space for subsequent searches is
essentially fixed. This allows computation power to be invested
in so-called acceleration searches for short-period binary
systems (see §
3.1.3). To date, searches have revealed 47 pulsars in globular
clusters (see Table
5
in the appendix for a list and the review by Kulkarni &
Anderson [123]). Highlights include the double neutron star binary in
M15 [195] and a low-mass binary system with a 95-min orbital period in
47 Tucanae [47], one of 20 millisecond pulsars currently known in this cluster
alone [81]. On-going surveys of clusters continue to yield new
discoveries [199,
62].