2.6 Where to look for binary and millisecond pulsars 

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.

2.6.1 All-sky searches 

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.

2.6.2 Searches close to the plane of our Galaxy 

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].

2.6.3 Searches at intermediate Galactic latitudes 

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.

2.6.4 Targeted searches of globular clusters 

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].

Binary and Millisecond Pulsars at the New Millennium Duncan R. Lorimer http://www.livingreviews.org/lrr-2001-5 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de