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2.9 Current and future binary and millisecond pulsar search strategies

2.9.1 All-sky searches

The oldest radio pulsars form a relaxed population of stars oscillating in the Galactic gravitational potential [113]. 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 began with the discovery of two recycled pulsars at high latitudes with the 305-m Arecibo telescope: the double neutron star binary B1534+12 [343Jump To The Next Citation Point] and the “planets pulsar” B1257+12 [346Jump To The Next Citation Point]. Surveys carried out at Arecibo, Parkes, Jodrell Bank and Green Bank (using the 140 ft telescope) by others in the 1990s (summarised in three review papers [475051]) have found many other millisecond and recycled pulsars in this way.

2.9.2 Searches close to the plane of our Galaxy

Young pulsars are most likely to be found near to their places of birth, before they have had time for their velocity to move them away, and hence they lie close to the Galactic plane (see Figure 6View Image). This was the target region of the main Parkes multibeam survey and has already resulted in the discovery of over 700 new pulsars [210221163123Jump To The Next Citation Point94Jump To The Next Citation Point], 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 4.8-hr orbit around a white dwarf [154Jump To The Next Citation Point241Jump To The Next Citation Point25Jump To The Next Citation Point127], PSR J1740-3052, a young pulsar orbiting an ~ 11Mo . star (probably a main sequence B-star [295Jump To The Next Citation Point296Jump To The Next Citation Point]), PSR 1638-4715, a young pulsar in a ~ 5 yr-eccentric orbit (e ~ 0.9) around around a 10- 20Mo . companion [193Jump To The Next Citation Point], several intermediate-mass binary pulsars [54Jump To The Next Citation Point], and two double neutron star binaries [199Jump To The Next Citation Point93Jump To The Next Citation Point]. Although the main survey has now been completed, extensions of the survey region, and re-analyses of existing data [94Jump To The Next Citation Point] will ensure further discoveries in the near future.

2.9.3 Searches at intermediate Galactic latitudes

To probe more deeply into the population of millisecond and recycled pulsars than possible at high Galactic latitudes, the Parkes multibeam system was also used to survey intermediate latitudes [9290Jump To The Next Citation Point]. Among the 69 new pulsars found in the survey, 8 are relatively distant recycled objects. Two of the new recycled pulsars from this survey [90Jump To The Next Citation Point] 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. Arecibo surveys at intermediate latitudes also continue to find new pulsars, such as the long-period binaries J2016+1948 and J0407+1607 [234Jump To The Next Citation Point190Jump To The Next Citation Point], and the likely double neutron star system J1829+2456 [62Jump To The Next Citation Point].

2.9.4 Targeted searches of globular clusters

Globular clusters have long been known to be breeding grounds for millisecond and binary pulsars [58Jump To The Next Citation Point]. The main reason for this is the high stellar density and consequently high rate of stellar interaction 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 [283]. To date, searches have revealed 103 pulsars in 24 globular clusters (see Table 5 and [23058]). Early highlights include the double neutron star binary in M15 [257Jump To The Next Citation Point] and a low-mass binary system with a 95 -min orbital period in 47 Tucanae [53Jump To The Next Citation Point], one of 22 millisecond pulsars currently known in this cluster alone [53Jump To The Next Citation Point183Jump To The Next Citation Point].

On-going surveys of clusters continue to yield new surprises [264Jump To The Next Citation Point77] including the discovery of the most eccentric binary pulsar so far - J0514-4002 is a 4.99 ms pulsar in a highly eccentric (e = 0.89) binary system in the globular cluster NGC 1851 [100Jump To The Next Citation Point]. The latest sensation, however, is the discovery [266Jump To The Next Citation Point233Jump To The Next Citation Point] of 28 millisecond pulsars in Terzan 5 with the Green Bank Telescope [232Jump To The Next Citation Point]. This brings the total known in this cluster to 26. The spin periods and orbital parameters of the new pulsars reveal that, as a population, they are significantly different to the pulsars of 47 Tucanae which have periods in the range 2-8 ms [183Jump To The Next Citation Point]. The spin periods of the new pulsars span a much broader range (1.67- 80 ms) including the second and third shortest spin periods of all pulsars currently known. The binary pulsars include two systems with eccentric orbits and likely white dwarf companions. No such systems are known in 47 Tucanae. The difference between the two pulsar populations may reflect the different evolutionary states and physical conditions of the two clusters. In particular, the central stellar density of Terzan 5 is about twice that of 47 Tucanae, suggesting that the increased rate of stellar interactions might disrupt the recycling process for the neutron stars in some binary systems and induce larger eccentricities in others.

2.9.5 Current and future surveys

Motivated by the successes at Parkes, a multibeam receiver has recently been installed at the Arecibo telescope [227Jump To The Next Citation Point]. A preliminary survey using this instrument began in 2004 and has so far discovered 10 pulsars [229]. The main virtue of this survey is the shorter integration times employed and resulting higher sensitivity to accelerated systems than the Parkes survey. Depending on the stamina of the observers, up to 1000 pulsars could be found in this survey over the next decade. Other surveys with the Green Bank Telescope [232] and with the Giant Metre Wave Radio Telescope [231] are also expected to make major contributions.

All surveys that have so far been conducted, or will be carried out in the next few years, are likely to be surpassed by the Square Kilometre Array [132Jump To The Next Citation Point] - the next generation radio telescope, planned to come online around 2020. Simulations suggest [162Jump To The Next Citation Point] that at least 104 pulsars, including 103 millisecond pulsars, could be detected in our Galaxy.

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