2.11 Pulsar searches

2.11.1 All-sky searches

The oldest radio pulsars form a relaxed population of stars oscillating in the Galactic gravitational potential [136]. The scale height for such a population is at least 500 pc [224], 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.

Motivated by the discovery of two recycled pulsars at high latitudes with the Arecibo telescope [400, 403], surveys carried out at Arecibo, Parkes, Jodrell Bank and Green Bank by others in the 1990s [59, 60, 61, 254, 247] discovered around 30 further objects. Although further searching of this kind has been carried out at Arecibo in the past decade [236], much of the recent efforts have been concentrated along the plane of our Galaxy and in globular clusters discussed below. Very recently, however, 12,000 square degrees of sky was surveyed using a new 350-MHz receiver on the Green Bank Telescope [1, 45]. Processing of these data is currently underway, with two millisecond pulsars found with around 10% of the dataset analysed.

2.11.2 Searches close to the plane of our Galaxy

Young pulsars are most likely to be found near to their places of birth close to the Galactic plane. This was the target region of the main Parkes multibeam survey and has so far resulted in the discovery of 783 pulsars [253, 265, 201, 149, 109, 227, 189, 51], almost half the number currently known! Such a large haul inevitably results in a number of interesting individual objects such as the relativistic binary pulsar J1141–6545 [186, 277, 26, 152, 37], a young pulsar orbiting an star (probably a main sequence B-star [342, 343]), a young pulsar in a  5 yr-eccentric orbit (e = 0.955; the most eccentric found so far) around a companion [239, 227], several intermediate-mass binary pulsars [58], and two double neutron star binaries [243, 108]. Further analyses of this rich data set are now in progress and will ensure yet more discoveries in the near future.

Motivated by the successes at Parkes, a multibeam survey is now in progress with the Arecibo telescope [294] and the Effelsberg radio telescope. The Arecibo survey has so far discovered 46 pulsars [294, 86] with notable finds including a highly relativistic binary [235] and an eccentric millisecond pulsar binary [72]. Hundreds more pulsars could be found in this survey over the next five years. A significant fraction of this yield are expected to be distant millisecond pulsars in the disk of our Galaxy. With the advent of sensitive low-noise receivers at lower observing frequencies, surveys of the Galactic plane are being carried out with the GMRT [174], Green Bank [142] and Westerbork [319]. At the time of writing, no new millisecond pulsars have been found in these searches, though significant amounts of data remain to be fully processed.

2.11.3 Searches at intermediate and high 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 [105, 103]. Among the 69 new pulsars found in the survey, 8 are relatively distant recycled objects. Two of the new recycled pulsars from this survey [103] 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 [269, 236], and the likely double neutron star system J1829+2456 [70].

Although the density of pulsars decreases with increasing Galactic latitude, discoveries away from the plane provide strong constraints on the scale height of the millisecond pulsar population. Two recent surveys with the Parkes multibeam system [51, 158] have resulted in a number of interesting discoveries. Pulsars at high latitudes are especially important for the millisecond pulsar timing array (Section 4.7.3) which benefits from widely separated pulsars on the sky to search for correlations in the cosmic gravitational wave background on a variety of angular scales.

2.11.4 Targeted searches of globular clusters

Globular clusters have long been known to be breeding grounds for millisecond and binary pulsars [64]. 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 [331]. To date, searches have revealed 140 pulsars in 26 globular clusters [300]. Early highlights include the double neutron star binary in M15 [295] and a low-mass binary system with a 95-min orbital period in 47 Tucanae [57], one of 23 millisecond pulsars currently known in this cluster alone [57, 226].

On-going surveys of clusters continue to yield new surprises [308, 92], with no less than 70 discoveries in the past five years [306]. Among these is the most eccentric binary pulsar in a globular cluster 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 [116]. The cluster with the most pulsars is now Terzan 5 which boasts 33 [309, 300], 30 of which were found with the Green Bank Telescope [366]. 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 [226]. The spin periods of the new pulsars span a much broader range (1.4 – 80 ms) including the first, third and fourth shortest spin periods of all pulsars currently known. The binary pulsars include six 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.

2.11.5 Targeted searches of other regions

While globular clusters are the richest targets for finding millisecond pulsars, other regions of interest have been searched. Recently, a search of error boxes from unidentified sources from the Energetic Gamma-Ray Experiment Telescope (EGRET) revealed three new binary pulsars J1614–2318, J1614–2230 and J1744–3922 [143, 91, 305]. None of these pulsars is likely to be energetic enough to be associated with their target EGRET sources [143]. While convincing EGRET associations with several young pulsars are now known [201], it is not clear whether millisecond pulsars are relevant to the energetics of these enigmatic sources [71]. Despite this lack of success, it is quite possible that the recent launches of the AGILE [2] and GLAST [126] gamma-ray observatories will provide further opportunities for follow-up.

Other targets of interest are X-ray point sources found with the Chandra [74] and XMM-Newton [404] observatories and TeV sources found with HESS [367]. The X-ray sources have been particularly fruitful targets for young pulsars, with a number of discoveries of extremely faint objects [55]. Although not directly relevant to the topic of this review, these searches are revolutionizing our picture of the young neutron star population and should provide valuable insights into the beaming fraction and birthrate of these pulsars.

2.11.6 Extragalactic searches

The only radio pulsars known outside of the Galactic field and its globular cluster systems are the 19 currently known in the Large and Small Magellanic Clouds [258, 90, 250]. The lack of millisecond pulsars in the sample so far is most likely due to the limited sensitivity of the searches and large distance to the clouds. Further surveys in the Magellanic clouds are warranted. Surveys of more distant galaxies have so far been fruitless. Current instrumentation is only sensitive to giant isolated pulsars of the kind observed from the Crab [135] and the millisecond pulsars [195]. While surveys for such events are on-going [197], detections of weaker periodic sources are likely to require the enhanced sensitivity of the next generation radio telescopes.

2.11.7 Surveys with new telescopes

All surveys that have so far been conducted, or will be carried out in the next few years, will ultimately be surpassed by the next generation of radio telescopes. The Allen Telescope Array in California [361] is now beginning operations and could allow large-area coverage of the 1–10 GHz sky for pulsars and transients. In Europe, the low-frequency array [369, 106] is set to discover hundreds of faint nearby pulsars [388] in the next five years. While the Square Kilometre Array [374] is not expected to be completed until 2020, a number of pathfinder instruments are now under development. In China, the Five hundred meter Aperture Spherical Telescope [111] is scheduled for completion in 2013 and will provide significant advances for pulsar research [266]. The Australian Square Kilometre Array Pathfinder, will have some applications as a pulsar instrument [167]. Very exciting wide-field search capabilities will be offered by the South African MeerKAT array of 80 dishes set to begin operations in 2012 [368].

"Binary and Millisecond Pulsars" Duncan R. Lorimer http://www.livingreviews.org/lrr-2008-8

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