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3.3 Millisecond pulsars

The population of known millisecond pulsars (MSPs) seems to be one of the fastest growing populations of relativistic binaries in globular clusters. In the past few years, improved searching techniques have revealed 47 pulsars in globular clusters [98Jump To The Next Citation Point]. The lion’s share of these are in 47 Tuc, with 20 - nine of which were recently discovered with the Parkes radio telescope [20Jump To The Next Citation Point]. Additional recent discoveries have been made in NGC 6544 [29Jump To The Next Citation Point30Jump To The Next Citation Point130Jump To The Next Citation Point], NGC 6266, NGC 6397, and NGC 6752 [29Jump To The Next Citation Point30].

An approach by Fruchter and Goss [51Jump To The Next Citation Point] uses deep multifrequency imaging to estimate the population of pulsars in globular clusters. In this approach, the expected number of pulsars beaming toward the earth, Npuls, is determined by the total radio luminosity observed when the radio beam width is comparable in diameter to the core of the cluster. If the minimum pulsar luminosity is Lmin and the total luminosity observed is Ltot, then, with simple assumptions on the neutron star luminosity function,

Npuls = -------Ltot-------. (11) Lminln (Ltot/Lmin)
In their observations of 7 globular clusters, Fruchter and Goss have recovered previously known pulsars in NGC 6440, NGC 6539, NGC 6624, and 47 Tuc [51Jump To The Next Citation Point]. Their estimates based on Equation (11View Equation) give evidence of a population of between 60 and 200 previously unknown pulsars in Terzan 5, and about 15 each in Liller 1 and NGC 6544 [51].

Chandra imaging of 47 Tuc has also recovered those 15 MSPs with precise radio positions [64Jump To The Next Citation Point]. One of these has now been identified with an optical counterpart that is almost certainly a He white dwarf [40]. The X-ray spectral and temporal characteristics of these suggest that over 50 of the 108 X-ray sources identified in 47 Tuc are millisecond pulsars [64].

To obtain a full understanding of the nature of millisecond pulsars, it is necessary to develop full timing solutions so that the orbits can be determined. Work has begun on the MSPs in 47 Tuc, and Freire et al. [48Jump To The Next Citation Point] have obtained timing solutions for 15 of the known pulsars, including 8 binary systems. The properties of all the globular cluster MSPs with orbital periods less than 1 day are given in Table  2, which is a subset of Table 5 in Lorimer [98Jump To The Next Citation Point].

Table 2: Short orbital period binary millisecond pulsars in globular clusters. Host clusters and orbital properties.












Pulsar
Pspin
Cluster
Porb
e
M2
Ref.
(ms)
(days)
M o .












J0024 -7204I 3. 485 47 Tuc 0. 23 < 0. 001 0. 01 [20Jump To The Next Citation Point48Jump To The Next Citation Point]
J0023 -7203J 2. 101 47 Tuc 0. 12 < 0. 0002 0. 02 [20Jump To The Next Citation Point48Jump To The Next Citation Point]
J0024 -7204O 2. 643 47 Tuc 0. 14 < 0. 004 0. 02 [20Jump To The Next Citation Point48Jump To The Next Citation Point]
J0024 -72R 3. 480 47 Tuc 0. 066 > 0. 0 0. 03 [20Jump To The Next Citation Point]
J0024 -7203U 4. 343 47 Tuc 0. 43 0. 0002 .. . [20Jump To The Next Citation Point48]
J0024 -72W 2. 352 47 Tuc 0. 11 > 0. 0 0. 15 [20]
B1718 -19 1004. 037 NGC 6342 0. 26 > 0. 0 0. 1 [100]
B1744 -24A 11. 563 Terzan 5 0. 08 > 0. 0 0. 1 [101114]
J1807 -24 3. 059 NGC 6544 0. 071 > 0. 0 0. 009 [29Jump To The Next Citation Point130]
J1910 -59 3. 266 NGC 6752 0. 865 > 0. 0 0. 19 [29]
J1910 +0004 3. 619 NGC 6760 0. 14 > 0. 0 0. 02 [35]
B2127 +11C 30. 529 M 15 0. 34 0. 68 0. 9 [6]












With the ongoing Parkes multi-beam surveys of globular clusters and the use of the acceleration technique [98Jump To The Next Citation Point108], the population of known MSPs will be expected to grow dramatically in the next few years. This will help improve the understanding of these objects and their progenitors as well as the dynamics of globular clusters.



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