3.3 Millisecond pulsars
The population of known millisecond pulsars (MSPs) is one of the fastest growing populations of
relativistic binaries in globular clusters. Several ongoing searches are continuing to reveal millisecond pulsars
in a number of globular clusters. Previous searches have used deep multifrequency imaging to estimate the
population of pulsars in globular clusters [73
]. In this approach, the expected number of pulsars beaming
toward the earth,
, 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
and the
total luminosity observed is
, then, with simple assumptions on the neutron star luminosity function,
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 [73
]. Their estimates based on Equation (12) 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 [73].
Current searches include the following: Arecibo, which is searching over 22 globular clusters [106
]; Green
Bank Telescope (GBT), which is working alone and in conjunction with Arecibo [124, 106]; the Giant
Metrewave Radio Telescope (GMRT), which is searching over about 10 globular clusters [68]; and Parkes,
which is searching over 60 globular clusters [42]. Although these searches have been quite successful, they
are still subject to certain selection effects such as distance, dispersion measure, and acceleration in compact
binaries [28
]. For an excellent review of the properties of all pulsars in globular clusters, see the review by
Camilo and Rasio [28
] and references therein. The properties of known pulsars in binary systems with
orbital period less than one day are listed in Table 2, which is a subset of Table 1 in Camilo and
Rasio [28
].
With the ongoing searches, it can be reasonably expected that the number of millisecond pulsars in
binary systems in globular clusters will continue to grow in the coming years.
Table 2: |
Short orbital period binary millisecond pulsars in globular clusters. Host clusters and orbital
properties. |
|