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5 Summary and Future Prospects

The main aim of this article was to review some of the many astrophysical applications provided by the present sample of binary and millisecond radio pulsars. The topics covered here, along with the bibliography and associated tables of observational parameters, should be useful to those wishing to delve deeper into the vast body of literature that exists.

Through an understanding of the Galactic population of radio pulsars summarised in Section 3 it is possible to predict the detection statistics of terrestrial gravitational wave detectors to nearby rapidly spinning neutron stars (see Section 3.3), as well as coalescing relativistic binaries at cosmic distances (see Section 3.4). Continued improvements in gravitational wave detector sensitivities should result in a number of interesting developments and contributions in this area. These developments and contributions might include the detection of presently known radio pulsars, as well as a population of coalescing binary systems which have not yet been detected as radio pulsars.

The phenomenal timing stability of radio pulsars leads naturally to a large number of applications, including their use as laboratories for relativistic gravity (see Section 4.4) and as natural detectors of gravitational radiation (see Section 4.5). Long-term timing experiments of the present sample of millisecond and binary pulsars currently underway appear to have tremendous potential in these areas and perhaps detect the gravitational wave background (if it exists) within the next decade.

These applications will benefit greatly from the continuing discovery of new systems by the present generation of radio pulsar searches which continue to probe new areas of parameter space. Based on the results presented in Section 3.3, it is clear that we are aware of only about 1% of the total active pulsar population in our Galaxy. It is therefore likely that we have not seen all of the pulsar zoo. More sensitive surveys using a multibeam system on the Arecibo telescope [227] are now being carried out. Future surveys with the Square Kilometre Array [132] will ultimately provide a far more complete census of the Galactic pulsar population. With the double pulsar now checked off the list of pulsar milestones given in the last version of this review [180], we look forward to new discoveries. Two possible “holy grails” of pulsar astronomy which may soon be found are the following ones:

A pulsar-black hole binary system:
Following the wide variety of science possible from the new double pulsar J0737-3039 (see Sections 2.6, 3.4.1 and 4.4), a radio pulsar with a black-hole companion would without doubt be a fantastic gravitational laboratory. Excellent places to look for such systems are globular clusters and the Galactic centre [247].

A sub-millisecond pulsar:
With the new discoveries in Terzan 5, it now appears that the original millisecond pulsar, B1937+21 is no longer the most rapidly rotating neutron star known [233Jump To The Next Citation Point]. Do neutron stars with kHz spin rates exist? Searches now have sensitivity to such objects [41] and a discovery of even one would constrain the equation of state of matter at high densities. As mentioned in Section 2.2, the R-mode instability may prevent neutron stars from reaching such high spin rates [36607].

Pulsar astronomy remains an extremely active area of modern astrophysics and the next decade will undoubtedly continue to produce new results from currently known objects as well as new surprises. In my opinion, pulsar research is currently limited by a shortage of researchers, and not necessarily resources. Keen graduate students more than ever are needed to help shape the future of this exciting and continually evolving field.

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