Summary and Future Prospects

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 that are relevant to gravitational physics. 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 4.5), constraining the equation of state of superdense matter (see Section 4.6) and as natural detectors of gravitational radiation (see Section 4.7). Long-term timing experiments of the present sample of millisecond and binary pulsars currently underway appear to have tremendous potential in these areas and may 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 a few per cent of the total active pulsar population in our Galaxy. In all likelihood, then, we have not seen all of the pulsar zoo. The current and futures surveys described in Section 2.11 will ultimately provide a far more complete census of the Galactic pulsar population. Three possible “holy grails” of pulsar astronomy which could soon be found are:

An X-ray/radio millisecond pulsar: While the link between millisecond pulsars and X-ray binaries appears to be compelling, despite intensive searches [49 ], no radio pulsations have been detected in these binaries. This could be a result of free-free absorption of any radio waves by the thick accretion disk, or perhaps quenching of the accelerating potential in the neutron star magnetosphere by infalling matter. The recent discovery of radio pulsations from two magnetars [63 , 62 ] demonstrates that radio emission can be found in seemingly unlikely sites. A millisecond pulsar in transition between an X-ray and radio phase would be another such remarkable discovery.
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 [285 ].

A sub-millisecond pulsar:: The most rapidly spinning radio pulsar currently known is the 1.39-ms PSR J1748–2446ad in Terzan 5 [144 ]. Do neutron stars with kHz spin rates exist? Searches now have sensitivity to such objects [48 ] 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 [40 , 68 ]. Of course, this does not stop observers from looking for such objects! The current generation of surveys summarized in Section 2.11 continues to be more and more sensitive to this putative source population.

The discovery of the eccentric millisecond binary pulsar J1903+0327 [72 ] (see Sections 2.9 and 4.6) was completely unprecedented within the framework of our theories for binary pulsar formation. It therefore serves as a stark reminder that we should “expect the unexpected”! Even though the study of radio pulsars is now 40 years old, these experiences tell us that many exciting discoveries lie ahead of us.

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. Although it is often stated that we are bound by available computational resources, in my opinion, pulsar research is currently limited by a shortage of researchers, and not necessarily computational resources. Keen students more than ever are needed to help shape the future of this exciting and continually evolving field.