7 Astrophysics with Gravitational Waves

Gravitational radiation plays an observable role in the dynamics of many known astronomical systems. In some, such as cataclysmic variables [157Jump To The Next Citation Point] and neutron-star–binary systems [358], the role of gravitational radiation has been understood for years. In others, such as young neutron stars [37Jump To The Next Citation Point] and low-mass X-ray binaries [79Jump To The Next Citation Point], the potential importance of gravitational radiation has been understood only recently. As further observations, particularly at X-ray wavelengths, become available, the usefulness of gravitational radiation as a tool for modelling astronomical systems should increase [385Jump To The Next Citation Point].

At this point in the progress of gravitational wave detection, the greatest emphasis in calculations of sources is on prediction: trying to anticipate what might be seen. Not only is this important in motivating the construction of detectors, but it also guides details of their design and, very importantly, the design of data analysis methods. Historically, many predictions of emission strengths and the capability of detectors to extract information from signals have relied on estimates using the quadrupole formula. This was justifiable because, given the uncertainties in our astrophysical understanding of potential sources, more accurate calculations would be unjustified in most cases.

But these rough estimates are now being replaced by more and more detailed source models where possible. This applies particularly in two cases. One is binary orbits, where the point-mass approximation is good over a large range of observable frequencies, so that fully relativistic calculations (using the post-Newtonian methods described above) are not only possible, but are necessary for the construction of sensitive search templates in the data analysis. The second exception is the numerical simulations of the merger of black holes and neutron stars, where the dynamics is so complex that none of our analytic approximations offers us reliable guidance. In fact, these two methods are currently being joined to produce uniform models of signal evolution over as long an observation time as the signal allows. From these models we not only improve detection algorithms, but we also learn much more about the kinds of information that detections will extract from the signals.

Once gravitational waves have been observed, there will of course be a welcome shift of emphasis to include interpretation. The emphasis will be on extracting observable parameters (waveforms, polarizations, source location, etc.) from noisy data or data where (in the case of LISA) there is source confusion. These issues need considerably more attention than they have so far received.

 7.1 Interacting compact binaries
  7.1.1 Resolving the mass-inclination degeneracy
 7.2 Black hole astrophysics
  7.2.1 Gravitational waves from stellar-mass black holes
  7.2.2 Stellar-mass black-hole binaries
  7.2.3 Intermediate-mass black holes
  7.2.4 Supermassive black holes
 7.3 Neutron star astrophysics
  7.3.1 Gravitational collapse and the formation of neutron stars
  7.3.2 Neutron-star–binary mergers
  7.3.3 Neutron-star normal mode oscillations
  7.3.4 Stellar instabilities
  7.3.5 Low-mass X-ray binaries
  7.3.6 Galactic population of neutron stars
 7.4 Multimessenger gravitational-wave astronomy

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