For almost 40 years, the de facto indirect evidence for the existence of gravitational waves has been precision measurements of the slow orbital inspiral of the Hulse-Taylor binary pulsar (PSR B1913+16). The slow loss of orbital energy and resultant shrinking of the orbit precisely matches the energy loss due to gravitational radiation emission predicted by general relativity.
Searches for more binary neutron star systems like PSR B1913+16 have only turned up a handful, including the Double Pulsar PSR J0737-3039 (for a discussion of the demographics of neutron star systems, including binaries, see the excellent review by Duncan Lorimer).
PSR B1913+16 has an orbital period of 7.75 hours, putting the peak frequency of its emitted gravitational waves below the expected observation window of most space-based gravitational wave detector concepts. One might imagine that other pulsar systems might exist in tighter orbits, making them prime candidates for a mission like eLISA or SGO. However, our expectations from population synthesis studies of the galaxy suggest that only a handful of NS-NS binaries might exist in a galaxy like the Milky Way. By contrast, there are expected to be some 10 million + binaries that are comprised of white dwarfs and a companion.
Many such systems have been discovered by electromagnetic telescopes, and are expected to be strong gravitational radiators in the millihertz band. These known binaries are called verification binaries (for a detailed listing, see Gijs Neleman’s verification binary wiki).
Recently, astronomers have replicated the binary pulsar feat with a close, white dwarf binary: they have measured the slow inspiral of the orbit in a WD-WD system due to the emission of gravitational radiation. As with the Hulse-Taylor pulsar, this is an indirect measurement of gravitational waves, but provides definitive confirmation that predicted sources for future gravitational wave observatories do exist and are behaving as general relativity predicts they will.
The system is known as SDSS J0651+2844, and contains two white dwarfs in a 12.75 minute orbit (arxiv:1208.5051 | ApJ). What makes J0651+2844 such a useful system is it is eclipsing— the orbital plane of the binary is oriented on the sky such that from the vantage point of earth, the stars pass in front of each other once every six minutes. This regular clockwork of eclipses makes precision timing with the light curve possible. Long term monitoring of the orbital period shows a slow but steady decrease in the orbital period; the two stars are shortening their orbital period by about 0.31 ms/yr. This slow inspiral is consistent with the loss of energy due to the emission of gravitational radiation. Given the orbital period, distance and physical character of this system, it should be a very loud source for space-based missions like eLISA or SGO.
We’ve known for a long time that there are ultracompact binaries in the galaxy; indeed there is a long history of simulating the ultra-compact binary population based on observations of known systems (see, for instance, the seminal work by Hils, Bender and Webbink ). The discovery of J0651+2844 is important because it validates our expectations from theory. Our theoretical expectations from general relativity are supported by a still-limited number of observations of systems where gravitational wave emission is important to the dynamics. In addition, the ultracompact binary population of the galaxy has been one of the principal science pillars for space-based gravitational wave observatories. They provide a detailed measure of the fossil record of Sun-like stars throughout the entire volume of the Milky Way, they provide a method to probe the interaction and evolutionary scenarios for doubly degenerate systems, and they are a progenitor population for most of the popular models that are being considered as viable candidates for Type Ia supernovae. Moreover, J0651+2844 is the first system that has been observed to be inspiralling due to the emission of gravitational radiation that will also be visible to a space-based gravitational wave observatory.