New Upper Limit on Gravity Wave Events in Our Galaxy

The International Gravitational Event Collaboration (IGEC) is the first ever network of cryogenic resonant-cylinder gravity wave detectors. It consists of five widely spaced detectors: one in the US (Baton Rouge), two in Italy (Legnaro and Frascati), one in Switzerland (at CERN), and one in Australia (Perth).

Searching for passing gravity waves is a delicate art since it involves sensing deformations much smaller than the size of an atomic nucleus in huge detectors meters or kilometers in size. In the resonant detector approach this means watching for longitudinal vibrations in chilled automobile-sized metal cylinders. In the interferometer approach (used at LIGO; see, for example, Update 442) the deformation is the change in the separation of distant mirrors attached to test masses. Gravity waves strong enough to be detected will most likely come from events such as the coalescence of black holes or neutron stars, and these are rare. IGEC reports now that in its first operational period it has observed no gravity waves. From this they calculate an upper limit of the order of one per year in the rate at which such gravity wave events occur in our galaxy.

GEC is not only striving to have the sensitivity to record gravity waves from events out to distances of 100 million light years but is also hoping to be able to locate the source of the waves in the sky. This would be accomplished partly by the use of more sensitive individual detectors and by improving the comparative time resolution of signals so that the network could be operated more as a true interferometer, much like networks of radio telescopes.

According to Giovanni Prodi (Universita di Trento, 39-0461-881-521, prodi@science.unitn.it) IGEC demonstrated that a network of many simultaneously operating detectors can achieve a negligible's false alarm rate, an indication of how well the detector network canit is possible to discriminate against spurius signals when looking for rare events. The false alarm rate is already as good as 10^-6 per year of observation with 4 detectors, a figure of merit thought to be satisfactory by most astronomers and those working with neutrino detectors. (Allen et al., Physical Review Letters, 11 December 2000; Select Articles; associated website.

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