(ed.). Gravitational waves (IOP, 2001)(422s).

38 Gravitational-wave detectorsfrequency from about 1 kHz up to 10 kHz, so suitably designed bars could, inprinciple, go after these interesting signals.A bar gets all of its sensitivity in a relatively narrow bandwidth, so if a barand an interferometer can both barely detect a burst of amplitude 10 −20 , thenthe bar has much greater sensitivity than the interferometer in its narrow band,and much worse at other frequencies. This has led recently to interest in bars asdetectors of continuous signals. If the signal frequency is in the observing band ofthe bar, it can do very well compared to interferometers. Signals from millisecondpulsars and possible signals from x-ray binaries are suitable if they have the rightfrequency. However, most known pulsars will radiate at frequencies rather lowcompared to the operating frequencies of present-day bars.The excellent sensitivity of bars in their narrow bandwidth also suits themto detecting stochastic signals. Cross-correlations of two bars or of bars withinterferometers can be better than searches with first-generation interferometers[9]. One gets no spectral information, of course, and in the long run expectedimprovements in interferometers will overtake bars in this regard.Today’s best bar detectors are orders of magnitude more sensitive than theoriginal Weber bar. Two ultra-cryogenic bars have been built and are operatingat thermodynamic temperatures below 100 mK: NAUTILUS [10] at Frascati, nearRome, and [11] in Legnaro. With a mass of several tons, these may be the coldestmassive objects ever seen anywhere in the universe. These are expected soon toreach a sensitivity of 10 −20 near 1 kHz. Already they are performing coincidenceexperiments with bars at around 4 K at Perth, Australia, and at LSU.Proposals exist in the Netherlands, Brazil, Italy, and the USA for sphericalor icosahedral detectors (see links from [10]). These detectors have moremass, so they could reach 10 −21 near 1 kHz. Because of their shape, theyhave omnidirectional antenna patterns; if they are instrumented so that all fiveindependent fundamental quadrupolar modes of vibration can be monitored, theycan do all-sky observing and determine directions as well as verify detectionsusing coincidences between modes of the same antenna.3.4 A detector in spaceAs we have noted earlier, gravitational waves from astronomical objects atfrequencies below 1 Hz are obscured by Earth-based gravity-gradient noise.Detectors must go into space to observe in this very interesting frequency range.The LISA [12] mission is likely to be the first such mission to fly. LISAwill be a triangular array of spacecraft, with arm lengths of 5 × 10 6 km, orbitingthe Sun in the Earth’s orbit, about 20 ◦ behind the Earth. The spacecraft will bein a plane inclined to the ecliptic by 60 ◦ . The three arms can be combined indifferent ways to form two independent interferometers. During the mission theconfiguration of spacecraft rotates in its plane, and the plane rotates as well, sothat LISA’s antenna pattern sweeps the sky.