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

100 Resonant detectors for gravitational waves and their bandwidthTable 8.4. Results of a coincidence search between data from Explorer and Nautilus. Seethe text for an explanation.Number of Number of Number of p poisson p expdays Explorer events Nautilus events 〈n〉 n c (%) (%)29.2 8527 5679 11.0 19 1.5 1.44technique is a powerful mean for reducing the noise. As an example we showin table 8.4 some results obtained recently [15] by searching for coincidencesbetween Explorer and Nautilus during 1995 and 1996.In the first column we give the number of days when both antennae wereoperating. The small number of useful days shows that it is difficult to keep a GWantenna in operation continuously with good behaviour. In future it should bepossible to increase the useful time to 70% of the total time, considering that sometime is always lost for cryogenic maintenance. In the second and third columnwe show the number of candidate events. The candidate events are obtained byintroducing a proper threshold on the data filtered with an optimum filter for shortburst detection. We notice the large number of candidate events that make itpractically impossible, using one detector alone, to search for a particular signaldue to a GW. A big improvement is obtained by the comparison of at least twodetectors.In the fourth column we give the expected number of accidental coincidencesmeasured by means of 10 000 shifts of the event times of one detector with respectto the other and using a coincidence window of w = ±0.29 s (one samplingtime for EXPLORER and NAUTILUS). This number of accidentals is smallenough to start considering the possibility of searching for a coincidence excess(though, according to astrophysical expectations, this excess should be muchsmaller than the observed accidentals). In this case the number of coincidences n c ,reported in column five, turns out to be slightly larger than the expected numberof accidentals.Finally, in column six we report the probability calculated with the Poissonformula and in column seven the experimental probability, obtained by countinghow many times we had a number of accidental coincidences equal or largerthan n c and dividing this number by the number of trials, 10 000. Theagreement between theoretical values and experimental values is good, indicatinga poissonian statistical behaviour of the data.A new and interesting result has recently been obtained by cross-correlatingthe data recorded with EXPLORER and NAUTILUS [17]. The measured spectralamplitudes of Explorer and Nautilus, shown in figure 8.2, were correlated. Theresult of such cross-correlation at a resonance of 907.2 Hz (the same for the twodetectors) has been the determination of an upper limit for that measures the