The limit improves by a factor of five our previosuly published result obtained with just one detector.
But let us remark again that the basic difference between the two cases, of one detector alone and of the correlation between two detectors, is not just that of improving the upper limit. In the first case, one detector alone, only an upper limit can be estimated, because our knowledge of the detector background would be never sufficiently good to subtract it from the signal. In the second case instead, crosscorrelation of the data of two detectors, a real measurement of the background is possible, which reduces to an upper limit estimation if the signal is null within the statistical error.
This is, in fact, the new result presented in this paper. Null value of the gravitational wave background, giving the above upper limit.
By extending the period of correlation to one year, we can obtain, with the detectors in operation now, an upper limit of less than unity. This would be already very interesting for the various theoretical scenarios of the gravitational wave background.
We finally remark the very interesting perspective of performing correlations between a large interferometric detector, one of those now being built in Europe and in the USA, and an advanced resonant detector located a few tens of km apart.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999