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Astron. Astrophys. 351, 811-814 (1999)

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1. Introduction

The measurement of a stochastic background of g.w. is very interesting as it might give information on the very early stages of the Universe. Various theories (Thorne 1987, Brustein et al. 1995) describe different scenarios for the generation of a stochastic background of g.w., where the intensities of the predicted phenomena are given in terms of [FORMULA], the ratio of the g.w. energy density to the critical density needed for a closed Universe.

To discriminate between the various models we need measurements over different frequency ranges, as provided by the different families of detectors that are now in operation or will start operating in the next future (Schutz 1997).

The Rome group at present has two detectors, Explorer (Astone et al. 1993) and Nautilus (Astone et al. 1997a), operating around 1 kHz. Their data have been used, separately, to put limits on [FORMULA] in this frequency range (Astone et al. 1996). The limit was [FORMULA].

The problem when using one detector only is that only an upper limit can be obtained. Instead, by crosscorrelating the data of two or more experiments a measurement of the stochastic background can be obtained, or an upper limit if the measurement turns out to be zero within the statistical error.

In the following we shall report on the result obtained when crosscorrelating the data obtained with Explorer and Nautilus.

We recall here that upper limits for the stochastic background, in the same frequency range, have been previously set also using bar detectors at room temperature in Glasgow, [FORMULA] (Hough et al. 1975, Zimmerman & Hellings 1980), interferometers [FORMULA] (Garching-Glasgow) (Compton et al. 1994), and recently the antenna Altair, operating at 1752 Hz, [FORMULA] (Astone et al. 1999).

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999