4 Conclusions

If a source of g.w. bursts located at a distance R produces g.w. with total energy $M_{\rm gw} c^{2}$(emitted uniformly over the solid angle $4~\pi$, and in a bandwidth $\Delta \nu= {1/\tau}$ where $\tau$ is the duration of the emission process) the corresponding $h_{\rm c}$ is given by (for the best orientation and at the resonant frequency $\frac{\omega_0}{2\pi}$) $h_{\rm c} = \sqrt {{2 G M_{\rm gw}} \over {c R^2 {\omega^2}_{0} \tau}}$.For the SN 1998bw, using the known source location (RA = 19 h 35 min 03.34 s, dec. $= - 52^{\circ}$ 50' 44.8''), an estimated distance R from the Earth of 38 Mpc (Kulkarni et al. 1998b; Galama et al. 1998), and $M_{\rm gw}=1\,M_{\odot}$, we have $h_{\rm c} = 8 \ 10^{-21}$, while the sensitivity of Explorer detector is 8  10^-19 (with $\rm SNR=1$), two orders of magnitude lower (The noise peak of 144 mK would correspond to an emission in g.w. of $\sim 1600\,M_{\odot}$). During that period two other g.w. detectors were in operation with a strain sensitivity close to Explorer within a factor 1.5: Auriga (Cerdonio et al. 1995) and Allegro (Mauceli et al. 1996). We are planning to do a concidence analysis between the three antennas, both for a short period, and for four days around the GRB 980425. This analysis will allow to give an upper limit for a g.w. pulse in the hypothesis that the SN 1998bw and the GRB 980425 occur simultaneously or not.

Acknowledgements

One of the authors (P.B.) wishes to thank Daniele Fargion for a stimulating talk that triggered this work.