1 Introduction  

GRB980425 was detected (Soffitta et al. 1998) on April 25.91 UT with one of the Wide Field Cameras (WFCs) and the Gamma Ray Burst Monitor (GRBM) on board BeppoSAX, and with the Burst and Transient Source Experiment (BATSE). With respect to its $\gamma$-ray properties, GRB980425 was not a remarkable event (Galama et al. 1998).

 

Figure 1: Digitized Sky Survey image (16$\hbox{$^\prime$}$ $\times$ 16$\hbox{$^\prime$}$)of the WFC error box of GRB980425 (large circle of radius 8$\hbox{$^\prime$}$). The left boundary of the IPN annulus is indicated as well as the error boxes of the two NFI X-ray sources (dashed circles of radius 90$\hbox{$^{\prime\prime}$}$) and the position of SN1998bw. The two X-ray sources as well as the position of SN1998bw are consistent with the WFC and IPN locations

  

Figure 2: Image on the left: NTT image (May 1 1998) of SN1998bw in the barred spiral galaxy ESO 184-G82. Image on the right: Pre-discovery image from 1976 (UK Schmidt Telescope, Australia)

SN1998bw was discovered by inspection of New Technology Telescope (NTT) images and the Digitized Sky Survey (Galama et al. 1998). Its position at R.A. = $19^{\rm h}35^{\rm m}03.34^{\rm s} \pm 0.02^{\rm s}$, Decl. = $-52^{\circ}50^{\prime}44.8^{\prime\prime} \pm$ $0.2^{\prime \prime}$ (J2000.0), 1$.\mkern-4mu^\prime$6 away from the center of the 8$^\prime$ radius WFC error box (Soffitta et al. 1998), is within the BATSE/Ulysses IPN annulus and coincides with that of the transient radio source in the WFC error box (Kulkarni et al. 1998) to within 0$.\!\!^{\prime\prime}$3 (see Fig. 1). It is located in an HII region in a spiral arm of the face-on barred spiral galaxy ESO 184-G82 at a redshift of 2550 km s^-1, in the DN 1931-529 group of galaxies (see Fig. 2).

The GRB and the supernova are spatially and temporally coincident; the time of occurrence of the core collapse and the GRB coincide to within $(+0.7,\,-2.0)$ days (Iwamoto et al. 1998). Any estimate of the probability that the supernova and the GRB coincided by chance (with respect to both time and direction) suffers from the problem of a posteriori statistics, i.e., that the parameters of the problem tend to be set by the observed phenomenon itself. In this case the parameters are the size of the error box, the peak magnitude of the supernova, and the time window within which the events can be considered as possibly related. Galama et al. (1998) made generous estimates of these parameters and find the probability of catching a SN in any of the 13 WFC GRB error boxes to be $9 \ 10^{-5}$. This estimate includes all supernovae with peak magnitudes two magnitudes below that of SN1998bw, and ignores the fact that SN1998bw is of a rare type. As a result, the notion that GRB980425 and SN1998bw are physically related becomes difficult to reject purely on the basis of the fact that afterglows observed so far from GRBs are very different from supernovae.