2 Data analysis and results

GRB 980425 triggered the BeppoSAX GRBM at 21:49:11 UT, and was simultaneously detected by the WFC unit 2 (Soffitta et al. 1998). The event had a duration of 31 s in the range 40-700 keV and of 40 s in the range 2-26 keV, and exhibited a single, non structured peak profile in both bands (Fig. 1). The fluences at $\gamma$- and hard X-ray energies are ($2.8 \pm 0.5$) $\ 10^{-6}$ erg cm^-2 and ($1.8 \pm 0.3)\ 10^{-6}$ erg cm^-2, respectively. (The Galactic absorption in the direction of GRB 980425, $N_{\rm HI} = 4\ 10^{20}$ cm^-2, is negligible at energies higher than 2 keV.) The BeppoSAX NFI were pointed at the $8^{\prime}$ radius error box determined by the WFC at three epochs starting 10 hours after the GRB (see Table 1; note that the first pointing has been split in two parts). The preliminary analysis of the LECS and MECS data of the first portion of the first pointing shows that inside the WFC error box, two point-like, previously unknown X-ray sources are detected with a positional uncertainty of $1\hbox{$.\mkern-4mu^\prime$}5$: $\rm 1SAXJ1935.0{-}5248$(hereafter S1), at RA = 19h 35m 05.9s and Dec = $-52^\circ$50$^{\prime}$ 03$^{\prime\prime}$, and $\rm 1SAXJ1935.3{-}5252$ (hereafter S2), at RA = 19h 35m 22.9s and Dec = $-52^\circ$ 53$^{\prime}$49$^{\prime\prime}$. Note that the coordinates distributed by Pian et al. (1998) have been revised in November 1998 (see to this regard Piro et al. 1998). The revised position of S1 is consistent within the uncertainty with the position of the optical and radio supernova SN 1998bw (Galama et al. 1998; Kulkarni et al. 1998), while the revised position of S2 is $\sim 4^{\prime}$ away from SN 1998bw, and therefore inconsistent with it (see Fig. 1 in Galama et al. 1999). The MECS count rates and upper limits for both sources during the three pointings are reported in Table 1. The upper limits have been estimated by taking into account, besides the normal photon statistics, also the fact that, at these flux levels, the MECS background may be dominated by the fluctuations of the cosmic X-ray background. The observation of November 1998 (taken about a week after the conclusion of this Conference) shows a decrease in the X-ray flux of S1 of approximately a factor of two with respect to the level measured in April-May and the suggestion of slightly extended X-ray emission around the source. During the second portion of the first pointing, as well as in the November pointing, S2 is not detected, while it is detected in the May pointing, at a marginally lower level than in the first observation (see Table 1).

  

Table 1: Journal of BeppoSAX-MECS observations