3 Discussion

  

Figure 1: BeppoSAX WFC (top) and GRBM (bottom) light curves of GRB 980425. The onset of the GRB, indicated by the zero abscissa, corresponds to 1998 April 25.909097 (i.e., 5 seconds earlier than the GRBM trigger time). The vertical bars represent the typical error associated with the individual flux points

The count rates in the first line of Table 1 correspond to $F_{\rm 2-10\ keV} \simeq 3 \ 10^{-13}$ erg s^-1 cm^-2 for S1 and to $F_{\rm 2-10\ keV} \simeq 1.6 \ 10^{-13}$ erg s^-1 cm^-2 for S2. The following data points show a decay for S1 of a factor of two in $\sim$6 months. Assuming, as suggested by the positional coincidence and by variability, that S1 is associated with SN 1998bw, the observed variation represents a lower limit on the amplitude of X-ray variability of SN 1998bw. In fact, the possible NFI detection of extended emission indicates that S1 might contain a non negligible contribution from the host galaxy of the supernova. This is the first detection of hard X-ray emission from a Type I supernova. At the distance of SN 1998bw, the luminosity observed in the range 2-10 keV, $5\ 10^{40}$ erg s^-1, is compatible with the luminosity observed in the 0.1-2.4 keV range for the Type Ic SN 1994I, the only case of soft X-ray Type I supernova emission so far detected (Immler et al. 1998). If SN 1998bw is the counterpart of GRB 980425, the production of $\gamma$-rays could be accounted for by the explosion of a very massive star ($\sim 40\ M_{\odot}$) and by the subsequent expansion of a relativistic shock, in which non thermal electrons are radiating photons of $\sim 100$ keV, provided the explosion is asymmetric, i.e. the GRB is produced in a relativistic jet (Iwamoto et al. 1998; Woosley et al. 1998; see however, Kulkarni et al. 1998). This raises the hypothesis that two classes of GRBs might exist, with apparently indistinguishable high energy characteristics, but with different progenitors. On the other hand, disregarding the extremely low probability of chance coincidence of GRB 980425 and SN 1998bw, one might consider S2 as the X-ray counterpart candidate of the burst. Assuming a power-law decay between the X-ray flux measured by the WFC in the 2-10 keV range during the GRB and the flux measured in the first NFI observation, we derive a power-law index of $\sim -1.4$. The X-ray flux measured in May is however a factor $\sim$10 larger than implied by the power-law decay. This behavior is unlike that of previously observed X-ray afterglows, although it could be still reconciled with it under the hypothesis of a re-bursting superposed on a "typical" power-law decline.

Acknowledgements

We thank the BeppoSAX Mission Planning Team and the BeppoSAX SDC and SOC personnel for help and support in the accomplishment of this project.