2 BeppoSAX NFIs observation and discussion

A follow-up observation performed with the BeppoSAX NFIs started less than 10 hrs after the trigger and a weak, rapidly decaying X-ray source was detected (Nicastro et al. 1998a). The decay was not monotonic, but the low counting rate did not allow us to reconstruct a detailed light curve.

Figure 2 shows the 2-10 keV (MECS) flux decay and three possible power-law decay fits $F_x \propto t^{-\delta}$.It can be seen that constraining the fit to connect with the last part of the WFC detected flux, gives a minimum decay slope $\delta = 1.67\pm 0.03$. On the other hand, if the first point of the NFIs observation is not a peak superimposed to the monotonic decay, then we have $\delta = 2.25\pm 0.22$. This can be considered a maximum decay slope. It is realistic to suppose that the real slope is close to $\delta = 1.83\pm 0.30$ obtained excluding the first NFIs point from the fit. In any case, this is the most rapid decay for all the 13 GRB afterglows detected so far, typical values ranging between $1.1 \div 1.4$.

  

Figure 3: LECS+MECS 0.1-10 keV spectrum of the GRB afterglow. It is $\alpha=2.8^{+0.6}_{-0.5}$ and $N_{\rm H}\approx 0.3-2 \ 10^{22}\,{\rm cm}^{-2}$

Spectral fitting of the 0.1-10 keV LECS+MECS data using an absorbed power-law gave a quite soft photon index of 2.8^+0.6_-0.5 and $N_{\rm H}$ in the range $0.3-2 \ 10^{22}\,{\rm cm}^{-2}$ (see Fig. 3). The 0.1-2 keV flux is $(7.9\,\pm \,2.5)\ 10^{-14}$ erg cm^-2 s^-1 while the 2-10 keV flux is $(1.4\,\pm\, 0.3)\, 10^{-13}$ erg cm^-2 s^-1. Further details are given in Nicastro et al. (1998b).

Optical observations, started as early as 8 hours after the GRB, resulted in the detection of the afterglow (OT) in $VR_{\rm c}I_{\rm c}$ bands. The power-law decays in all these bands were all consistent with $\delta \simeq 2.0$.Deep observations performed $\simeq 66$ days after the burst with the 6-m telescope of the SAO-RAS revealed that at the position of the OT there is a faint object, possibly a galaxy, of magnitude $R_{\rm c} \simeq 26$(Sokolov et al. 1998b). It is worth to note that for all 9 optically identified GRBs, there are indications of the presence of an underlying host galaxy (Hogg & Fruchter 1998; Sokolov et al. 1998a).

The afterglow was also detected in the radio band by the VLA (Frail et al. 1998) at R.A. = 23$^{\rm h}$22$^{\rm m}$$21\hbox{$.\!\!^{\rm s}$}49$, Dec. = $+77^\circ15'43\hbox{$.\!\!^{\prime\prime}$}2$ (J2000, $\pm 0\hbox{$.\!\!^{\rm s}$}1$).

Acknowledgements

This research is supported by the Italian Space Agency (ASI) and Consiglio Nazionale delle Ricerche (CNR). BeppoSAX is a major program of ASI with participation of the Netherlands Agency for Aerospace Programs (NIVR). All authors warmly thank the extraordinary teams of the BeppoSAX Scientific Operation Center and Operation Control Center for their enthusiastic support to the GRB program. K.H. is grateful to the US SAX Guest Investigator program for support.