4 Afterglow sources

Some of the TOO pointings gave the detection of a fading source, clearly associated with the burst, while some did not. In the Table 1. we show the presence of a clear or ambiguous afterglow source in the X, Optical and Radio Bands. Remembering that three of the 14 positions were not followed up, we have 7 unambiguous detections and four doubtful results. This can derive from the absence of afterglow, e.g. for an high absorption, proving the evidence of a high density environment, as suggested to explain the absence of optical transient associated to the GRB. But this could come as well from the quick decay, too fast for the pointing capability of BeppoSAX NFI. Which of the two is real is of high importance and these GRBs without afterglow deserve a short discussion.

4.1 Unambiguous detection

We consider unambiguous a detection when during the first part of the NFI observation a source is present, in the error box of WFC, with an average luminosity that makes unlikely a random occurrence of such a source in solid angle included in the error box. Moreover we require that the source has a well established fading behaviour. In the Fig. 1 we show the decay of the most luminous afterglow sources. On the left side we have indicated, as a reference, the average flux in the 2-10 keV band during the main burst itself.

  

Figure 1: X-ray afterglows

4.2 Ambiguous detection

Here we report the ambiguous afterglow detection (one or more) found in the WFCs positions and the associated probability of a random occurrence based on the cumulative distribution of ASCA-GIS (Cagnoni et al. 1998).

• GRB 970111: WFC error box = 8 square arcminutes. TOO delay 16h, J21528.1+1937 $\rm flux = 1.2 \ 10^{-13}\,erg\, cm^{-2}\,s^{-1}$, probability of random occurrence in such a field = 2.5%; suspect fading,
• GRB 971227: WFC error box = 6 arcmin radius (the Window Back skeleton covers a part of the error box), TOO delay 16h, J1257.5+5915 $\rm flux = 1.2 \ 10^{-13}\,erg\, cm^{-2}\,s^{-1}$,probability of random occurrence in such a field = 40%, J1257.3+5924, $\rm flux = 3 \ 10^{-13}\, erg\, cm^{-2}\,s^{-1}$,possible fading, probability of random occurrence in such a field = 6.5%;
• GRB 980425: WFC error box 8 arcmin radius, TOO delay 10h, J1935.0+5248 $\rm flux = 3 \ 10^{-13}\, erg\, cm^{-2}\,s^{-1}$,likely associated with SN1998bw (or its host), probability of random occurrence in such a field = 13%, J1935.3-5252 $\rm flux = 1.6 \ 10^{-13}\, erg\, cm^{-2}\,s^{-1}$,fading, probability of random occurrence in such a field = 40%;
• GRB 980515: WFC error box = 8 arcmin radius, TOO delay 10h, J2116.8-6712, $\rm flux = 2.3 \ 10^{-13}\, erg\, cm^{-2}\,s^{-1}$,fading, probability of random occurrence in such a field = 20%.

We conclude that 1 or 2 of these sources may be field sources. The statement that there are GRBs without an X-ray afterglow is not excluded, but is not supported at all by BeppoSAX data. On the other side the possibility that all the poor detections derive from fast decay afterglow should be seriously considered. It is a matter of fact that fast afterglows do exist and all the previous cases of ambiguity belong to:
1) Late TOOs.
2) Bad attitude reconstruction.
It is very likely that the sources detected in the error boxes of GRB 970111 (Feroci et al. 1998) and GRB 971227 (Antonelli et al. 1999), are really associated to the GRB afterglow, because of the small error box and of the clear fading respectively. The situation of GRB 980425 is much more intriguing and is discussed in detail in Pian et al. (1999).