Tables 6, 7 and Fig. 12 are only available in electronic form at http://www.edpsciences.org

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

Present address: Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... GRBM

In the case of GRB 020410, GRBM was actually switched off at the time of the burst. A $\gamma$ -ray signal was detected by Konus (Nicastro et al. 2004).

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... (within 1 day

2 days for GRB 000926.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... version 4.3

See http://heasarc.gsfc.nasa.gov/docs/xanadu/ximage/ximage.html

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... burst

In the cases of GRB 000926 and GRB 020427, we have used data gathered by the Chandra X-ray observatory to constrain the decay index (see Gendre et al. 2006; Piro et al. 2001). For GRB 011121, we have used the last WFC data points (see Piro et al. 2005).

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... time

In the case of GRB 970111 and GRB 970402, better results were obtained by using local background.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

... $\sigma_{S\gamma}=0.77$

GRB 980425 has not been included in this calculation or in the successive ones because of its peculiarity.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

...)

As for GRB 000214, z=0.44 was adopted.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.