3 Results and discussion

We identify 68 bursts in our sample as apparently consisting of a single pulse. Of these, 30 are NHE bursts and eight of these NHE bursts have profiles similar to the broad profile of GRB 980425. The BATSE time profiles for the eight bursts are shown in Fig. 1. We judge only four of the eight to have sufficiently high s/n to make the statistical determination of a single-pulse structure reliable. Their BATSE trigger numbers are 2510, 2665, 6673, and 6707 itself. Thus the proportion of events in our sample that fit the two distinguishing criteria of the putative S-GRB class as defined by their exemplar GRB 980425 smooth, single-pulse, and NHE is 4/1573 (0.25%). This fraction is possibly as high as 8/1573 (0.5%).

Attempts to match positions and times of occurrence of these eight bursts with those of SNe have yielded no matches other than already reported for GRB 980425 and SN1998bw. Further, no subset of our single-pulse GRBs shows a tendency to cluster near the Supergalactic Plane, which one might expect if the sources of such bursts lie within the nearby supercluster ([Norris et al. 1999]).

Follow-up observations ([Pian et al. 1998b]) of the original BeppoSax WFC error circle have also failed to detect a recurrence of the fading X-ray source (1SAXJ1935.3-5252) indicating that the X-ray decay signature for GRB 980425 remains similar to that for other GRBs if in fact it was the true counterpart. In that case, of the more than twenty bursts detected by BeppoSAX with X-ray or optical afterglows, none besides GRB 980425 has single-pulse morphology in its prompt emission. Future BeppoSAX detections of X-ray afterglows of broad single-pulse, NHE events like this would clearly constitute additional evidence against the putative S-GRB class.