2 Single-pulse gamma-ray bursts

The pulse in GRB 980425 exhibited the dominant GRB spectral evolutionary mode: At lower energies pulses tend to peak later and exhibit longer decay timescales ([Norris 1995]; [Norris et al. 1996]). Since this behavior is observed in most GRBs, it is not a constraining diagnostic. Also, as noted by [Wang et Wheeler (1998)], GRB 980425 had no high-energy (NHE) emission above 300 keV (see [Pendleton et al. 1998]). Most bursts exhibit significant high energy (HE) emission, but approximately one-third, mostly dim bursts, do not. In fact, it is likely that the apparent NHE class results from brightness bias effects ([Bonnell & Norris 1999]). Nevertheless, for the putative S-GRB class, the NHE characteristic may be a distinguishing signature. Thus if gamma-ray diagnostics are of any value in recognizing S-GRB events, the salient feature is a solitary smooth, broad pulse, and possibly NHE.

We have culled single-pulse GRBs from a total sample of 1573 BATSE GRBs with four energy channel LAD data presently available at the Compton GRO Science Support Center archive. The background fitting and sample selection are described in detail in []. Discrimination between single and multiple pulse bursts was based on statistical fits to a single, lognormal pulse model ([Brock et al. 1994]) using data binned at a 256-ms timescale. Signal to noise equalized profiles were also examined for statistically significant HE emission as described in Norris et al.