*Cumulative Counts*

Figure 6 shows that, for the *Swift*/BAT sample, Group 1 and Group 2 GRBs, identified within the T<sup>1</sup> interval, mostly consist of bursts from the classical short-duration (T<sup>90</sup> < 2 s) and long-duration (T<sup>90</sup> > 2 s) samples, respectively. However, there are some 'strays', as shown in Table 3 and discussed in Section 5.

The counts measured in Band 3 (50–100 keV) of the first second of *Swift*/BAT 4 ms light curves are summed and normalised by the number of light curves, to obtain an average cumulative counts measure for bursts in each group in Table 3 (Figure 11). The cumulative counts of Group 1 and Group 2 bursts track those of short and long GRBs, respectively, during the first second.

**Figure 11.** Normalised cumulative counts of Band 3 (50–100 keV) *Swift*/BAT 4 ms light curves. Short (T<sup>90</sup> < 2 s) and long (T<sup>90</sup> > 2 s) duration bursts, and those within the Groups 1 and 2 identified from the first 1 s of prompt emission, are shown.

The results of this analysis suggest that the behaviour of GRB pulses in the first second carries essential information, which is needed to classify GRBs in the vast majority of cases, independent of their duration. The characteristics of the 'long' Group 1 and 'short' Group 2 bursts suggests that they have not been misclassified, but are duration outliers of their identified class. Group 1 and Group 2 bursts evolve in a similar way to the traditional short and long classes, respectively.

Previous studies have interpreted the cumulative GRB light-curves slope as a measure of the cumulative power output of the central engine [143]. Combined with the association of Group 1 and 2 bursts with kilonovae and supernovae, respectively (Section 6), the cumulative counts behaviour in the first second suggests that Group 1 and 2 represent distinct progenitors, namely, the merger and collapsar populations.
