*4.1. 64 ms Results*

The t-SNE plots, coloured by burst duration (T90), are shown in Figure 3 for the T<sup>100</sup> intervals of bursts from BATSE, *Swift*/BAT and *Fermi*/GBM. t-SNE plots produce a mapping onto an arbitrary space, whereby the scale of the axes have no units or physical meaning. Thus, the t-SNE plots presented in this paper do not label the X and Y axes, and the precise position of points along the axes is not significant. However, the structure within the t-SNE space is significant and is identified. A separate group of shorter-duration bursts is evident in Figure 3b for Swift/BAT, while for BATSE and *Fermi*/GBM, the separation is not as clear.

GMM clustering, applied to the t-SNE map for *Swift*/BAT (Figure 3b), identified four clusters of bursts. However, the distribution is complex and is likely unsuitable for model-based clustering. When coloured by duration, it is clear that two groups of bursts were identified within the T<sup>100</sup> intervals of *Swift*/BAT light curves: one consisting primarily of short bursts and a larger group of longer duration bursts.

**Figure 3.** 2D t-SNE representation of the extracted wavelet and PCA features from the 64 ms light curves from T<sup>0</sup> to T100, coloured by burst duration T90, for (**a**) BATSE, (**b**) *Swift*/BAT and (**c**) *Fermi*/GBM.

## *4.2. 4 ms Results*

The t-SNE plots from the analysis of the 4 ms light curves are shown in the animations in Figure 4, coloured by burst duration (T90). The video animations are available to download in the Supplementary Materials. The intervals shown in each iteration of the t-SNE plot increase by 0.1 s, starting from the burst trigger time, T0. For *Swift*/BAT, a small group of shorter-duration bursts, begins to form and separate from the larger group of longer bursts within T<sup>0</sup> + 0.2 s. This shorter group of bursts grows and detaches from the longer group by T<sup>0</sup> to T<sup>0</sup> + 1.004 s, remaining detached up to the first 3 s post-trigger, which is the maximum interval available at 4 ms resolution. For BATSE and *Fermi*/GBM, the distinction between groups is not as clear, but a similar pattern is observed—a group of shorter bursts begins to form at ∼T<sup>0</sup> + 0.2 s and grows, separating itself from the larger, longer-duration group. We conclude that the time at which the two clusters of bursts become clearly separated is T<sup>0</sup> + 1.004 s.

**Figure 4.** Animation of t-SNE projections for different GRB light-curve time-intervals at 4 ms resolution for (**a**) BATSE (**b**) *Swift*/BAT and (**c**) *Fermi*/GBM, coloured by their T<sup>90</sup> duration. The title indicated on the top axis of each figure denotes the analysed time interval, since the burst trigger. The video files are available in the Supplementary Materials.
