*5.5. Collapsar 'Contamination'*

The fraction of *Swift*/BAT bursts within Groups 1 and 2 can be compared to the expected distributions that are specific to the *Swift*/BAT detector.

Bromberg et al. [32] quantified the contamination of short GRB samples by collapsar bursts by fitting the duration distribution of *Swift* bursts with a function representing the merger and collapsar duration distributions. The model is based on the plateau in the duration distribution for shorter durations than the jet breakout time, which is predicted by the collapsar model [106]. According to this model ∼40% of *Swift*/BAT bursts with durations <2 s are collapsar bursts. *Swift*/BAT is more sensitive to soft GRBs, meaning that low-fluence long GRBs contaminate the short GRB population to a greater extent than they do for BATSE (∼10%) and *Fermi*/GBM (∼15%).

There are 27 bursts for which Bromberg et al. [32] assigns a probability of being a non-Collapsar of >90%. The majority (23) of these are classified as Group 1 in this analysis,

indicating that Group 1 primarily consists of bursts arising from mergers. The collapsar (Group 2) contamination of *Swift*/BAT bursts with durations <2 s from our analysis of the first 1 s of prompt emission is 31.8%, or 34/107 bursts (Table 3), consistent within 1*σ* with the predictions in Bromberg et al. [32]. The collapsar contamination of short-duration bursts for the T<sup>100</sup> analysis is significantly lower, at ∼15%. These results suggest that classification based on the T<sup>1</sup> interval may be useful for identifying collapsar 'imposters' in short GRB samples.
