The Gamma-Ray Burst Intrinsic Duration Distribution: Progenitor Insights across Cosmic Time

We present the distribution of the intrinsic duration of gamma-ray burst (GRB) emission. This expands upon the analysis of O. Bromberg et al. (2012, 2013), who showed evidence for collapsar progenitors based on the presence of a plateau in the distribution of T₉₀, the duration over which 90% of the prompt emission is observed. We confirm the presence of this plateau in the distribution of duration corrected for cosmological time dilation but shifted to smaller timescales by a factor of 1/(1 + z_av) ∼ 1/3, where z_av is the average GRB redshift. More significantly, we show that this plateau is only present in the sample of GRBs with redshifts greater than (1 + z) ∼ 2. This result aligns with suggestions that the low-redshift population of GRBs has a significant contribution from noncollapsar progenitors (while the high-redshift sample is dominated by collapsars). We also show the difference in this distribution between spectrally hard and soft GRBs, confirming that a plateau is only present for the soft subset of GRBs. However, when we separate the soft GRBs into low- and high-redshift subsets, we find that only the high-redshift soft GRBs show evidence of a plateau, while the low-redshift soft GRBs do not. This suggests that there exists a significant subset of spectrally soft noncollapsar progenitors at low redshift. Finally, we use the end time of the plateau to constrain the GRB progenitor density profile and radius and show that the maximum size of a collapsar is a few tenths of a solar radius.