Fermi/GBM GRB minimum timescales

The tightest constraints on progenitor radii derived from these timescales are obtained from light curves in the hardest energy channel. In the softer bands --or from measurements of the same GRBs in the hard X-rays from Swift-- we show that variability timescales tend to be a factor of two to three longer.

Applying a survival analysis to account for detections and upper limits, we find median minimum timescale in the rest frame for long-duration and short-duration GRBs of 45 and 10ms, respectively. Less than 10% of GRBs show evidence for variability on timescales below 2ms. These shortest timescales require Lorentz factors >~400 and imply typical emission radii R~1x10^14^cm for long-duration GRBs and R~3x10^13^cm for short-duration GRBs.