The Energy-Dependence of GRB Minimum Variability Timescales

TL;DR

This study analyzes the minimum variability timescales in 938 GRBs observed by Fermi/GBM, revealing median timescales, energy dependence, and implications for progenitor sizes and Lorentz factors, with potential evolution over cosmic time.

Contribution

It provides the largest dataset of GRB variability timescales, applying survival analysis to distinguish between long and short GRBs and their physical emission regions.

Findings

Median minimum timescale: 45 ms for long GRBs, 10 ms for short GRBs

Less than 10% of GRBs show variability below 2 ms

Variability timescales suggest high Lorentz factors and specific emission radii

Abstract

We constrain the minimum variability timescales for 938 GRBs observed by the Fermi/GBM instrument prior to July 11, 2012. 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 2--3 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 ms and 10 ms, respectively. Fewer than 10% of GRBs show evidence for variability on timescales below 2 ms. These shortest timescales require Lorentz factors $\gtrsim 400$ and imply typical emission radii $R \approx 1 {\times} 10^{14}$ cm for long-duration GRBs and $R \approx 3 {\times} 10^{13}$ cm for short-duration GRBs. We discuss implications for the GRB fireball model and investigate whether GRB minimum timescales evolve with cosmic time.