Evidence for sub-millisecond structure in a γ-ray burst

Abstract

GAMMA-RAY bursts (GRBs) vary in duration from hundreds of seconds down to several milliseconds. Early studies¹ suggested that bursts with durations of <100 ms form a distinct class, accounting for a few per cent of the total number of detected bursts, and there is some evidence² for a break in the distribution of GRB durations at ∼600 ms, perhaps implying separate physical mechanisms for long and short bursts. Recently the estimated number of short GRBs has risen substantially. The shortest burst recorded so far is GRB820405, with duration ∼12ms (ref. 3), and the shortest spike within a burst, an unresolved feature with width <5 ms, was in GRB841215 (refs 4–7). GRB790305 had the shortest rise-time, 0.2ms. We report here that GRB910711, with apparently the shortest duration (∼8 ms) yet seen by the Burst and Transient Source Experiment (BATSE), has a time profile that shows significant submillisecond structure. The responses to this burst in the different BATSE detectors, from both direct and Earth-scattered γ-rays, show that the burst is both narrower and of higher energy than is indicated by a light-curve summed over all detectors. We detected a narrow spike of duration 200 μs in the light curve; variations on this timescale have not previously been observed in GRBs, and their explanation should be a stringent test of any GRB theory.