Pulse Width Evolution of Late Time X-rays Flares in GRBs: Evidence For Internal Shocks

We study the duration and variability of late time X-ray flares following gamma-ray bursts (GRBs) observed by the narrow field X-ray telescope (XRT) aboard the {\it Swift} spacecraft. These flares are thought to be indicative of late time activity by the central engine that powers the GRB and produced by means similar to those which produce the prompt emission. We use a non-parametric procedure to study the overall temporal properties of the flares and a structure function analysis to look for an evolution of the fundamental variability time-scale between the prompt and late time emission. We find a strong correlation in 28 individual x-ray flares in 18 separate GRBs between the flare duration and their time of peak flux since the GRB trigger. We also find a qualitative trend of decreasing variability as a function of time since trigger, with a characteristic minimum variability timescale $\Delta t/t=0.1$ for most flares. We interpret these results as evidence of internal shocks at collision radii that are larger than those that produced the prompt emission. Contemporaneous detections of high energy emission by GLAST could be a crucial test in determining if indeed these X-ray flares originate as internal shocks behind the afterglow, as any X-ray emission originating from behind the afterglow is expected to undergo inverse Compton scattering as it passes through the external shock.
Comment: 26 pages, 5 figures, 1 table. Submitted to ApJ. This work expands upon and formalizes our previous report at the October 2006 AAS HEAD Meeting of the discovery of pulse width evolution