On the nature of late X-ray flares in Swift gamma-ray bursts

Context. Previously detected in only a few gamma-ray bursts (GRBs), X-ray flares are now observed in $\sim $50% of Swift GRBs, though their origins remain unclear. Most flares are seen early on in the afterglow decay, while some bursts exhibit flares at late times of 104 to 105 s, which may have implications for flare models. Aims. We investigate whether a sample of late time ($\gtrsim $ $1 \times 10^4$ s) flares are different from previous samples of early time flares, or whether they are merely examples on the tail of the early flare distribution. Methods. We examine the X-ray light curves of Swift bursts for late flares and compare the flare and underlying temporal power-law properties with those of early flares, and the values of these properties predicted by the blast wave model. Results. The burst sample shows late flare properties consistent with those of early flares, where the majority of the flares can be explained by either internal or external shock, though in a few cases one origin is favoured over the other. The underlying power-laws are mainly consistent with the normal decay phases of the afterglow. Conclusions. If confirmed by the ever growing sample of late time flares, this would imply that, in some cases, prolonged activity out to a day or a restarting of the central engine is required.