Context. One of the most intriguing features revealed by the Swift satellite are flares that are superimposed on the gamma-ray burst (GRB) X-ray light curves. The vast majority of flares occurs before 1000?s, but some of them can be found up to 106?s after the main event. Aims. We shed light on late-time (i.e. with peak time tpk???1000?s) flaring activity. We address the morphology and energetic of flares in the window ?~103?106?s to put constraints on the temporal evolution of the flare properties and to identify possible differences in the mechanism producing the early and late-time flaring emission, if any. This requires the complete understanding of the observational biases affecting the detection of X-ray flares superimposed on a fading continuum at t?>?1000?s. Methods. We consider all Swift GRBs that exhibit late-time flares. Our sample consists of 36 flares, 14 with redshift measurements. We inherit the strategy of data analysis from Chincarini et?al. (2010) in order to make a direct comparison with the early-time flare properties. Results. The morphology of the flare light curve is the same for both early-time and late-time flares, but they differ energetically. The width of late-time flares increases with time similarly to the early-time flares. Simulations confirmed that the increase of the width with time is not owing to the decaying statistics, at least up to 104?s. The energy output of late-time flares is one order of magnitude lower than the early-time flare one,and is ?~1%Eprompt. The evolution of the peak luminosity as well as the distribution of the peak-flux-to-continuum ratio for late-time flares indicate that the flaring emission is decoupled from the underlying continuum, differently from early-time flares/steep decay. A sizable fraction of late-time flares are compatible with afterglow variability. Conclusions. The internal shock origin seems the most promising explanation for flares. However, some differences that emerge between late- and early-time flares suggest that there could be no unique explanation about the nature of late-time flares.
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