Gamma-ray burst long lasting X-ray flaring activity

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 t_pk ≳ 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% E_prompt. 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.