On the average Gamma-Ray Burst X-ray flaring activity

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Gamma-ray burst X-ray flares are believed to mark the late time activity of the central engine. We compute the temporal evolution of the average flare luminosity < L > in the common rest frame energy band of 44 GRBs taken from the large Swift 5-years data base. Our work highlights the importance of a proper consideration of the threshold of detection of flares against the contemporaneous continuous X-ray emission. In the time interval 30 s<t<1000s we find < L >∝ t⁻².⁷± ⁰.¹; this implies that the flare isotropic energy scaling is Eiso,flare∝ t⁻¹.⁷. The decay of the continuum underlying the flare emission closely tracks the average flare luminosity evolution, with a typical flare to steep-decay luminosity ratio which is Lflare/Lsteep=4.7: this suggests that flares and continuum emission are deeply related to one another. We infer on the progenitor properties considering different models. According to the hyper-accreting black hole scenario, the average flare luminosity scaling can be obtained in the case of rapid accretion (tacc≪ t) or when the last 0.5 M of the original 14 M progenitor star are accreted. Alternatively, the steep ∝ t⁻².⁷ behaviour could be triggered by a rapid outward expansion of an accretion shock in the material feeding a convective disk. If instead we assume the engine to be a rapidly spinning magnetar, then its rotational energy can be extracted to power a jet whose luminosity is likely to be between the monopole (L∝ e⁻²ᵗ) and dipole (L∝ t⁻²) cases. In both scenarios we suggest the variability, which is the main signature of the flaring activity, to be established as a consequence of different kinds of instabilities.