Observations of X-ray emission from GRBs at late times: Flares

Swift-XRT observations of the X-ray emission from gamma ray bursts (GRBs) and during the GRB afterglow have led to many new results during the past two years. These results have included the first ever detection of an afterglow from short GRBs, the detection of X-ray emission from extremely high redshift GRBs (including one at z=6.3), the detection of many X-ray afterglows that have no evidence of a late-time jet break, the characterization of a new multi-phase light curve following the initial GRB prompt emission (sometimes including the initial prompt emission itself), and the detection of X-ray flares that sometimes occur at very late times relative to the initial prompt emission. While each of these results is exciting, this paper will concentrate exclusively on the X-ray flares from GRBs. The mean fluence of the X-ray flares is ≈ 10× less than that of the initial prompt emission, but in some cases the flare is as energetic as the prompt emission itself. The flares also display a fast rise and decay at very late times relative to the prompt emission (sometimes as late as 10 ⁵ s after T ₀), and they have very high peak fluxes relative to the underlying afterglow decay that has clearly begun prior to some flares. The properties of the flares tend to favor models in which they are due to the same GRB internal engine processes that spawned the prompt GRB emission. Therefore, GRB internal engine models must be capable of producing high fluences in the X-ray band at very late times.