X-ray flares in the early Swift observations of the possible naked gamma-ray burst 050421

Abstract

We present the Swift observations of the faint burst GRB 050421. The X-ray light-curve shows at least two flares: the first flare peaking at ~110 s after the BAT trigger (T0) and the second one peaking at ~ 154 s. The first flare presents a flux variation of δF/Fpeak ~3.7 and a short timescale ratio δt/tpeak.~ 0.07. The second flare is smaller and presents a flux variation of δF/Fpeak~ 1.7 and a short timescale ratio δt/tpeak 0.03. We argue that the mechanism producing these flares is probably late internal shocks. The X-ray light-curve is consistent with a rapid decline with a temporal index a ~ 3.1, which decays from ~10-9 erg cm-2 s-1 at T0 + 100s to <7 × 10-13 erg cm-2 s-1 at T 0 + 900s. A possible spectral softening is also observed with time, from β~ 0.1 to ~ 1.2. A good joint fit to the BAT and XRT spectra before T0 + 171 s with βXRT-BAT ~ 0.2 indicates that the early X-ray and Gamma-ray emissions are likely produced by the same mechanism. We argue that the X-ray spectral softening, if any, is due to a shift of the peak of the prompt emission spectrum down to lower energies, and that the rapid decline of the X-ray emission is probably the tail of the prompt emission. This suggests that the X-ray emission is completely dominated by high latitude radiation and the external shock, if any, is extremely faint and below the detection threshold. GRB 050421 is likely the first "naked burst" detected by Swift. © ESO 2006.