2006A&A...452..819G

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 ∼110s after the BAT trigger (T₀) and the second one peaking at ∼154s. The first flare presents a flux variation of δF/F_peak∼3.7 and a short timescale ratio δt/t_peak∼0.07. The second flare is smaller and presents a flux variation of δF/F_peak∼1.7 and a short timescale ratio δt/t_peak∼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 α∼3.1, which decays from ∼10^–9erg/cm2/s at T₀+100s to <7x10^–13erg/cm2/s at T₀+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+171s with β_XRT–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.