GRB 050421: A possible naked burst with X-ray flares.

We present Swift observations of the faint burst GRB 050421. The X-ray light-curve shows two flares: the first peaking at ∼ 110 s after the BAT trigger (T0) and the second one peaking at ∼ 154 s. We argue that the mechanism producing these flares is probably late internal shocks. The X-ray light-curve is consistent with a power-law with a temporal index α1 ∼ 3.1. The X-ray flux decays from ∼ 10-9 erg cm-2 s-1 at T0 + 100 s to < 7 × 10-13 erg cm-2 s-1 at T0 + 1000 s. An X-ray spectral softening is also observed with time from β ∼ 0.1 to ∼ 1.2, where β is the spectral index such as Fv ∝ v-βt-α. A good joint fit to the BAT and XRT spectra indicates that the early X-ray and Gamma-ray emissions are produced by the same mechanism. The X-ray spectral softening is likely due to a shift down to lower energies of the peak of the prompt emission, and 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. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]