GRB 090111 : extra soft steep decay emission and peculiar re - brightening

We present a detailed study of GRB090111, focusing on its extra soft power-law photon index Γ > 5 at the very steep decay phase emission (power-law index α = 5.1, steeper than 96% of GRBs detected by Swift) and the following peculiar X-ray rebrightening. Our spectral analysis supports the hypothesis of a comoving Band spectrum with the the peak of the νFν spectrum evolving with time to lower values: a period of higher temporal variability in the 1-2 keV light-curve ends when the Epeak evolves outside the energy band. The X-ray re-brightening shows extreme temporal properties when compared to a homogeneous sample of 82 early flares detected by Swift. While an internal origin cannot be excluded, we show these properties to be consistent with the energy injection in refreshed shocks produced by slow shells colliding with the fastest ones from behind, well after the internal shocks that are believed to give rise to the prompt emission have ceased.