GRB090111: Extra soft steep-decay emission and peculiar rebrightening

We present a detailed study of GRB 090111, 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 per cent of gamma-ray bursts detected by Swift) and the following peculiar X-ray rebrightening. Our spectral analysis supports the hypothesis of a comoving band spectrum with the peak of the νF_ν spectrum evolving with time to lower values: a period of higher temporal variability in the 1-2 keV light curve ends when the E_peak evolves outside the energy band. The X-ray rebrightening 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.