GRB090111: extra soft steep decay emission and peculiar re-brightening

TL;DR

This paper analyzes GRB090111's unique soft steep decay and re-brightening, suggesting evolving spectral peaks and energy injection via refreshed shocks, providing insights into complex GRB emission behaviors.

Contribution

It presents a detailed spectral and temporal analysis of GRB090111, proposing a model involving evolving peak energies and refreshed shocks to explain its peculiar features.

Findings

Gamma > 5 during steep decay phase

X-ray re-brightening consistent with energy injection by refreshed shocks

Spectral evolution indicates a shifting nu*F_nu peak over time

Abstract

We present a detailed study of GRB090111, focusing on its extra soft power-law photon index Gamma>5 at the very steep decay phase emission (power-law index alpha=5.1, steeper than 96% of GRBs detected by Swift) and the following peculiar X-ray re-brightening. Our spectral analysis supports the hypothesis of a comoving Band spectrum with the the peak of the nu*F_nu 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 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.