The “ ultra-long ” Gamma Ray Burst GRB 111209A at redshift z=0.677 , is so far the longest GRB ever observed , with rest frame prompt emission duration of \sim 4 hours .
In order to explain the burst exceptional longevity , a low metallicity blue supergiant progenitor has been invoked .
In this work , we further constrain the phenomenology and progenitor properties of this peculiar GRB by performing a multi-band temporal and spectral analysis of both the prompt and the afterglow emission .
We use proprietary and publicly available data from Swift , Konus Wind , XMM-Newton , TAROT as well as from other ground based optical and radio telescopes .
We find some peculiar properties that are possibly connected to the exceptional nature of this burst , namely : i ) an unprecedented large optical delay of 410 \pm 50 s between the peak time in gamma-rays and the peak time in optical of a marked multiwavelength flare ; ii ) multiwavelength prompt emission spectral modelling requires a certain amount of dust in the circumburst environment , with rest frame visual dust extinction of A _ { V } = 0.3 - 1.5 mag , that may undergo to destruction at late times ; iii ) we detect the presence of a hard spectral extra power law component at the end of the X-ray “steep decay phase” and before the start of the X-ray afterglow , which was never revealed so far in past GRBs .
The optical afterglow shows more usual properties , with a flux power law decay with index 1.6 \pm 0.1 and a late re-brightening feature observed at \sim 1.1 day after the first BAT trigger .
We discuss our findings in the context of several possible interpretations given so far to the complex multi-band GRB phenomenology and propose a binary channel formation for the blue supergiant progenitor .