Author: Bruce Gendre, G. Stratta, J.L. Atteia, M. Boer, A. Klotz, for a larger collaboration
Abstact: GRB 111209A is the longest ever recorded burst. This burst was detected by Swift and Konus-Wind, and we obtained TOO time from XMM-Newton as well as prompt data from TAROT. We made a common reduction using data from these instruments together with other ones. This allows for the first time a precise study at high signal-to-noise ratio of the prompt to afterglow transition. We show that several mechanisms are responsible of this phase. In its prompt phase, we show that its duration is longer than 20 000 seconds. This, combined with the fact that the burst fluence is among the top 5% of what is observed for other events, makes this event extremely energetic. We discuss the possible progenitors that could explain the extreme duration properties of this burst as well as its spectral properties. We present evidences that this burst belong to a new, previously unidentified, class of GRBs. The most probable progenitor of this new class is a low metalicity blue super-giant star. We show that selection effects could prevent the detection of other bursts at larger redshift and conclude that this kind of event is intrinsically rare in the local Universe. Finally, the afterglow presents similar features to other normal long GRBs and it shows the presence of an early bump and a late rebrightening in the optical wavelengths, as observed in other long GRBs. We have studied these rebrightenings using optical-to-X-ray SEDs in order to investigate on the possible phenomena which are at work in this source. A last broad band SED from radio to X-rays at late times does not show significant deviations from the expected standard fireball afterglow synchrotron emission.