The ultra-long GRB 111209A - II. Prompt to afterglow and afterglow properties

摘要

The "ultra-long" Gamma Ray Burst GRB 111209A at redshift z=0.677, is so farthe longest GRB ever observed, with rest frame prompt emission duration of ~4hours. In order to explain the bursts exceptional longevity, a low metallicityblue supergiant progenitor has been invoked. In this work, we furtherinvestigate this peculiar burst by performing a multi-band temporal andspectral analysis of both the prompt and the afterglow emission. We useproprietary and publicly available data from Swift, Konus Wind, XMM-Newton,TAROT as well as from other ground based optical and radio telescopes. We findsome peculiar properties that are possibly connected to the exceptional natureof this burst, namely: i) an unprecedented large optical delay of 410+/-50 s ismeasured between the peak epochs of a marked flare observed also in gamma-raysafter about 2 ks from the first Swift/BAT trigger; ii) if the optical andX-ray/gamma-ray photons during the prompt emission share a common origin, assuggested by their similar temporal behavior, a certain amount of dust in thecircumburst environment should be introduced, with rest frame visual dustextinction of AV=0.3-1.5 mag; iii) at the end of the X-ray "steep decay phase"and before the start of the X-ray afterglow, we detect the presence of a hardspectral extra power law component never revealed so far. On the contrary, theoptical afterglow since the end of the prompt emission shows more commonproperties, with a flux power law decay with index alpha=1.6+/-0.1 and a latere-brightening feature at 1.1 day. We discuss our findings in the context ofseveral possible interpretations given so far to the complex multi-band GRBphenomenology. We also attempt to exploit our results to further constrain theprogenitor nature properties of this exceptionally long GRB, suggesting abinary channel formation for the proposed blue supergiant progenitor.