The Ultra-Long GRB 111209a. Ii. Prompt to Afterglow and Afterglow Properties

Abstract

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 ∼ 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 ± 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±0.1 and a late re-brightening feature observed at ∼ 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.2 Stratta et al.

Magnetars in Ultra-Long Gamma-Ray Bursts and GRB 111209A

“…Our best fit was β o = 1.08 ± 0.07 with a rest frame V-band extinction magnitude of A V = 0.10 ± 0.05. This measured β o is almost identical to the value of 1.07 ± 0.15 found for the single SED at T 0 + 63 ks by Stratta et al (2013), though their result is obtained with no intrinsic extinction. Our result is compatible with each of their findings for β o during the optical afterglow.…”

“…Similarly, the temporal indices of α x = 1.43 ± 0.06 and α o = 1.30 ± 0.05 also strongly suggest a cooling break, though they are a little further away from the theoretically expected difference of 0.25 (α x − α o = 0.13 ± 0.08). The predicted offset is even more closely recovered if the X-ray temporal indices of Gendre et al (2013) (α x = 1.51±0.08) or Stratta et al (2013) (α x = 1.52 ± 0.06) are adopted. The need for a cooling break between the bands is highlighted in Figure 2, where the well-measured X-ray spectral index would greatly over-estimate the observed optical flux if no break is present.…”

Individual power density spectra ofSwiftgamma-ray bursts

“…It might be wondered whether this holds for so-called ultralong events (Levan et al 2014;Gendre et al 2013;Stratta et al 2013;Virgili et al 2013;Zhang et al 2014;Boer et al 2015). We therefore report our results on 130925A (Evans et al 2014;Piro et al 2014), although Swift-BAT data do not cover it in its entirety.…”

The Third Swift Burst Alert Telescope Gamma-Ray Burst Catalog

“…Within these detections, 7 GRBs are previously classified as ultralong GRBs, which are GRB121027A, GRB111215A, GRB111209A, GRB101225A, GRB100316D, GRB 090417B, GRB060218 (e.g., Gendre et al 2013;Virgili et al 2013;Levan et al 2014;Boër et al 2015). Studies usually refer "ultra-long GRBs" to bursts with durations  kiloseconds (e.g., Gendre et al 2013;Stratta et al 2013;Evans et al 2014;Levan et al 2014;Boër et al 2015), however, no unified definition has been adopted. In this paper, an ultra-long GRB is referred to those events with observed durations longer than 1000 s in BAT energy band.…”