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Post Info TOPIC: GRB 110731a
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RE: GRB 110731a
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Title: Multiwavelength observations of GRB 110731A: GeV emission from onset to afterglow
Authors: The Fermi-LAT Collaboration, Fermi-GBM Collaboration, Swift Collaboration, GROND Collaboration, the MOA Collaboration

We report on the multiwavelength observations of the bright, long gamma-ray burst GRB, by the Fermi and Swift observatories, and by the MOA and GROND optical telescopes. The analysis of the prompt phase reveals that GRB shares many features with bright Large Area Telescope bursts observed by Fermi during the first 3 years on-orbit: a light curve with short time variability across the whole energy range during the prompt phase, delayed onset of the emission above 100 MeV, extra power law component and temporally extended high-energy emission. In addition, this the first GRB for which simultaneous GeV, X-ray, and optical data are available over multiple epochs beginning just after the trigger time and extending for more than 800 s, allowing temporal and spectral analysis in different epochs that favour emission from the forward shock in a wind-type medium. The observed temporally extended GeV emission is most likely part of the high-energy end of the afterglow emission. Both the single-zone pair transparency constraint for the prompt signal, and the spectral and temporal analysis of the forward shock afterglow emission, independently lead to an estimate of the bulk Lorentz factor of the jet Gamma ~ 500 - 550.

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Blobrana


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Title: Two distinct phases in the first 13 seconds of GRB110731a prompt emission
Authors: Mohammad A. F. Basha

In this work, the time-resolved BAT/GBM/LAT joint spectral analysis of GRB110731A during the prompt phase from GBM trigger and up to 13 seconds showed that, at the very early phase of prompt emission, the emission mechanism is the closest to the standard fireball model which over-predicts the thermal photospheric emission and used to contradict observations. Lightcurves at different energy bands revealed two distinguishable phases that may be from different regions, an early phase, which is not detected by LAT, is dominated by lower energies, which arises from the photospheric emissions without any emissions involved from dissipation mechanisms and characterised by low Lorentz factor and high radiation efficiency, this is followed by a later phase having more complex structure that remarkably follow the same track in all energy bands and is attributed to emissions from internal shocks. This burst is a good candidate to study both thermal and non-thermal emissions since the two phases can be clearly separated in lightcurve and spectrum. The rapid variation of Lorentz factor and the values of photospheric radii that are relatively farther away from the central engine in Phase 2 are more consistent with the mechanism of collisional heating in baryonic jets. Further information can be revealed by combining more wavelengths with the help of the other detectors.

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