Direct GRB fireball Lorentz factor measurements through REM early afterglow observations |
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| Authors: | Susanna D Vergani |
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| Institution: | (1) Dunsink Observatory, DIAS, Dublin, Ireland;(2) School of Physical Sciences and NCPST, Dublin City University, Dublin, Ireland |
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| Abstract: | The huge energies involved in gamma-ray bursts (GRBs) coupled with the short emission time scales unavoidably imply that the
emitting source is moving relativistically, with a speed close to that of light. Here we present the REM telescope observations
of the early-time near-infrared light curves of the GRB 060418 and GRB 060607A afterglows. The detection of the afterglow
peak provides for the first time a direct measurement of the initial Lorentz factor Γ0 of the radiating material. We find that the emitting region was indeed highly relativistic in the first seconds after the
explosions, with Γ0∼400. Comparison with the Lorentz factor as determined at later epochs provides direct evidence that the emitting shell is
decelerating and confirms that the afterglow emission is powered by the dissipation of bulk kinetic energy. The deceleration
radius was inferred to be R
dec≈1017 cm. This is much larger than the internal shocks radius (believed to power the prompt emission), thus providing further evidence
for a different origin of the prompt and afterglow stages of the GRB.
Susanna D. Vergani on behalf of the REM collaboration. |
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| Keywords: | Gamma rays: bursts Relativity |
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