REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination
Abstract
Context: Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs.
Aims: Currently, only lower limits were securely set to the initial fireball Lorentz factor Γ_0. We aim to provide a direct measure of Γ_0.
Methods: The early-time afterglow light curve carries information about Γ_0, which determines the time of the afterglow peak. We have obtained early observations of the near-infrared afterglows of GRB 060418 and GRB 060607A with the REM robotic telescope.
Results: For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of Γ_0∼ 400, fully confirming the highly relativistic nature of GRB fireballs. 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.
- Publication:
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Astronomy and Astrophysics
- Pub Date:
- July 2007
- DOI:
- 10.1051/0004-6361:20077388
- arXiv:
- arXiv:astro-ph/0612607
- Bibcode:
- 2007A&A...469L..13M
- Keywords:
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- gamma rays: bursts;
- relativity;
- Astrophysics
- E-Print:
- 6 pages, 3 color figures, 3 tables (including two online)