REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination

• Published in: Astronomy and astrophysics (Berlin) Vol. 469; no. 1; pp. L13 - L16
• Main Authors: Molinari, E., Vergani, S. D., Malesani, D., Covino, S., D'Avanzo, P., Chincarini, G., Zerbi, F. M., Antonelli, L. A., Conconi, P., Testa, V., Tosti, G., Vitali, F., D'Alessio, F., Malaspina, G., Nicastro, L., Palazzi, E., Guetta, D., Campana, S., Goldoni, P., Masetti, N., Meurs, E. J. A., Monfardini, A., Norci, L., Pian, E., Piranomonte, S., Rizzuto, D., Stefanon, M., Stella, L., Tagliaferri, G., Ward, P. A., Ihle, G., Gonzalez, L., Pizarro, A., Sinclaire, P., Valenzuela, J.
• Format: Journal Article
• Language: English
• Published: EDP Sciences 01.07.2007
• Subjects: Astrophysics; Cosmology and Extra-Galactic Astrophysics; gamma rays: bursts; Physics; relativity; Sciences of the Universe
• DOI: 10.1051/0004-6361:20077388
• ISSN: 0004-6361

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 $\Gamma_0$. We aim to provide a direct measure of $\Gamma_0$. Methods.The early-time afterglow light curve carries information about $\Gamma_0$, 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 $\Gamma_0\sim$ 400, fully confirming the highly relativistic nature of GRB fireballs. The deceleration radius was inferred to be $R_{\rm dec} \approx$ 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.