2008 Supplement: Frascati Workshop 2007 / Covino

The Afterglow Onset for GRB 060418 and GRB 060607A

S. Covino, S. D. Vergani, D. Malesani, E. Molinari, P. D'Avanzo, G. Chincarini, F. M. Zerbi, L. A. Antonelli, P. Conconi, V. Testa, G. Tosti, F. Vitali, F. D'Alessio, G. Malaspina, L. Nicastro, E. Palazzi, D. Guetta, S. Campana, P. Goldoni, N. Masetti, E. J. A. Meurs, A. Monfardini, L. Norci, E. Pian, S. Piranomonte, D. Rizzuto, M. Stefanon, L. Stella, G. Tagliaferri, P. A. Ward, G. Ihle, L. Gonzalez, A. Pizarro, P. Sinclair, J. Valenzuela

Abstract

Gamma-ray burst are thought to be produced by highly relativistic
outflows. Although upper and lower limits for the outflow initial
Lorentz factor γ_0 are available, observational efforts to derive
a direct determination of γ_0 have so far failed or provided
ambiguous results. As a matter of fact, the shape of the early-time
afterglow light curve is strongly sensitive on γ_0 which
determines the time of the afterglow peak, i.e. when the outflow and the
shocked circumburst material share a comparable amount of energy. We now
comment early-time observations of the near-infrared afterglows of GRB
060418 and GRB 060607A performed by the REM robotic telescope. For both
events, the afterglow peak was singled out and allowed us to determine
the initial fireball Lorentz, γ_0˜400.

Keywords


gamma rays: bursts relativity

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