The afterglow onset for GRB060418 and GRB060607A - AstrophysicsReport as inadecuate

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Abstract: Gamma-ray burst are thought to be produced by highly relativistic outflows.Although upper and lower limits for the outflow initial Lorentz factor$\Gamma 0$ are available, observational efforts to derive a directdetermination of $\Gamma 0$ have so far failed or provided ambiguous results.As a matter of fact, the shape of the early-time afterglow light curve isstrongly sensitive on $\Gamma 0$ which determines the time of the afterglowpeak, i.e. when the outflow and the shocked circumburst material share acomparable amount of energy. We now comment early-time observations of thenear-infrared afterglows of GRB 060418 and GRB 060607A performed by the REMrobotic telescope. For both events, the afterglow peak was singled out andallowed us to determine the initial fireball Lorentz, $\Gamma 0\sim 400$.

Author: S. Covino, S.D. Vergani, D. Malesani, E. Molinari, P. D'Avanzo, G. Chincarini, F.M. Zerbi, L.A. Antonelli, P. Conconi, V. Te


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