Molinari 2006 用红外余辉定伽玛暴火球的膨胀速度

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astro-ph/0612607 [abs, ps, pdf, other] :

Title: A direct measurement of the relativistic expansion velocity of gamma-ray burst fireballs
Authors: E. Molinari, S.D. Vergani, D. Malesani, S. Covino, 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
Comments: 14 pages, 2 color figures

Gamma-ray bursts are among the most powerful and remote events in the universe. It is commonly thought that these explosions originate inside fireballs expanding ultra-relativistically. Here we report a direct measurement of the relativistic expansion velocity of the fireball. Using the robotic telescope REM located in Chile we were able to catch the infrared afterglow of two gamma-ray bursts before the early maximum. By measuring the delay between the burst onset and the peak of the afterglow light curve, we determine the fireball initial Lorentz factor to be about 400, corresponding to a velocity >99.999% of the light speed.