Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-02-25
Astrophys.J.595:950-954,2003
Astronomy and Astrophysics
Astrophysics
emulateapj style, 6 pages, 1 figure. Expanded version accepted for publication in ApJ Part 1
Scientific paper
10.1086/377363
Early optical afterglows have been observed from GRB 990123, GRB 021004, and GRB 021211, which reveal rich emission features attributed to reverse shocks. It is expected that Swift will discover many more early afterglows. Here we investigate in a unified manner both the forward and the reverse external shock emission components, and introduce a straightforward recipe for directly constraining the initial Lorentz factor of the fireball using early optical afterglow data. The scheme is largely independent of the shock microphysics. We identify two types of combinations of the reverse and forward shock emission, and explore their parameter regimes. We also discuss a possible diagnostic for magnetized ejecta. There is evidence that the central engine of GRB 990123 is strongly magnetized.
Kobayashi Shiho
Meszaros Peter
Zhang Bing
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