(Enlargement of the plateau phase)
Our observation first revealed the presence of a plateau phase preceding the ordinary decay of the afterglow. According to the fireball model, the optical afterglow is predicted to start rapid fading when the observed frequency becomes larger than the typical synchrotron frequency, which rapidly moves to lower-frequency regions with time. On the other hand, during a very early phase when the optical flux is still dominated by the emission under the typical synchrotron frequency, the flux is predicted to increase, as propto t^(1/2) in a case that the ambient medium has a constant density distribution (Sari et al. 1998). The time when the typical frequency passes through the optical range has been theoretically estimated to be ~0.1 d after the burst with typical physical parameters for GRB afterglows (Sari and Piran 1999). We propose that the plateau phase of GRB 021004 corresponds to a part of such an early phase of the ordinary afterglow from the forward shock. On the nature of the plateau phase, an alternative scenario may be possible that it is one of wiggles observed during the late phase (>1 d), which are apparently unusual compared with other GRB afterglows. However, the observed transition time from the early plateau to the decay phase is just what is expected from the theoretical calculation, which favors our scenario for the plateau phase. The fading trend cannot actually be described by a simple power-law between the first fading around 0.01 d and the late afterglow after 0.1 d (Malesani et al. 2002, GCN No. 1645). The late wiggles in the afterglow light curve may have just been overlooked in previous GRBs because of their sparse sampling. The increase of the fading rate around 5 days can be naturally interpreted as a normal break as seen in other GRB afterglows.
For more details, see the following paper (PASJ Letters in press, astro-ph/0303119).
Nature letter (T. Kato and H. Yamaoka being the co-authors)
NASA press release
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