Gamma-Ray Burst: GRB 021004

GRB 021004: First Detection of a Plateau Phase in the Early Gamma-Ray Burst Afterglow


(Enlargement of the plateau phase)

We report optical observations during the first hour of the gamma-ray burst (GRB) afterglow of GRB 021004. Our observation revealed the existence of a short plateau phase, in which the afterglow remained at almost constant brightness, before an ordinary rapid fading phase. This plateau phase lasted for about 2 hours from 0.024 to 0.10 d after the burst, which corresponds to a missing blank of the early afterglow light curve of GRB 990123. We propose that the plateau phase can be interpreted as the natural evolution of synchrotron emission from the forward shock region of a blast wave. The time when the typical frequency of the synchrotron emission passes through the optical range has been predicted to be about 0.1 d after the burst, which is consistent with the observed light curve. Our scenario hence implies that the observed feature in GRB 021004 is a common nature of GRB afterglows.


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).

Uemura, Makoto; Kato, Taichi; Ishioka, Ryoko; Yamaoka, Hitoshi
Discovery of a short plateau phase in the early evolution of a gamma-ray burst afterglow
PASJ, in press
[VSNET preprint] [PDF]

Links to other pages on GRB021004

Nature letter (T. Kato and H. Yamaoka being the co-authors)

MIT page

NASA press release

HETE-2 page (RIKEN)

HETE-2 page (MIT)

HETE-2 page (LANL)

Links to other GRB pages on VSNET

GRB 990123

GRB 010222 (Kyoto first detection)

GRB 030229 (Super-bright GRB!)

Supernovae in general

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