Possible geometries of afterglow generation in the gamma-ray burst GRB990705

Details Possible geometries of afterglow generation in the gamma-ray burst GRB990705 Possible geometries of afterglow generation in the gamma-ray burst GRB990705

2002-07-20

arxiv.org/abs/astro-ph/0207439v5

Astron.Astrophys. 401 (2003) 593-597

Astronomy and Astrophysics

Astrophysics

5 pages, 1 figures, final version, Astronomy & Astrophysics, in press

Scientific paper

10.1051/0004-6361:20030094

The absorption feature detected in the prompt X-ray emission of GRB990705 has important consequences for its circum-burst environment and therefore on its afterglows. Here we investigate whether the circum-burst environment constrained by the absorption feature could be consistent with the observed H-band afterglow, which exhibits an earlier power law decay but a much faster decay about one day after the burst. Two possible geometries of the afterglow-emitting regions are suggested: 1) afterglow emission produced by the impact of the fireball on the surrounding torus, which serves as the absorbing material of the X-ray feature, as would be expected in the models involving that a supernova explosion precedes the gamma-ray burst by some time; 2)afterglow emission produced in the dense circum-burst medium inside the torus. In case 1), the faster decay at the later time is attributed to the disappearance of the shock due to the counter-pressure in the hot torus illuminated by the burst and afterglow photons. For case 2), the circum-burst medium density is found to be very high ($n\ga 10^4-10^5 {\rm cm^{-3}}$) if the emitting plasma is a jet or even higher if it is spherical. Future better observations of afterglows of GRBs that have absorption features might make it possible to make a more definite choice between these two scenarios.

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