A&A 401, 593-597 (2003)
Possible geometries of afterglow generation in the gamma-ray burst GRB 990705X. Y. Wang, Z. G. Dai and T. Lu
Department of Astronomy, Nanjing University, Nanjing 210093, PR China
(Received 23 July 2002 / Accepted 9 January 2003)
The absorption feature detected in the prompt X-ray emission of GRB 990705 has important consequences for its circum-burst environment and therefore on its afterglow. 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 ( ) 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.
Key words: gamma rays: bursts -- line: formation -- radiation mechanism: nonthermal
Offprint request: X. Y. Wang, email@example.com
© ESO 2003