Issue A&A Volume 401, Number 2, April II 2003 Page(s) 593 - 597 Section Formation, structure and evolution of stars DOI http://dx.doi.org/10.1051/0004-6361:20030094

A&A 401, 593-597 (2003)
DOI: 10.1051/0004-6361:20030094

Possible geometries of afterglow generation in the gamma-ray burst GRB 990705

X. Y. Wang, Z. G. Dai and T. Lu

Department of Astronomy, Nanjing University, Nanjing 210093, PR China
    e-mail: daizigao@public1.ptt.js.cn;tlu@nju.edu.cn
(Received 23 July 2002 / Accepted 9 January 2003)

Abstract
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, xywang@nju.edu.cn

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