Possible Geometries for Afterglow of GRB990705

Abstract

We investigate whether the circum-burst environment constrained by the absorption feature of GRB990705 could be consistent with the observed H-band afterglow. Two possible geometries of the afterglow-emitting regions are suggested: 1) afterglow emission produced by the impact of the fireball on the surrounding torus; 2)afterglow emission produced in the dense circum-burst medium inside the torus. In case 1), the faster decay at 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 ≳ 104 − 105cm−3).