2001MNRAS.324L..11D

The afterglow emission from a spreading jet expanding in a circumstellar cloud is discussed. Prompt X-ray radiation and a strong UV flash from the reverse shock produced by the interaction of the jet with the cloud may destroy and clear the dust out to about 30pc within the initial solid angle of the jet. As the sideways expansion of the jet becomes significant, most of the optical radiation from the high-latitude part of the jet may be absorbed by the dust outside the initial solid angle of the jet, but only the radiation from the part within the initial solid angle can be observed. We analytically show that the flux of the observational radiation decays as ∝t^–(p+1) (where p is the power-law index of the electron distribution) in the relativistic phase. This preliminary result motivates us to perform numerical calculations. Our results show that one break in the optical afterglow light curve extends over a factor of ∼3 in time rather than one decade in time in the previous jet model. These results may provide a way to judge whether gamma-ray bursts are located in dense clouds or not. Finally, we carry out a detailed modelling for the R-band afterglow of GRB 000926.