Acad Sinica, Purple Mt Observ, Nanjing 210008, Peoples R China
; Chinese Acad Sci, Natl Astron Observ, Beijing 100864, Peoples R China
; Nanjing Univ, Dept Astron, Nanjing 210093, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Lab Cosm Ray & High Energy Astrophys, Beijing, Peoples R China
Whether gamma-ray bursts are highly beamed or not is a very important question, as it has been pointed out that the beaming will lead to a sharp break in the afterglow light curves during the ultrarelativistic phase, with the breaking point determined by Gammasimilar to1/theta (0) , where Gamma is the bulk Lorentz factor and theta (0) is the initial half opening angle of the ejecta, and such a break is claimed to be present in the light curves of some GRBs. In this paper we will examine whether all the observed breaks in GRB afterglow light curves can be explained by jet effects. Here we present a detailed calculation of the jet evolution and emission, and have obtained a simple formula of bulk Lorentz factor evolution. We show that the light curves are very smoothly steepened by jet effect, and the shape of the light curve is determined by only one parameter -, where E and n are the fireball energy and surrounding medium density, respectively. We find that for GRB 990123 and GRB 991216, the jet model can approximately fit their light curves, and the values of are about 0.17 and 0.22, respectively. On the other hand, the light curves of GRB 990510, GRB 000301c, GRB 000926 and GRB 010222 cannot be fitted by the jet model, which suggests that the breaks may be caused by some other reasons, and the jet effect should be not the unique reason.