Multicolor observations of the GRB 000926 afterglow

We present multicolor light curves of the optical afterglow of gamma-ray bu rst (GRB) 000926. Beginning similar to1.5 days after the burst, the light c urves of this GRB steepen measurably. The existence of such achromatic brea ks is usually taken to be an important observational signature that the eje cta are not expanding isotropically but rather have a collimated jetlike ge ometry. If we interpret the data in this context, we derive an opening angl e of 5 degrees, which reduces the energy release compared with an isotropic model by a factor of similar to 275, to 1.7 x 10(51) ergs. To fit the data with a simple jet model requires extinction along the line of sight. The d erived is in the range A(nu) 0.11-0.82 mag, depending on the adopted extinc tion law and whether the electrons giving rise to the optical emission are undergoing synchrotron cooling or not. Since this is in excess of the expec ted extinction from our Galaxy, we attribute this to the GRB host. We note that this extinction is typical of a galactic disk, and therefore the event likely took place in the disk of its host.