Multicolor Observations of the GRB 000926 Afterglow

We present multicolor light curves of the optical afterglow of gamma-ray burst (GRB) 000926. Beginning ~1.5 days after the burst, the light curves of this GRB steepen measurably. The existence of such achromatic breaks is usually taken to be an important observational signature that the ejecta are not expanding isotropically but rather have a collimated jetlike geometry. If we interpret the data in this context, we derive an opening angle of 5°, which reduces the energy release compared with an isotropic model by a factor of ~275, to 1.7 × 10⁵¹ ergs. To fit the data with a simple jet model requires extinction along the line of sight. The derived A_V is in the range 0.11-0.82 mag, depending on the adopted extinction law and whether the electrons giving rise to the optical emission are undergoing synchrotron cooling or not. Since this is in excess of the expected 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.