Optical Spectropolarimetry of the GRB 020813 Afterglow

The optical afterglow of gamma-ray burst 020813 was observed for 3 hours withthe LRIS spectropolarimeter at the Keck-I telescope, beginning 4.7 hours afterthe burst was detected by HETE-2. The spectrum reveals numerous metalabsorption lines that we identify with two systems at z=1.223 and z=1.255. Wealso detect an O II 3727 emission line at z=1.255 and we identify this galaxyas the likely host of the GRB. After a correction for Galactic interstellarpolarization, the optical afterglow has a linear polarization of 1.8-2.4%during 4.7-7.9 hours after the burst. A measurement of p = 0.80% +/- 0.16% onthe following night by Covino et al. demonstrates significant polarizationvariability over the next 14 hours. The lack of strong variability in theposition angle of linear polarization indicates that the magnetic field in thejet is likely to be globally ordered rather than composed of a number ofrandomly oriented cells. Within the framework of afterglow models withcollimated flows, the relatively low observed polarization suggests that themagnetic field components perpendicular and parallel to the shock front areonly different by about 20%.Comment: To appear in ApJ Letters. 6 pages including 2 figure