TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1477 SUBJECT: Spectropolarimetry of GRB 020813 DATE: 02/08/13 15:21:40 GMT FROM: Josh Bloom at CIT Spectropolarimetry of GRB 020813 A. J. Barth (Caltech), M. H. Cohen (Caltech), R. W. Goodrich (Keck), P.A. Price (RASAA, ANU), J. S. Bloom (Harvard/CfA, Caltech), and D. W. Fox (Caltech) report: We have observed the afterglow of GRB 020813 (Fox, Blake, & Price, GCN #1470) with the dual-beam LRIS spectropolarimeter on the Keck I 10 m telescope. GCN #1475 discusses absorption features seen in these data. A preliminary reduction of one hour of polarization data reveals significant linear polarization of 3.0% +/- 0.1% at a p.a. of 166 +/- 1 degrees from 3500-5500 Ang., and 2.8% +/- 0.1% at p.a. 158 +/- 1 degrees from 6000-9000 Ang. The fractional polarization is effectively flat and featureless with wavelength and the polarization angle exhibits a slight decrease towards the red. This message may be cited.