Spectropolarimetry of GRB 021004 - Evidence for High Velocity Lyman-alpha Absorptions

Abstract

We present spectropolarimetry observations of GRB 021004 obtained at the ESO VLT. We detect a remarkable increase of the degree of polarization blueward of the host galaxy rest frame Ly-alpha to about 5-10% that corresponds to a decrease in the continuum flux below the extrapolation of a power law. An increase of the degree of polarization is perhaps also recorded in some narrow lines, but of lower statistical significance. The broad polarization feature blueward of the Ly-alpha absorption line at 4040 Åis at least partially produced by hydrogen-rich material with velocities approaching 40,000 \kms\ located beyond the front of the afterglow shock, rather than due entirely to intergalactic Ly-alpha absorption systems on the line of sight to the GRB. The presence of the broad Ly-alpha absorption provides evidence that the GRB occurred inside the ejecta from a hydrogen-rich supernova that exploded before the GRB.