Dust and gas in the local environments of gamma-ray bursts

From spectral analysis on the XRT/UVOT Spectral Energy Distribution (SED) on a sample of Swift GRBs we determine the host galaxy hydrogen column density, N-H,N-X, and visual dust-extinction, AV. All but one GRBs have host galaxy dust properties that are most consistent with an SMC or LMC dust extinction law, with a range in AV from 0.11 +/- 0.07 to 0.88(-0.20)(+0.23) mags, although selection effects distort the true AV distribution. We estimate that GR13s with no afterglow detected blueward of 5500 angstrom to have an average rest-frame extinction almost seven times the average in our sample. This may help account for the similar to 1/3 of GRBs observed by Swift that have no afterglow detected by UVOT