Signatures of Extragalactic Dust in Pre‐SwiftGRB Afterglows

We present the results of a systematic analysis of gamma-ray burst afterglowspectral energy distributions (SEDs) in the optical/near-infrared bands. Ourinput list includes the entire world sample of afterglows observed in thepre-Swift era by the end of 2004 that have sufficient publicly available data.We apply various dust extinction models to fit the observed SEDs (Milky Way,Large Magellanic Cloud and Small Magellanic Cloud) and derive the correspondingintrinsic extinction in the GRB host galaxies and the intrinsic spectral slopesof the afterglows. We then use these results to explore the parameter space ofthe power-law index of the electron distribution function and to derive theabsolute magnitudes of the unextinguished afterglows.Comment: Submitted to ApJ 25 May 2005, accepted for publication 16 December 2005, updated 22 December 2005. 50 Pages, 12 Figures, 5 Tables. Figures 1a to 1af (30 subfigures) merged into three subfigures with downgraded resolution. Figures 7 and 8 severly downsampled in resolutio