Determining the Dust Extinction of Gamma-ray Burst Host Galaxies: A Direct Method Based on Optical and X-ray Photometry

Abstract

We propose a straightforward method to determine the extinction of GRB host galaxies by comparing the observed optical spectra to the intrinsic ones extrapolated from the X-ray spectra. The derived extinction curves are gray and do not resemble any extinction curves of local galaxies. The amount of extinction is rather large. We model the derived extinction curves in terms of the silicate-graphite interstellar grain model and obtain the dust size distribution skewed to large grains. We determine, for the first time, the local dust-to-gas ratios of GRB host galaxies using the model-derived dust parameters and the hydrogen column densities determined from X-ray absorptions.