We present the first results of a program to systematically study the optical-to-X-ray spectral energy distribution (SED) of Swift GRB afterglows with known redshift. The goal is to study the properties of the GRB explosion and of the intervening absorbing material. In this report we present the preliminary analysis on 23 afterglows. Thanks to Swift, we could build the SED at early times after the GRB (minutes to hours). We derived the Hydrogen column densities and the spectral slopes from the X-ray spectrum. We then constrained the visual extinction by requiring that the combined optical/X-ray SED is due to synchrotron, namely either a single power law or a broken power law with a slope change by 0.5. We confirm a low dust-to-metal ratio, smaller than in the SMC, even from the analysis of data taken significantly earlier than previously possible. Our analysis does not support the existence of ``grey dust. We also find that the synchrotron spectrum works remarkably well to explain afterglow SEDs. We clearly see, however, that during the X-ray steep decay phases and the flares, the X-ray radiation cannot be due only to afterglow emission.
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