Absorption in gamma-ray burst afterglows

We studied the X-ray and optical absorption properties of 13 gamma-ray burst (GRB) afterglows observed by BeppoSAX. We found that X-ray absorption in addition to the Galactic one along the line of sight is highly statistically significant in the two cases with the best statistics (probability >99.9%). In three other cases the presence of X-ray absorption is marginally significant (probability similar to97%). Measured rest-frame equivalent column densities of hydrogen, N-H, range from 0.1 x 10(22) to 10.0 x 10(22) cm(-2) (at 90% confidence level) assuming a solar metal abundance. X-ray absorption may be common, although the quality of present data does not allow us to reach a firm conclusion. We found that the rest-frame column densities derived from XMM and Chandra data as quoted in the literature are in good agreement with the BeppoSAX estimated rest-frame N-H range, supporting our result. For the same GRB afterglow sample we evaluated the rest-frame visual extinction A We fitted the optical-NIR afterglow photometry with a power-law model corrected at short wavelengths by four different extinction curves. By comparing X-ray absorptions and optical extinction, we found that if a Galactic-like dust grain size distribution is assumed, a dust-to-gas ratio lower than the one observed in the Galaxy is required by the data. A dust-to-gas ratio similar to1/10 that of the Galactic one, as in the Small Magellanic Cloud (SMC) environment, has been tested using the SMC extinction curve, which produces good agreement between the best-fit N-H I and A(Vr). We note, however, that the best-fit N-H values have been obtained by assuming solar metal abundances, while the metallicity of the SMC ISM is similar to1/8 the solar one (Pei 1992). If such low metallicity were assumed, the best-fit N-H values would be higher by a factor of similar to7, providing a significant increase of the chi(2). Alternative scenarios to explain simultaneously the optical and X-ray data involve dust with grain size distributions biased toward large grains. Possible mechanisms that can bring about such a grain size distribution are discussed.