2011A&A...532A..45S

Dust is an important tracer of chemical enrichment in primeval galaxies and it has also important implications for their evolution. So far, at z>6, close to the reionization epoch, the presence of dust has only been firmly established in quasar host galaxies, which are rare objects associated with enormous star formation rates. The only non-quasar host galaxy, with modest star formation rate, for which dust extinction has been tentatively detected at these early cosmic epochs, is the host of gamma ray burst GRB 050904 at z=6.3. However, the claim of dust extinction for this GRB has been debated in the past. We suggest that the discrepant results occur primarily because most of previous studies have not simultaneously investigated the X-ray to near-IR spectral energy distribution of this GRB. The difficulty with this burst is that the X-ray afterglow is dominated by strong flares at early times and is poorly monitored at late times. In addition, the Z band photometry, which is the most sensitive to dust extinction, has been found to be affected by strong systematics. In this paper we carefully re-analyze the Swift/XRT afterglow observations of this GRB, using extensive past studies of X-ray flare properties when computing the X-ray afterglow flux level and exploiting the recent reanalysis of the optical (UV rest frame) data of the same GRB. We extract the X-ray to optical/near-IR afterglow SED for the three epochs where the best spectral coverage is available: 0.47, 1.25, and 3.4 days after the trigger. A spectral power-law model has been fitted to the extracted SEDs. We discuss that no spectral breaks or chromatic temporal breaks are expected in the epochs of interest. To fit any UV rest-frame dust absorption, we tested the Small Magellanic Cloud (SMC) extinction curve, the mean extinction curve (MEC) found for a sample of QSO at z>4 and its corresponding attenuation curve, the starburst attenuation curve, and the extinction curve consistent with a supernova dust origin (SN-type). The SMC extinction curve and the SN-type one provide good fit to the data at all epochs, with an average amount of dust absorption at λ_rest=3000Å of A₃₀₀₀=0.25±0.07mag. These results indicate that the primeval galaxy at z=6.3 hosting this GRB has already enriched its ISM with dust.