Is GRB 050904 at z=6.3 absorbed by dust?
  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, as
well as a 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 $\lambda_{rest} = 3000 \AA$ of $A_{3000} = 0.25\pm 0.07$ mag.
These results indicate that the primeval galaxy at $z = 6.3$ hosting this GRB
has already enriched its ISM with dust.
