The optical afterglow spectrum of GRB 050401 ( at z = 2.8992 \pm 0.0004 ) shows the presence of a damped Ly \alpha absorber ( DLA ) , with \log { N _ { H \textsc { i } } } = 22.6 \pm 0.3 .
This is the highest column density ever observed in a DLA , and is about five times larger than the strongest DLA detected so far in any QSO spectrum .
From the optical spectrum , we also find a very large Zn column density , allowing us to infer an abundance of [ Zn/H ] = -1.0 \pm 0.4 .
These large columns are supported by the early X-ray spectrum from Swift -XRT which shows a column density ( in excess of Galactic ) of \log { N _ { H } } = 22.21 ^ { +0.06 } _ { -0.08 } assuming solar abundances ( at z = 2.9 ) .
The comparison of this X-ray column density , which is dominated by absorption due to \alpha -chain elements , and the H i column density derived from the Ly \alpha absorption line , allows us to derive a metallicity for the absorbing matter of [ \alpha /H ] = -0.4 \pm 0.3 .
The optical spectrum is also substantially reddened and can be well reproduced with a power-law with SMC extinction , where A _ { V } = 0.62 \pm 0.06 .
But the total optical extinction can also be constrained in a way which is independent of the shape of the extinction curve : from the optical-to-X-ray spectral energy distribution we find , 0.5 \lesssim { A } _ { V } \lesssim 4.5 .
However , even this upper limit , independent of the shape of the extinction curve , is still well below the dust column that is inferred from the X-ray column density , i.e .
A _ { V } = 9.1 ^ { +1.4 } _ { -1.5 } .
This discrepancy might be explained by a small dust content with high metallicity ( low dust-to-metals ratio ) .
‘ Grey ’ extinction can not explain the discrepancy since we are comparing the metallicity to a measurement of the total extinction ( without reference to the reddening ) .
Little dust with high metallicity may be produced by sublimation of dust grains or may naturally exist in systems younger than a few hundred Myr .
From these results it is clear that dust extinction properties in GRBs derived from comparisons of optical reddening and metallicity are unreliable .