A log <i>N</i>_{H <scp>i</scp>}= 22.6 Damped Lyα Absorber in a Dark Gamma‐Ray Burst: The Environment of GRB 050401

The optical afterglow spectrum of GRB 050401 (at z=2.8992+/-0.0004) shows the presence of a large damped Ly-alpha absorber (DLA), with log N(HI)=22.5+/-0.3. This is the highest column density DLA ever observed, and is nearly an order of magnitude larger than any DLA observed in QSO spectra. The X-ray spectrum shows absorption above Galactic of log N(H)=22.21^{+0.06}_{-0.08} (assuming solar abundances and a redshift z=2.8992). The comparison of the X-ray column density measurement, which is dominated by metal absorption, and the optical H(I) column, allows us to derive [X/H]=-0.3+/-0.3. From the optical spectrum, we infer a zinc abundance of [Zn/H] = -0.8+/-0.4. We can then derive [alpha/Zn] = 0.5+/-0.2, suggestive of an overabundance of alpha-elements in the absorber, consistent with alpha-element overabundances observed in DLAs with similar Fe-group metallicities. There is evidence of dust depletion in Fe, Si and Cr and the optical spectrum is very reddened. This can be well reproduced with an SMC extinction curve, with A_V=0.62+/-0.06. However the total extinction can also be constrained from the optical--X-ray SED, to be 0.5&gt;A_V&gt;4.5, independent of assumptions about the shape of the extinction curve. These limits are less than the A_V=9.1^{+1.4}_{-1.5} inferred from the soft X-ray absorption. This discrepancy implies a low dust-to-metals ratio. 'Grey' extinction cannot explain the discrepancy, even with an arbitrarily flat extinction curve (i.e. large absorption with little reddening), but a large alpha-element overabundance can. If such overabundance is common in afterglow absorbers it may explain the high X-ray column densities with little optical reddening observed in afterglow spectra, and would necessitate a cautious approach to dust content based on metallicity arguments in these environments.