A log NH i= 22.6 Damped Lyα Absorber in a Dark Gamma‐Ray Burst: The Environment of GRB 050401

The optical afterglow spectrum of GRB050401 (at z=2.8992+/-0.0004) shows thepresence of a DLA, with log(nHI)=22.6+/-0.3. This is the highest column densityever observed in a DLA, and is about five times larger than the strongest DLAdetected so far in any QSO spectrum. From the optical spectrum, we also find avery large Zn column density, allowing us to infer an abundance of[Zn/H]=-1.0+/-0.4. These large columns are supported by the X-ray spectrum fromSwift-XRT which shows a column density (in excess of Galactic) oflog(nH)=22.21^{+0.06}_{-0.08} assuming solar abundances (at z=2.9). Thecomparison of this X-ray column density, which is dominated by absorption dueto alpha-chain elements, and the HI column density derived from the Ly-alphaabsorption line, allows us to derive a metallicity for the absorbing matter of[alpha/H]=-0.4+/-0.3. The optical spectrum is reddened and can be wellreproduced with a power-law with SMC extinction, where A_V=0.62+/-0.06. But thetotal optical extinction can also be constrained in a way which is independentof the shape of the extinction curve: from the optical-to-X-ray spectral energydistribution we find, 0.5<~A_V<~4.5. However, even this upper limit,independent of the shape of the extinction curve, is still well below the dustcolumn 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 dustcontent with high metallicity (low dust-to-metals ratio). `Grey' extinctioncannot explain the discrepancy since we are comparing the metallicity to ameasurement of the total extinction (without reference to the reddening).Little dust with high metallicity may be produced by sublimation of dust grainsor may naturally exist in systems younger than a few hundred Myr.