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

Abstract

The optical afterglow spectrum of GRB050401 (at z=2.8992+/-0.0004) shows the\npresence of a DLA, with log(nHI)=22.6+/-0.3. This is the highest column density\never observed in a DLA, and is about five times larger than the strongest DLA\ndetected so far in any QSO spectrum. From the optical spectrum, we also find a\nvery large Zn column density, allowing us to infer an abundance of\n[Zn/H]=-1.0+/-0.4. These large columns are supported by the X-ray spectrum from\nSwift-XRT which shows a column density (in excess of Galactic) of\nlog(nH)=22.21^{+0.06}_{-0.08} assuming solar abundances (at z=2.9). The\ncomparison of this X-ray column density, which is dominated by absorption due\nto alpha-chain elements, and the HI column density derived from the Ly-alpha\nabsorption line, allows us to derive a metallicity for the absorbing matter of\n[alpha/H]=-0.4+/-0.3. The optical spectrum is reddened and can be well\nreproduced with a power-law with SMC extinction, where A_V=0.62+/-0.06. But the\ntotal optical extinction can also be constrained in a way which is independent\nof the shape of the extinction curve: from the optical-to-X-ray spectral energy\ndistribution we find, 0.5&lt;~A_V&lt;~4.5. However, even this upper limit,\nindependent of the shape of the extinction curve, is still well below the dust\ncolumn that is inferred from the X-ray column density, i.e.\nA_V=9.1^{+1.4}_{-1.5}. This discrepancy might be explained by a small dust\ncontent with high metallicity (low dust-to-metals ratio). `Grey' extinction\ncannot explain the discrepancy since we are comparing the metallicity to a\nmeasurement of the total extinction (without reference to the reddening).\nLittle dust with high metallicity may be produced by sublimation of dust grains\nor may naturally exist in systems younger than a few hundred Myr.\n