A Very Metal-poor Damped Lyman-alpha System Revealed Through the Most Energetic GRB 090926A

We present VLT/FORS2 spectroscopy and Gamma-ray Optical/Near-Infrared Detector optical/near-IR photometry of the afterglow of the bright Fermi/LAT GRB 090926A. The spectrum shows prominent Lyman-α absorption with ^N_HI=10^21.73±0.07 cm–2 and a multitude of metal lines at a common redshift of z = 2.1062 ± 0.0004, which we associate with the redshift of the gamma-ray burst (GRB). The metallicity derived from S II is log(Z/Z ☉) ≈ –1.9, one of the lowest values ever found in a GRB Damped Lyman-α (DLA) system. This confirms that the spread of metallicity in GRB-DLAs at z ≈ 2 is at least two orders of magnitude. We argue that this spread in metallicity does not require a similar range in abundances of the GRB progenitors, since the neutral interstellar medium probed by the DLA is expected to be at a significant distance from the explosion site. The hydrogen column density derived from the Swift/XRT afterglow spectrum (assuming log(Z/Z ☉) ≈ –1.9) is approx. ≈100 times higher than the ^N_HI obtained from the Lyman-α absorptions. This suggests either a large column density of ionized gas or a higher metallicity of the circum-burst medium compared to the gas traced by the DLA. We also discuss the afterglow light curve evolution and energetics. The absence of a clear jet-break like steeping until at least 21 days post-burst suggests a beaming-corrected energy release of E γ>3.5 × 10⁵² erg, indicating that GRB 090926A may have been one of the most energetic bursts ever detected.