A VERY METAL-POOR DAMPED LYMAN-α 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-alpha absorption with N-H I = 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(circle dot)) approximate to -1.9, one of the lowest values ever found in a GRB Damped Lyman-alpha (DLA) system. This confirms that the spread of metallicity in GRB-DLAs at z approximate to 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(circle dot)) approximate to -1.9) is approx. approximate to 100 times higher than the N-H I obtained from the Lyman-alpha 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-gamma > 3.5 x 10(52) erg, indicating that GRB 090926A may have been one of the most energetic bursts ever detected.