A very metal-poor damped Lyman-α system revealed through the most energetic GRB 090926A

We present VLT/FORS2 spectroscopy and GROND optical/near-IR photometry of the afterglow of the bright Fermi/LAT GRB 090926A. The spectrum shows prominent Lyman-� absorption with NHI = 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 GRB. The metallicity derived from SII is log(Z/Z⊙) � 1.9, one of the lowest values ever found in a GRB Damped Lyman-� (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 Swift/XRT afterglow spectrum (assuming log(Z/Z⊙) � 1.9) is approx. � 100 times higher than the NHI 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 21days post-burst suggests a beaming corrected energy release of E > 3.5×10 52 erg, indicating that GRB 090926A may have been one of the most energetic bursts ever detected. Subject headings: gamma rays: bursts – X-rays: individual (GRB 090926A)