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We present VLT/FORS2 spectroscopy and GROND optical/near-IR photometry of theafterglow of the bright Fermi/LAT GRB 090926A. The spectrum shows prominentLyman-alpha absorption with N_HI = 10^(21.73 +/- 0.07) cm^-2 and a multitude ofmetal lines at a common redshift of z=2.1062 +/- 0.0004, which we associatewith the redshift of the GRB. The metallicity derived from SII is log(Z/Z_sun)~ -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~2 is atleast two orders of magnitude. We argue that this spread in metallicity doesnot require a similar range in abundances of the GRB progenitors, since theneutral interstellar medium probed by the DLA is expected to be at asignificant distance from the explosion site. The hydrogen column densityderived from Swift/XRT afterglow spectrum (assuming log(Z/Z_sun) ~ -1.9) isapprox. 100 times higher than the N_HI obtained from the Lyman-alphaabsorptions. This suggests either a large column density of ionized gas or ahigher metallicity of the circum-burst medium compared to the gas traced by theDLA. We also discuss the afterglow light curve evolution and energetics. Theabsence of a clear jet-break like steeping until at least 21 days post-burstsuggests a beaming corrected energy release of E_gamma>3.5x10^52erg, indicatingthat GRB090926A may have been one of the most energetic bursts ever detected.

A very metal poor Damped Lyman-alpha system revealed through the most energetic GRB 090926A