A very metal poor Damped Lyman-alpha 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-alpha 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 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 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_sun) ~ -1.9) is
approx. 100 times higher than the N_HI 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>3.5x10^52erg, indicating
that GRB090926A may have been one of the most energetic bursts ever detected.
