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

Understand

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.