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

TL;DR

This study analyzes the spectrum of GRB 090926A, revealing an extremely metal-poor Damped Lyman-alpha system and providing insights into the burst's energy and the interstellar medium's properties at high redshift.

Contribution

First detailed spectroscopic analysis of a very metal-poor DLA in a GRB afterglow, revealing the lowest metallicity in such systems and implications for GRB energetics.

Findings

Metallicity in the DLA is log(Z/Z_sun) ~ -1.9, one of the lowest in GRB-DLAs.

The hydrogen column density from XRT is much higher than from Lyman-alpha, indicating ionized gas or higher metallicity near the burst.

GRB 090926A's energy release exceeds 3.5x10^52 erg, making it one of the most energetic GRBs observed.

Abstract

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.