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arXiv · 1004.3261

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

Arne Rau·S. Savaglio·T. Krühler·P. Afonso·J. Greiner·S. Klose·P. Schady·S. McBreen·R. Filgas·F. Olivares E.·A. Rossi·A. Updike
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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.

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Arne Rau, S. Savaglio, T. Krühler, P. Afonso, J. Greiner, S. Klose, P. Schady, S. McBreen, R. Filgas, F. Olivares E., A. Rossi, A. Updike. 2010-07-14. A very metal poor Damped Lyman-alpha system revealed through the most energetic GRB 090926A. https://doi.org/10.1088/0004-637x%2F720%2F1%2F862

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