A Very Metal-poor Damped Lyman-alpha System Revealed Through the Most Energetic GRB 090926A

A. 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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We present VLT/FORS2 spectroscopy and Gamma-ray Optical/Near-Infrared Detector optical/near-IR photometry of the afterglow of the bright Fermi/LAT GRB 090926A. The spectrum shows prominent Lyman-α absorption with NHI=1021.73±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 gamma-ray burst (GRB). The metallicity derived from S II is log(Z/Z ☉) ≈ –1.9, one of the lowest values ever found in a GRB Damped Lyman-α (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 the Swift/XRT afterglow spectrum (assuming log(Z/Z ☉) ≈ –1.9) is approx. ≈100 times higher than the NHI obtained from the Lyman-α 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 γ>3.5 × 1052 erg, indicating that GRB 090926A may have been one of the most energetic bursts ever detected.
Original languageEnglish
Pages (from-to)862-871
Number of pages10
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 13 Aug 2010


  • gamma-ray burst: general, X-rays: individual: GRB 090926A, Astrophysics - Cosmology and Extragalactic Astrophysics


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