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 N_H I = 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 gamma-ray burst (GRB). The metallicity derived from S II is log(Z/Z sun) ≈ -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 sun) ≈ -1.9) is approx. ≈100 times higher than the N_H I 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.

An extremely metal poor Damped Lyman-α system revealed through GRB 090926A

SAVAGLIO, SANDRA;
2010

Abstract

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 N_H I = 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 gamma-ray burst (GRB). The metallicity derived from S II is log(Z/Z sun) ≈ -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 sun) ≈ -1.9) is approx. ≈100 times higher than the N_H I 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.
Cosmic Chemical Evolution; Interstellar medium at high redshift; Gamma-ray bursts
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.11770/135161
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