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VLT/X-shooter spectroscopy of the GRB 090926A afterglow
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2010 (Engelska)Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 523, s. 36-Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Aims: The aim of this paper is to study the environment and intervening absorbers of the gamma-ray burst GRB 090926A through analyzing optical spectra of its afterglow. Methods: We analyzed medium-resolution spectroscopic observations (R = 10 000, corresponding to 30 km s-1, S/N = 15-30 and wavelength range 3000-25 000) of the optical afterglow of GRB 090926A, taken with X-shooter at the VLT ~22 h after the GRB trigger. Results: The spectrum shows that the ISM in the GRB host galaxy at z = 2.1071 is rich in absorption features, with two components contributing to the line profiles. In addition to the ground state lines, we detect C ii, O i, Si ii, Fe ii, and Ni ii-excited absorption features, which we used to derive information on the distance between the host absorbing gas and the site of the GRB explosion. The distance of component I is found to be 2.40 ± 0.15 kpc, while component II is located far away from the GRB, possibly at ~5 kpc. These values are compatible with those found for other GRBs. The hydrogen column density associated to GRB 090926A is log NH/cm -2 = 21.60 ± 0.07, and the metallicity of the host galaxy is in the range [X/H] = -2.5 to -1.9 with respect to the solar values, i.e., among the lowest values ever observed for a GRB host galaxy. A comparison with galactic chemical evolution models has suggested that the host of GRB090926A is likely to be a dwarf-irregular galaxy. No emission lines were detected, but a Hα flux in emission of 9 × 10-18 erg s-1 cm-2 (i.e., a star-formation rate of 2 M_ȯ yr-1), which is typical of many GRB hosts, would have been detected in our spectra, and thus emission lines are well within the reach of X-shooter. We put an upper limit to the H molecular fraction of the host galaxy ISM, which is f < 7 × 10-7. The continuum has been fitted assuming a power-law spectrum, with a spectral index of β = 0.89-0.02+0.02. The best fit does essentially not require any intrinsic extinction because EB-V < 0.01 mag adopting a SMC extinction curve. Finally, the line of sight of GRB 090926A presents four weak intervening absorption systems in the redshift range 1.24 < z < 1.95. Based on observations collected at the European Southern Observatory, ESO, the VLT/Kueyen telescope, Paranal, Chile, during the science verification phase, proposal code: 060-9427(A).

Ort, förlag, år, upplaga, sidor
2010. Vol. 523, s. 36-
Nyckelord [en]
ISM: abundances, ISM: atoms, galaxies: star formation, line: profiles, techniques: spectroscopic, atomic processes
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Identifikatorer
URN: urn:nbn:se:su:diva-49531DOI: 10.1051/0004-6361/201015216ISI: 000285346600040OAI: oai:DiVA.org:su-49531DiVA, id: diva2:378130
Tillgänglig från: 2010-12-15 Skapad: 2010-12-15 Senast uppdaterad: 2017-12-11Bibliografiskt granskad

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