TY - JOUR
T1 - VLT/X-shooter spectroscopy of the GRB 090926A afterglow
AU - D'Elia, V.
AU - Fynbo, J. P.U.
AU - Covino, S.
AU - Goldoni, P.
AU - Jakobsson, P.
AU - Matteucci, F.
AU - Piranomonte, S.
AU - Sollerman, J.
AU - Thöne, C. C.
AU - Vergani, S. D.
AU - Vreeswijk, P. M.
AU - Watson, D. J.
AU - Wiersema, K.
AU - Zafar, T.
AU - De Ugarte Postigo, A.
AU - Flores, H.
AU - Hjorth, J.
AU - Kaper, L.
AU - Levan, A. J.
AU - Malesani, D.
AU - Milvang-Jensen, B.
AU - Pian, E.
AU - Tagliaferri, G.
AU - Tanvir, N. R.
PY - 2010/11/15
Y1 - 2010/11/15
N2 - 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 $M-\odot$ 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.
AB - 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 $M-\odot$ 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.
KW - Atomic processes
KW - Galaxies: star formation
KW - ISM: abundances
KW - ISM: atoms
KW - Line: profiles
KW - techniques: spectroscopic
UR - http://www.scopus.com/inward/record.url?scp=78449245484&partnerID=8YFLogxK
U2 - 10.1051/0004-6361/201015216
DO - 10.1051/0004-6361/201015216
M3 - Article
AN - SCOPUS:78449245484
SN - 0004-6361
VL - 523
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
IS - 2
M1 - A36
ER -