Letter to the Editor

H I column densities of z > 2 Swift gamma-ray bursts

¹ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
³ Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland
⁴ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁶ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁷ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁸ Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Campus Box 3255, Chapel Hill, NC 27599, USA
⁹ Instituto de Astrofísica de Andalucía (CSIC), Apartado de Correos 3004, 18080 Granada, Spain
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received: 15 September 2006
Accepted: 10 October 2006

Abstract

Context.Before the launch of the Swift satellite, the majority of the gamma-ray burst (GRB) afterglows for which Lyα was redshifted into the observable spectrum showed evidence for a damped Lyα  absorber. This small sample indicated that GRBs explode either in galaxies, or regions within them, having high neutral hydrogen column densities.

Aims.To increase the spectroscopic sample of GRBs with and hence establish the distribution along GRB lines-of-sight.

Methods.We have obtained six GRB afterglow spectra and fitted the Lyα absorption line in each case to determine . This has been complemented with 12 other Swift values from the literature.

Results.We show that the peak of the GRB  distribution is qualitatively consistent with a model where GRBs originate in Galactic-like molecular clouds. However, a systematic difference, in particular an excess of low column-density systems compared to the predictions, indicates that selection effects and conditions within the cloud (e.g. strong ionization) influence the observed range. We also report the discovery of Lyα emission from the GRB 060714 host, corresponding to a star-formation rate of approximately 0.8 . Finally, we present accurate redshifts of the six bursts: (GRB 050319), (GRB 050922C), (GRB 060526), (GRB 060707), (GRB 060714) and (GRB 060906).