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A&A 442, L21-L24 (2005)
DOI: 10.1051/0004-6361:200500181

Letter

Gas and dust properties in the afterglow spectra of GRB 050730

R. L. C. Starling1, P. M. Vreeswijk2, S. L. Ellison3, E. Rol4, K. Wiersema1, A. J. Levan4, 5, N. R. Tanvir5, R. A. M. J. Wijers1, C. Tadhunter6, J. R. Zaurin6, R. M. Gonzalez Delgado7 and C. Kouveliotou8

1  Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
    e-mail: starling@science.uva.nl
2  European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
3  Dept. of Physics and Astronomy, University of Victoria, Elliott Building, 3800 Finnerty Rd, Victoria, BC, V8P 1A1, Canada
4  Dept. of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
5  Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Herts. AL10 9AB, UK
6  Dept. of Physics and Astronomy, The Hicks Building, University of Sheffield, Sheffield S3 7RH, UK
7  Instituto de Astrofisica de Andalucia (CSIC), PO Box 3004, 18080 Granada, Spain
8  NASA Marshall Space Flight Center, NSSTC, XD-12, 320 Sparkman Drive, Huntsville, AL 35805, USA

(Received 11 August 2005 / Accepted 3 September 2005 )

Abstract
We present early WHT ISIS optical spectroscopy of the afterglow of gamma-ray burst GRB 050730. The spectrum shows a DLA system with the highest measured hydrogen column to date: N(H ${\small\rm $) = $22.1 \pm 0.1$ at the third-highest GRB redshift z = 3.968. Our analysis of the Swift XRT X-ray observations of the early afterglow show X-ray flares accompanied by decreasing X-ray absorption. From both the optical and the X-ray spectra we constrain the dust and gas properties of the host galaxy. We find the host to be a low metallicity galaxy, with low dust content. Much of the X-ray absorbing gas is situated close to the GRB, whilst the H I absorption causing the DLA is most likely located further out.


Key words: gamma rays: bursts -- galaxies: distances and redshifts -- cosmology: observations

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© ESO 2005


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