Letter to the Editor

Gas and dust properties in the afterglow spectra of GRB 050730

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 ) = 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.