Evidence for intrinsic absorption in the Swift X-ray afterglows

¹ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy e-mail: campana@merate.mi.astro.it
² JILA, Campus Box 440, University of Colorado, Boulder, CO 80309-0440, USA
³ INAF – Istituto di Astrofisica spaziale e Fisica Cosmica, via Bassini 15, 20133, Milano, Italy
⁴ Università di Milano-Bicocca, Dipartimento di Fisica, Piazza della Scienza 3, 20126 Milano, Italy
⁵ INAF – Istituto di Astrofisica spaziale e Fisica Cosmica, via La Malfa 153, 90146 Palermo, Italy
⁶ Agenzia Spaziale Italiana, Science Data Center, via Galileo Galilei, 00044, Frascati, Italy
⁷ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Roma, Italy
⁸ Department of Astronomy and Astrophysics, 525 Davey Lab., Pennsylvania State University, University Park PA 16802, USA
⁹ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
¹⁰ Space Research Centre, University of Leicester, Leicester LE1 7RH, UK
¹¹ NASA/Goddard Space Flight Center, Greenbelt MD 20771, USA
¹² Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore MD 21218, USA

Received: 13 July 2005
Accepted: 22 November 2005

Abstract

Gamma-ray burst (GRB) progenitors are observationally linked to the death of massive stars. X-ray studies of the GRB afterglows can deepen our knowledge of the ionization status and metal abundances of the matter in the GRB environment. Moreover, the presence of local matter can be inferred through its fingerprints in the X-ray spectrum, i.e. the presence of absorption higher than the Galactic value. A few studies based on BeppoSAX and XMM-Newton found evidence of higher than Galactic values for the column density in a number of GRB afterglows. Here we report on a systematic analysis of 17 GRBs observed by Swift up to April 15, 2005. We observed a large number of GRBs with an excess of column density. Our sample, together with previous determinations of the intrinsic column densities for GRBs with known redshift, provides evidence for a distribution of absorption consistent with that predicted for randomly occurring GRB within molecular clouds.