Evidence for intrinsic absorption in the Swift X-ray afterglows

S Campana,O Godet,J A Kennea,P Romano,V La Parola,N Gehrels,J P Osborne,M R Goad,C Pagani,V Mangano,D N Burrows,G Cusumano,A P Beardmore,A De Luca,K L Page,J L Racusin,G Chincarini,G Tagliaferri,M Capalbi,M Perri,J A Nousek,P Giommi,L A Antonelli,P T O’brien ,J E Hill,L Angelini,S Covino,David C Morris ,Davide Lazzati ,A Wells ,A Moretti,T Mineo

doi:10.1051/0004-6361:20053823

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.

Highlights

Evidence has been accumulating in recent years that at least a subclass of Gamma-ray bursts (GRBs), the ones with a long (>∼2 s) burst event, are associated with deaths of massive stars (e.g. Woosley 1993; Paczynski 1998; MacFadyen & Woosley 1999)
We have analyzed the X-ray spectra of 17 GRB afterglows observed with Swift up to April 15, 2005
In at least 10 of them we find significant evidence that the observed column density is higher than the Galactic value

Summary

Introduction

Evidence has been accumulating in recent years that at least a subclass of Gamma-ray bursts (GRBs), the ones with a long (>∼2 s) burst event, are associated with deaths of massive stars (e.g. Woosley 1993; Paczynski 1998; MacFadyen & Woosley 1999). Stratta et al (2004) presented a systematic analysis of a sample of 13 bright afterglows observed with BeppoSAX narrow field instruments They found a significant detection of additional intervening material in only two cases (namely, GRB 990123 and GRB 010222), but, owing to the limited photon statistics, they could not exclude that intrinsic X-ray absorption is present in the other bursts. XMM-Newton observed 9 GRB afterglows (for a review see De Luca et al 2005 and Gendre et al 2005) These are mainly INTEGRAL GRBs, the large majority of which has been discovered close to the Galactic plane (i.e. are characterized by a relatively high Galactic column density). From XMM-Newton data one can gather evidence that at least several GRBs occur in high density regions within their host galaxies (e.g., De Luca et al 2005).

Swift data

Discussion

Summary and conclusions