Probing the ambient medium of GRB 090618 with XMM-Newton observations

Abstract

Long gamma-ray bursts (GRBs) signal the death of massive stars. The afterglow emission can be used to probe the progenitor ambient through a detailed study of the absorption pattern imprinted by the circumburst material as well as the host galaxy interstellar medium on the continuum spectrum. This has been done at optical wavelengths with impressive results. Similar studies can in principle be carried out in the X-ray band, allowing us to shed light on the material metallicity, composition and distance of the absorber. We start exploiting this route through high-resolution spectroscopy XMM-Newton observations of GRB 090618. We find a high-metallicity absorbing medium (Z≳ 0.2 Z_⊙) with possible enhancements of S and Ne with respect to the other elements (improving the fit at a level of >3.4σ). Including the metallicity effects on the X-ray column density determination, the X-ray and optical evaluations of the absorption are in agreement for a Small Magellanic Cloud extinction curve.