Modeling the oxygen K absorption in the interstellar medium: an {\it XMM}-Newton view of Sco X-1

Abstract: We investigate the absorption structure of the oxygen in the interstellar medium by analyzing {\it XMM}-Newton observations of the low mass X-ray binary Sco X-1. We use simple models based on the O {\sc i} atomic cross section from different sources to fit the data and evaluate the impact of the atomic data in the interpretation of astrophysical observations. We show that relatively small differences in the atomic calculations can yield spurious results. We also show that the most complete and accurate set of atomic cross sections successfully reproduce the observed data in the $21-24.5$ {\AA} wavelength region of the spectrum. Our fits indicate that the absorption is mainly due to neutral gas with an ionization parameter of $\xi=10^{-4}$ erg cm s$^{-1}$, and an oxygen column density of $N_{\mathrm{O}}\approx 8-10\times 10^{17}$ cm$^{-2}$. Our models are able to reproduce both the K edge and the K$\alpha$ absorption line from O {\sc i}, which are the two main features in this region. We find no conclusive evidence for absorption by other than atomic oxygen.