Interstellar oxygen along the line of sight of Cygnus X-2

Abstract

Interstellar dust permeates our Galaxy and plays an important role in many physical processes in the diffuse and dense regions of the interstellar medium (ISM). High-resolution X-ray spectroscopy, coupled with modelling based on laboratory dust measurements, provides a unique probe for investigating the interstellar dust properties along our line of sight towards Galactic X-ray sources. Here, we focus on the oxygen content of the ISM through its absorption features in the X-ray spectra. To model the dust features, we perform a laboratory experiment using the electron microscope facility located at the University of Cadiz in Spain, where we acquire new laboratory data in the oxygen K-edge. We study 18 dust samples of silicates and oxides with different chemical compositions. The laboratory measurements are adopted for our astronomical data analysis. We carry out a case study on the X-ray spectrum of the bright low-mass X-ray binary Cygnus X-2, observed by XMM−Newton. We determine different temperature phases of the ISM and parameterise oxygen in both gas (neutral and ionised) and dust form. We find Solar abundances of oxygen along the line of sight towards the source. Due to both the relatively low depletion of oxygen into dust form and the shape of the oxygen cross section profiles, it is challenging to determine the precise chemistry of interstellar dust. However, silicates provide an acceptable fit. Finally, we discuss the systematic discrepancies in the atomic (gaseous phase) data of the oxygen edge spectral region using different X-ray atomic databases as well as consider future prospects for studying the ISM with the Arcus concept mission.

The Long Stare at Hercules X-1. I. Emission Lines from the Outer Disk, the Magnetosphere Boundary, and the Accretion Curtain

“…Residuals in this energy range were present in multiple previous RGS studies of the O K edge (e.g., Costantini et al 2012;Pinto et al 2013). They were later discussed in particular by Psaradaki et al (2020), who interpret them as instrumental features in RGS (see Figure 6 of their paper). Regardless of the origin of the features, we decided to ignore the 22.25-24 Å band in this study.…”

Ray-tracing Simulations and Spectral Models of X-Ray Radiation in Dusty Media

“…The interference pattern created by the photoelectron waves during the absorption of an X-ray photon in dust grains can in fact give rise to X-ray absorption fine structures (XAFSs). These features are expected to be commonly associated with Mg, Si, andO (e.g., Zeegers et al 2017, 2019;Costantini et al 2019;Rogantini et al 2019Rogantini et al , 2020Psaradaki et al 2020) and have been identified in the X-ray spectra of X-ray-bright Galactic sources thanks to high-resolution XMM-Newton and Chandra observations (e.g., Lee et al 2002;Ueda et al 2005;Kaastra et al 2009;Lee 2010;Pinto et al 2010Pinto et al , 2013Costantini et al 2012;Valencic & Smith 2013;Psaradaki et al 2023). The presence of dust in the ISM could lead to significant biases when measuring the column density in the direction of X-ray sources with models that do not consider dust absorption and scattering (Corrales et al 2016;Smith et al 2016).…”

Ray-tracing simulations and spectral models of X-ray radiation in dusty media

“…The interference pattern created by the photoelectron waves during the absorption of a X-ray photon in dust grains can in fact give rise to X-ray absorption fine structures (XAFS). These features are expected to be commonly associated to Mg, Si and O (e.g., Costantini et al 2019;Rogantini et al , 2020Psaradaki et al 2020;Zeegers et al 2017Zeegers et al , 2019, and have been identified in the X-ray spectra of X-ray bright Galactic sources thanks to high-resolution XMM-Newton and Chandra observations (e.g., Lee et al 2002;Lee 2010;Ueda et al 2005;Kaastra et al 2009;Pinto et al 2010Pinto et al , 2013Costantini et al 2012;Valencic & Smith 2013;Psaradaki et al 2022). The presence of dust in the ISM could lead to significant biases when measuring the column density in the direction of X-ray sources with models that do not consider dust absorption and scattering (Corrales et al 2016;Smith et al 2016).…”