Absorption and scattering by interstellar dust in the silicon K-edge of GX 5-1

Abstract

Context. We study the absorption and scattering of X-ray radiation by interstellar dust particles, which allows us to access the physical and chemical properties of dust. The interstellar dust composition is not well understood, especially on the densest sight lines of the Galactic Plane. X-rays provide a powerful tool in this study. Aims. We present newly acquired laboratory measurements of silicate compounds taken at the Soleil synchrotron facility in Paris using the Lucia beamline. The dust absorption profiles resulting from this campaign were used in this pilot study to model the absorption by interstellar dust along the line of sight of the low-mass X-ray binary (LMXB) GX 5-1. Methods. The measured laboratory cross-sections were adapted for astrophysical data analysis and the resulting extinction profiles of the Si K-edge were implemented in the SPEX spectral fitting program. We derive the properties of the interstellar dust along the line of sight by fitting the Si K-edge seen in absorption in the spectrum of GX 5-1. Results. We measured the hydrogen column density towards GX 5-1 to be 3.40 ± 0.1 × 10 22 cm −2 . The best fit of the silicon edge in the spectrum of GX 5-1 is obtained by a mixture of olivine and pyroxene. In this study, our modeling is limited to Si absorption by silicates with different Mg:Fe ratios. We obtained an abundance of silicon in dust of 4.0 ± 0.3 × 10 −5 per H atom and a lower limit for total abundance, considering both gas and dust, of > 4.4 × 10 −5 per H atom, which leads to a gas to dust ratio of > 0.22. Furthermore, an enhanced scattering feature in the Si K-edge may suggest the presence of large particles along the line of sight.

Absence of Reflection Features in NuSTAR Spectra of the Luminous Neutron Star X-Ray Binary GX 5–1

“…However, the lower limit on the Fe abundance reported by Zeegers et al (2017), >0.76, rules out a highly subsolar Fe abundance.…”

Investigating the interstellar dust through the Fe K-edge

“…The crystalline counterparts of these compounds were also artificially produced by slowly cooling the silicate mixture. The Si-bearing compounds present in the sample set correspond to the silicates already presented in detail by Zeegers et al (2017). The two iron sulfides have different origins.…”

“…Thus, using the X-ray spectra we can also determine abundances in different ISM environments, thereby opening a window on the study of grain evolution and the cycle between diffuse and dense or dark clouds. Dust in diffuse regions along the Galactic plane has been modelled in the X-rays by Lee et al (2009);Costantini et al (2012); Pinto et al (2013); Valencic & Smith (2015) and Zeegers et al (2017) for several lines of sight. The best targets for this kind of study are low mass X-ray binaries (LMXB), which generally emit a featureless continuum X-ray spectrum.…”

“…In this way we can determine k as a function of energy (or wavelength) in detail around the edge (see also Zeegers et al 2017). …”

Tetrahedral hydrocarbon nanoparticles in space: X-ray spectra

“…Studies of X-ray spectra for materials in the astrophysical context are rather new, and till now done mainly for iron and silicate dust compounds using experimental synchrotron based methods (Lee & Ravel 2005;Lee et al 2009;Zeegers et al 2017;Rogantini et al 2018). Calculations of scattering and absorption of Xrays (based on realistic dielectric functions) by carbonaceous and silicate dust grains are also available (Draine 2003).…”