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

TL;DR

This study uses laboratory measurements and X-ray observations to analyze interstellar dust properties, focusing on silicon absorption features in the spectrum of GX 5-1, revealing dust composition and size distribution.

Contribution

It introduces new laboratory-derived absorption profiles for silicate dust and applies them to astrophysical data, improving understanding of dust composition along the Galactic Plane.

Findings

Silicon abundance in dust is approximately 4.0×10^{-5} per H atom.

Best fit dust model includes olivine and pyroxene silicates.

Evidence suggests the presence of large dust particles along the line of sight.

Abstract

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. 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. 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. We measured the hydrogen column density towards GX 5-1 to be $3.40\pm0.1\times10^{22} \rm cm^{-2}$. 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 $4.0\pm0.3\times10^{-5}$ per H atom and a lower limit for total abundance, considering both gas and dust, of $>4.4\times10^{-5}$ 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.