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