Dust grain chemistry in the diffuse ISM towards the black hole transient GX 339-4

We present results on X-ray absorption and the dust grain chemistry in the diffuse interstellar medium (ISM), based on a new Cycle 25 Chandra High Energy Transmission Grating Spectrometer (HETGS) observational campaign targeting the black hole transient GX 339-4. The X-ray source offers an optimal combination of moderate hydrogen column density and high X-ray flux, enabling the first detailed simultaneous fitting of the photoabsorption edges of Fe, O, Si, and Mg which are key elemental constituents of interstellar dust. We performed a joint spectral analysis of Chandra/HETGS data and archival observations from the Reflection Grating Spectrometer (RGS) on board XMM-Newton. We found that the dust grain chemical composition along this diffuse Galactic line of sight is best described by the silicate Mg-rich amorphous pyroxene (Mg0.75Fe0.25SiO3) and metallic iron. We also discuss the elemental abundances and depletions of Fe, O, Si, and Mg, and the presence of absorption features in the X-ray spectrum of this source associated with highly ionised plasma.