Investigating Interstellar dust in the Milky Way through X-ray scattering and absorption

Interstellar dust strongly affects astronomical observations across the entire electromagnetic spectrum. In the optical and ultraviolet bands, it produces extinction and reddening that must be corrected for both Galactic and extragalactic sources. At longer wavelengths, its thermal emission contaminates observations, particularly those related to studies of the cosmic microwave background. In X-ray astronomy, dust impacts observations by both absorbing and scattering photons. Despite extensive studies at multiple wavelengths, several fundamental aspects of interstellar dust—its composition, size distribution, and large-scale spatial distribution within the Milky Way—remain uncertain. The aim of this thesis is to use X-ray absorption and scattering as complementary probes to investigate the nature of interstellar dust and derive accurate geometry-based distances to Galactic structures. The first part of this work (Chapters 1 and 2) provides an overview of the interstellar medium (ISM) and of the physical principles governing X-ray absorption and scattering by dust grains. In the third chapter, we analyze the twenty-one X-ray dust-scattering rings generated by GRB 221009A, the brightest gamma-ray burst ever recorded. Using observations from XMM-Newton and The Neil Gehrels Swift Observatory, we mapped the ISM along the burst direction with higher resolution than existing optical and infrared three-dimensional dust maps—both in the plane of the sky (a few arcminutes) and along the line of sight (from ≃1 pc for nearby dust clouds up to ≃100 pc for distant structures). This analysis allowed us to revise previous estimates of the GRB soft X-ray fluence and to constrain the absorption within its host galaxy. The fourth chapter focuses on the smallest X-ray rings produced by GRB221009A, originating from the most distant interstellar dust layers. Using Chandra and late time XMM-Newton observations, we accurately constrained the distance of the Perseus, Outer, and Outer Scutum–Centaurus arms towards the GRB direction. This analysis was further extended to the other two gamma-ray bursts where X-ray rings associated with dust beyond 5 kpc had previously been detected. Through a detailed re-analysis of XMM-Newton and Chandra archival data, we obtained the most precise geometric distance measurements to these Galactic spiral arms to date. These results provide independent constraints on the Milky Way’s large-scale structure, derived without relying on kinematic models and assumptions about the Galactic rotation curve. The final part of the thesis investigates the composition and size distribution of interstellar dust along the line of sight to the bright low-mass X-ray binary GX13+1, using high-resolution Chandra/HETG spectroscopy. By simultaneously modeling the Mg K and Si K absorption edges with different dust size distributions, we ruled out scenarios of both very diffuse and very dense ISM, favoring grain populations typical of average Galactic conditions, in this direction. Along this sightline, the dust composition is found to be dominated by amorphous olivine, with a minor crystalline fraction of about 2%. In conclusion, this thesis shows that X-ray scattering and absorption provide powerful tools to constrain both the physical properties of interstellar dust and the large-scale structure of the Milky Way, offering a perspective complementary to that obtained from radio, optical, and infrared observations.