SRG/eROSITA 3D mapping of the ISM using X-ray absorption spectroscopy

We present a detailed study of the hydrogen density distribution in the local interstellar medium (ISM) using the X-ray absorption technique. Hydrogen column densities were precisely measured by fitting X-ray spectra from coronal sources observed during the initial {\it eROSITA} all-sky survey (eRASS1). Accurate distance measurements were obtained through cross-matching Galactic sources with the {\it Gaia} third data release (DR3). Despite the absence of a discernible correlation between column densities and distances or Galactic longitude, a robust correlation with Galactic latitude was identified. This suggests a decrease in ISM material density along the vertical direction away from the Galactic plane. To further investigate, we employed multiple density laws to fit the measured column densities, revealing constraints on height scale values ($8 < h_{z} < 30$~pc). Unfortunately, radial scales and central density remain unconstrained due to the scarcity of sources near the Galactic center. Subsequently, a 3D density map of the ISM was computed using a Gaussian processing approach, inferring hydrogen density distribution from hydrogen column densities. The results unveiled the presence of multiple beams and clouds of various sizes, indicative of small-scale structures. Large density regions were identified at approximately 100~pc, consistent with findings in dust reddening studies, potentially associated with the Galactic Perseus arm. Moreover, high-density regions were pinpointed in proximity to the Orion star-forming region and the Chamaeleon molecular complex, enriching our understanding of the intricate structure of the local interstellar medium.