Probing the structure of the gas in the Milky Way through X-ray high-resolution spectroscopy

We have developed a new X-ray absorption model, called IONeq, which computes the optical depth τ(E) simultaneously for ions of all abundant elements, assuming ionization equilibrium and taking into account turbulent broadening. We use this model to analyse the interstellar medium (ISM) absorption features in the Milky Way for a sample of 18 Galactic (LMXBs) and 42 extragalactic sources (mainly Blazars). The absorbing ISM was modelled as a combination of three components/phases - neutral (T ≲ 1 × 10⁴ K), warm (T ∼ 5 × 10⁴ K) and hot (T ∼ 2 × 10⁶ K). We found that the spatial distribution of both, neutral and warm components, are difficult to describe using smooth profiles due to non-uniform distribution of the column densities over the sky. For the hot phase we used a combination of a flattened disc and a halo, finding comparable column densities for both spatial components, of the order of ∼6-7 × 10¹⁸ cm^-2, although this conclusion depends on the adopted parametrization. If the halo component has sub-solar abundance Z, then the column density has to be scaled up by a factor of Z_{⊙}/Z. The vertically integrated column densities of the disc components suggest the following mass fractions for these three ISM phases in the Galactic disc: neutral ∼ 89 per cent, warm ∼ 8 per cent and hot ∼ 3 per cent components, respectively. The constraints on the radial distribution of the halo component of the hot component are weak.