ISM composition through X-ray spectroscopy of LMXBs
  The diffuse interstellar medium (ISM) is an integral part of the evolution of
the entire Galaxy. Metals are produced by stars and their abundances are the
direct testimony of the history of stellar evolution. However, the interstellar
dust composition is not well known and the total abundances are yet to be
accurately determined. We probe ISM dust composition, total abundances, and
abundance gradients through the study of interstellar absorption features in
the high-resolution X-ray spectra of Galactic low-mass X-ray binaries (LMXBs).
We use high-quality grating spectra of nine LMXBs taken with XMM-Newton. We
measure the column densities of O, Ne, Mg, and Fe with an empirical model and
estimate the Galactic abundance gradients. The column densities of the neutral
gas species are in agreement with those found in the literature. Solids are a
significant reservoir of metals like oxygen and iron. Respectively, 15-25 % and
65-90 % of the total amount of O I and Fe I is found in dust. The dust amount
and mixture seem to be consistent along all the lines-of-sight (LOS). Our
estimates of abundance gradients and predictions of local interstellar
abundances are in agreement with those measured at longer wavelengths. Our work
shows that X-ray spectroscopy is a very powerful method to probe the ISM. For
instance, on a large scale the ISM appears to be chemically homogeneous showing
similar gas ionization ratios and dust mixtures. The agreement between the
abundances of the ISM and the stellar objects suggests that the local Galaxy is
also chemically homogeneous.
