High-resolution X-ray spectroscopy of the interstellar medium : XMM-Newton observation of the LMXB GS 1826−238

Aims. The interstellar medium (ISM) has a multiphase structure characterized by gas, dust, and molecules. The gas can be found in different charge states: neutral, weakly ionized (warm) and highly ionized (hot). It is possible to probe the multiphase ISM through the observation of its absorption lines and edges in the X-ray spectra of background sources. Methods. We present a high-quality RGS spectrum of the low-mass X-ray binary GS 1826−238 with an unprecedentedly detailed treatment of the absorption features caused by the dust and both the neutral and ionized gas of the ISM. We constrain the column density ratios within the different phases of the ISM and measure the abundances of elements such as O, Ne, Fe, and Mg. Results. We found significant deviations from the protosolar abundances: oxygen is over-abundant by a factor 1.23 ± 0.05, neon by 1.75±0.11, iron by 1.37±0.17, and magnesium by 2.45±0.35. The abundances are consistent with the measured metallicity gradient in our Galaxy: the ISM appears to be metal-rich in the inner regions. The spectrum also shows the presence of warm and hot ionized gas. The gas column has a total ionization degree of less than 10%. We also show that dust plays an important role as expected from the position of GS 1826−238: most iron appears to be bound in dust grains, while 10−40% of oxygen consist of a mixture of dust and molecules.