Interstellar medium composition through X-ray spectroscopy of low-mass X-ray binaries
Astronomical Institute, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands.Received 2 October 2012 / Accepted 7 January 2013ABSTRACTContext.The diﬀuse interstellar medium (ISM) is an integral part of the evolution of the entire Galaxy. Metals are produced by starsand their abundances are the direct testimony of the history of stellar evolution. However, the interstellar dust composition is not wellknown and the total abundances are yet to be accurately determined.Aims.We probe ISM dust composition, total abundances, and abundance gradients through the study of interstellar absorption featuresin the high-resolution X-ray spectra of Galactic low-mass X-ray binaries (LMXBs).Methods.We use high-quality grating spectra of nine LMXBs taken withXMM-Newton. We measure the column densities of O, Ne,Mg, and Fe with an empirical model and estimate the Galactic abundance gradients.Results.The column densities of the neutral gas species are in agreement with those found in the literature. Solids are a signiﬁca ntreservoir of metals like oxygen and iron. Respectively, 15–25% and 65–90% of the total amount of O iand Feiis found in dust.The dust amount and mixture seem to be consistent along all the lines-of-sight (LOS). Our estimates of abundance gradients andpredictions of local interstellar abundances are in agreement with those measured at longer wavelengths.Conclusions.Our work shows that X-ray spectroscopy is a very powerful method to probe the ISM. For instance, on a large scalethe ISM appears to be chemically homogeneous showing similar gas ionization ratios and dust mixtures. The agreement between theabundances of the ISM and the stellar objects suggests that the local Galaxy is also chemically homogeneous.Key words. ISM: abundances – ISM: dust, extinction – ISM: molecules – IS M: structure – X-rays: ISM