Ne and Fe abundances in the ISM: Archival Study of Fe-L and Ne-K edges in Chandra and XMM-Newton

Abstract

The abundance of elements in the interstellar medium (ISM) is a key facet for many fields of astrophysical study. In the soft X-ray spectra, absorption by interstellar gas can result in deep absorption features that affect continuum measurements. In this paper, we focus on measuring the abundance of interstellar iron and neon from the column densities observed in soft spectra from XMM-Newton and Chandra for various low mass X-ray binaries (LMXBs), which allows for a direct probe of elemental abundances. As a noble gas, neon will not deplete into solid form, thus providing a benchmark with abundances determined via UV spectroscopy. We find that, when assuming Fe is 90\% depleted into grains, [Fe/Ne]$ = -0.523\pm0.025$, [Fe/H]$ + 12 = 7.482\pm0.016$, and [Ne/H]$ + 12 = 8.012\pm0.022$, which are the tightest observational constraints on these abundances to date, while being consistent with literature which uses protosolar abundances. We also test how depletion into solid grains and scattering affect the results. The choice of depletion fraction can affect the abundance measurement by roughly $5\%$, and that the inclusion of a scattering component can affect abundance measurements by $\sim1-7\%$.