High-resolution X-ray spectroscopy of the interstellar medium

Abstract

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.

Interstellar medium composition through X-ray spectroscopy of low-mass X-ray binaries

“…These sources are well suited for the analysis of the ISM because of their brightness and column densities N H ∼ 1−5 × 10 25 m −2 (see Table 3), which are high enough to produce prominent O, Fe, and Ne absorption edges in the soft X-ray energy band. This paper provides a significant extension of the previous work on the ISM in the line-of-sight (LOS) toward GS 1826−238 done by Pinto et al (2010). We have used updated atomic and molecular database and extended the analysis to several LOS (see Fig.…”

“…Absorption structure due to ionized gas and dust was found around the interstellar oxygen K-shell absorption edge in the spectra of several sources (Paerels et al 2001;de Vries et al 2003;Juett et al 2004;Costantini et al 2005Costantini et al , 2012Kaastra et al 2009;Pinto et al 2010Pinto et al , 2012a.…”

“…The presence of different ionization states constrains the multiphase temperature structure of the ISM. A summary of the results obtained by X-ray spectroscopy on the ISM in the last decades is given by Pinto et al (2010).…”

Absorption and scattering by interstellar dust in the silicon K-edge of GX 5-1

“…Studies of the diffuse ISM show variation of the composition of the ISM along different lines of sight (Lee et al 2009;Pinto et al 2010;Costantini et al 2012;Valencic & Smith 2013). From studies of the Fe L and O K-edge, Pinto et al (2010) concluded that the dust composition of interstellar dust is probably chemically inhomogeneous, since they found indications for iron rich silicates toward X-ray binary GS 1826-238. Lee et al (2009), on the contrary, find that Fe oxides provide a better fit to the data of Cyg X-1.…”

“…In this way, a large range of column densities can be investigated and it is possible to analyze dust in various regions in the Galaxy. Dust in diffuse regions along the Galactic plane has been studied in the X-rays by Lee et al (2009), Pinto et al (2010, Costantini et al (2012), and Pinto et al (2013) for several sight lines. The dense ISM has been less extensively studied in the X-rays.…”