1. Netherlands Organisation for Scientific Research High resolution X-ray spectroscopy of the Interstellar Medium (ISM) C. Pinto (SRON), J. S. Kaastra (SRON), E. Costantini (SRON) Abstract The interstellar medium (ISM) has a multiphase structure characterized by gas, dust and molecules. The gas can be found in different states: cold/warm neutral gas, warm/hot ionized gas (Ferrière et al. 2001). It is possible to probe the ISM through observation of its absorption lines and edges in X-ray spectra of background sources. We present a high quality RGS spectrum of the low-mass X-ray binary GS 1826-238 with a detailed treatment of the absorption features due to both the neutral and the ionized gas of the ISM. For some metals we found significant deviations from the proto-solar abundances (Lodders 2003): oxygen is over-abundant by a factor of 1.3, neon ~ 2.2, iron 1.6 and magnesium 1.9. The high metallicity could be due to the location of the target near the center of the Milky Way, where we expect such enhanced metal abundances (see e.g. Esteban et al. 2005). The spectrum of GS 1826-238 also suggests the presence of warm/hot gas, highlighted by the O VII and Ne IX absorption line (Fig 1, 2). Essentials Data reduction: we filtered background flares and source bursts. For each burst we removed 50 s before the peak to 250 s after it, (Kong et al. 2007). After filtering, the total exposure time is ~ 145 ks. Continuum determination: we fitted both EPIC and RGS spectra, selecting 0.5-10 keV and 0.4-1.77 keV bands respectively. The continuum as been modeled with a black body plus two comptonization components (see Pinto et al. 2009 in prep.). ISM abundances: we fitted RGS freezing the continuum and enabling the abundances of O, Ne, Mg and Fe to vary. The references are the proto-solar abundances estimated by Lodders (2003). We estimate a column ratio Ne/O = 0.26 ± 0.02, consistent with the Crab nebula spectrum, which is located in a different region of the Galaxy (Kaastra et al. 2009). This suggests that the conditions for the stellar evolution remain the same within the Galaxy. ISM diagnostic: we used 2 additional absorbers to model the warm and\or hot ionized phases of the interstellar gas (Table 1). The fit improves (Δ C stat ~ 80 for just 6 additional parameters). Abundances gradient: we compare our results with the ones obtained by Kaastra et al. (2009); oxygen shows an overabundance of 30% with respect to Crab (see Table 1). Such a gradient is in agreement with the one expected for a source at distance ~ 6-7 kpc in the direction of the Galactic center (Esteban et al. 2005). Conclusion We have shown that X-ray spectroscopy is a powerful tool to investigate the ISM. Column densities and abundances of elements such as O, Ne, Fe, Mg have been measured showing a high gradient towards the Galactic center. The oxygen gradient fully agrees with Esteban et al. (2005). The neon and oxygen abundances differ from those obtained towards the Crab nebula (Kaastra et al. 2009), but the ratio of their column densities are consistent, which means that the stellar evolution in the different parts of the Galaxy remains the same. Our work confirms that the ISM gas can be well represented by a mixture of 3 components: cold neutral gas (88%) with T ~ 7·10 3 K, warm gas (~11%) with a low ionization degree and T ~ 6.5·10 4 K and hot ionized gas (~1%) with T ~ 2 million K. This agrees with the current state of the art (Ferrière et al. 2001). Fig. 1 RGS spectrum best fit for the cold gas model: there are strong absorption features due to the K-edges of oxygen (23 Å), neon (14.3 Å) and magnesium (9.5 Å), plus the L-edge of iron (17.4 Å). The fit residuals show small features attributed to O II absorption at 23.35 Å, dust features around 23 Å (see also Fig. 2), O VII line at 21.6 Å. Interestingly there is an excess at higher (~17.6 Å) wavelength as also found by Juett et al. (2006) using Chandra. Table 1 The column densities N H are expressed in units of 10 21 cm -2, the temperatures T in K. Abundances are referred to Lodders (2003). The cold gas shows traces of Fe II and Mg II. The warm component is slightly ionized, typical ions are O II-III and Ne II-III. The hot ionized gas accounts for O VII-VIII, Ne IX and Mg XI. RGS local fit 3 phases gas: Neon edge: the edge is well fitted. The Ne IX line is expected at 13.45 Å, and it is the signature of warmer gas. Oxygen edge: the O I 1s-2p line is well reproduced at 23.5 Å and there are also traces of O II and O VII. The broad feature (22.7-23.2 Å) which is not well modeled is probably due to absorption by oxygen bound in dust grains. References Esteban et al. 2005, ApJ, 618, L95 Ferrière et al. 2001, RMP, 73, 1031 Juett et al. 2006, ApJ, 648, 1066 Kaastra et al. 2009, A&A, 497, 291 Lodders 2003, ApJ, 591, 1220 O VII K O I K O II Fe II L 3 Mg XI Ne IX Ne III Mg XI See Fig. 2 O I K Fe I L Ne I K Mg I K Neon edge Oxygen edge Fig. 2