1. Netherlands Organisation for Scientific Research Probing interstellar dust through X-ray spectroscopy C. Pinto *, J. S. Kaastra * †, E. Costantini *, F. Verbunt * † Abstract The interstellar medium (ISM) has a multiphase structure characterized by gas, dust and molecules. The interstellar gas consists of different charge states: neutral, weakly-ionized and highly-ionized. Dust is mainly found in carbonaceous and silicates compounds, and molecules like CO and CH are also common. Through the study of the X-ray absorption lines in the spectra of background sources it is possible to probe the different phases of the ISM to estimate the abundances of the most abundant elements, such as oxygen, iron, and neon. We present our results on high-quality spectra taken with the Reflection Grating Spectrometer on-board XMM-Newton along different lines of sight with a detailed analysis of the absorption features due to the neutral and ionized gas of the ISM, as well as absorption by dust. Towards the innermost regions of the Galaxy the abundances exceed the Solar values in agreement with the metallicity gradient measured at longer wavelengths. The bulk of the interstellar gas is neutral, but a significant fraction of the interstellar matter is found in dust grains. We show how the fine structure of the absorption edges contains clues on the chemical composition of the dust grains. For instance, most of the solid oxygen is found in silicates compounds of pyroxene MgSiO 3. Essentials Data: the best targets for our analysis are the low-mass X-ray binaries because of their simple and bright spectrum (plus novae, nebulae and AGN), see Fig. 1. ISM structure: we model the ISM with a 3-phase gas plus a dust phase which takes into account depletion for elements such as O, Mg, Si, and Fe. The gas is assumed to be in collisional equilibrium. H-columns and abundances of O, Ne, Mg and Fe are free parameters. Abundances are referred to the proto-solar values of Lodders & Palme (2009). The column ratio within the different ISM phases is consistent along different lines of sight with a few deviations. Indeed, sources near the Galactic disk have more dust than ionized gas, while at higher latitudes the column of ionized gas is definitely larger than the dust column. On average, the neutral gas accounts the bulk of the oxygen, while ionized gas and dust together provide ~ 20% of the total O-column. Abundance gradients: we compare abundances of O, Ne, Mg and Fe measured along different sightlines. They all increase towards the Galactic Center in agreement with the metallicity gradient found in literature (see Fig. 3, Esteban et al. 2005, Pedicelli et al. 2009). Conclusion X-ray spectroscopy is a powerful tool to investigate the ISM. Abundances of O, Ne, Fe, Mg have been measured and show a different gradient towards the Galactic Center. All the gradients agree with the values found at different wavelengths (e.g. Esteban et al. 2005 and Pedicelli et al. 2009). The ISM can be well represented by a mixture of dust silicates and 3 gaseous components: cold neutral gas (88%) with T < 10 4 K, warm gas (~10%) with a low ionization degree and T ~ 1-5·10 4 K and highly-ionized gas (~2%) with T ~ 2·10 6 K. This confirms the current state of the art (Ferrière et al. 2001). Fig. 1 Targets map Color code refers to the position of each source in the Galaxy and its line of sight: red refers to the sources near the Center, purple to those sources at an intermediate longitude, and blue to the sources more external than the Sun. The line of sight towards Mrk 509 is shown to give an idea of the crossed region. Fig. 2 Oxygen K edge Simulation of 4 alternative ISM models with warm gas (O III - IV ) or molecules. Cold and hot gas parameters are kept constant. The template spectrum we have used is that of LMXB 4U 1735-44. The spectral features are different for the models and optimal in searching and disentangling the molecules. Best fit of the full gas + dust model for the spectrum of GS 1826-238. Pyroxene silicates are clearly detected (see also Fig. 4). We estimate that ~ 10-15% of oxygen is bound in dust grains, mostly consisting of silicates. The analysis of the other edges provides an even higher depletion for both iron and magnesium (~ 90%). Iron is bound in oxides, such as hematite, and in the metallic phase. The gas is mostly neutral as less than 10% is ionized. References Esteban et al. 2005, ApJ, 618, L95 Lodders & Palme 2009, M&PSA 72, 5154 Ferrière et al. 2001, RMP, 73, 1031 Pedicelli et al. 2009, A&A, 504, 81 Kaastra et al. 2009, A&A, 497, 291 Pinto et al. 2010, A&A, 521, A79 Fig. 4 Transmission of different components of the ISM near the OI edge Both Cold gas (O I-II) and total ISM are multiplied by a factor of 4 for clarity. Warm gas provides O III-IV. Hot gas provides O VII-VIII. Silicates provide solid O I. Most of the interstellar absorption is provided by the neutral gas, but ionized gas and dust also furnish an important contribution. Fig. 3 Abundance gradients ISM average abundances. The averages are taken between sources with comparable location and line of sight in the Galaxy (see color code in Fig. 1). All elements are more abundant in the innermost regions of the Galaxy. But the gradients appear to be different. Fe and Mg show a steeper gradient, which might have clues on the SNe Types Ia/CC gradient. OXYGEN EDGE Targets map projected on the disk Typical error bar Towards * SRON – Netherlands Institute for Space Research † Utrecht University, The Netherlands