XMM-Newton observation of 4U 1820-30 Broad band spectrum and the contribution of the cold interstellar medium

We present an analysis of the bright X-ray binary 4U 1820-30, thet is based mainly on XMM-Newton-RGS data, in addition to complementary data from XMM-Newton-EPIC-pn, INTEGRAL, and Chandra-HETG, to investigate different aspects of the source. The broad band continuum is fitted well by a classical combination of black body and Comptonized emission. The continuum shape and the high flux of the source (L/ L-Edd similar to 0.16) are consistent with a " high state" of the source. We do not find any significant evidence of iron emission at energies >= 6.4 keV. The soft X-ray spectrum contain a number of absorption features. Here we focus on the cold and mildly ionized gas. The neutral gas column density is N-H similar to 1.63 x 1021 cm-2. The detailed study of the oxygen and iron edge reveals that those elements are depleted, which is defined here as the ratio of the dust to the total ISM cold phase, by a factor 0.20 +/- 0.02 and 0.87 +/- 0.14, respectively. Using the available dust models, the best-fit points to a major contribution of Mg-rich silicates, with metallic iron inclusion. Although we find that a large fraction of Fe is in dust form, the fit shows that Fe-rich silicates are unlikely to be present. The measured Mg: Fe ratio is 2.0 +/- 0.3. Interestingly, this modeling may provide additional support for a well studied dust constituent (GEMS), which is sometimes proposed as a silicate constituent in our Galaxy. Oxygen and iron are found to be slightly over-and under-abundant, respectively (1.23 and 0.85 times the solar value), along this line of sight. We also report the detection of two absorption lines, which are tentatively identified as part of an outflow of mildly ionized gas (xi similar to -0.5) at a velocity of similar to 1200 km s(-1).