XMM-Newton observation of 4U 1820-30: Broad band spectrum and the
  contribution of the cold interstellar medium
  We present the analysis of the bright X-ray binary 4U 1820-30, based mainly
on XMM-Newton-RGS data, but using complementary data from XMM-Epic, Integral,
and Chandra-HETG, to investigate different aspects of the source. The broad
band continuum is well fitted by a classical combination of black body and
Comptonized emission. The continuum shape and the high flux of the source
(L/L_Edd\sim0.16) are consistent with a "high state" of the source. We do not
find 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-mildly ionized gas. The neutral gas column density is N_H\sim1.63x10^21
cm^-2. The detailed study of the oxygen and iron edge reveals that those
elements are depleted, defined here as the ratio between dust and the total ISM
cold phase, by a factor 0.20\pm0.02 and 0.87\pm0.14, 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 disfavored. The
measured Mg:Fe ratio is 2.0\pm0.3. Interestingly, this modeling may point to a
well studied dust constituent (GEMS), 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,
tentatively identified as part of an outflow of mildly ionized gas
(\xi\sim-0.5) at a velocity of \sim1200 km/s.
