XMM-Newton observation of 4U 1820-30

Broad band spectrum and the contribution of the cold interstellar medium

¹ SRON, Netherlands Institute for Space Research, Sorbonnelaan, 2, 3584 CA Utrecht, The Netherlands
e-mail: e.costantini@sron.nl
² Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching bei München, Germany
⁴ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV, Groningen, The Netherlands
⁵ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, PO Box 94249, 1090 GE Amsterdam, The Netherlands

Received: 3 August 2011
Accepted: 16 December 2011

Abstract

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 ~ 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 ~ 1.63 × 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 (ξ ~ −0.5) at a velocity of  ~1200 km s^-1.