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Issue A&A
Volume 418, Number 3, May II 2004
Page(s) 1061 - 1072
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20034550



A&A 418, 1061-1072 (2004)
DOI: 10.1051/0004-6361:20034550

Discovery of X-ray absorption features from the dipping low-mass X-ray binary XB 1916-053 with XMM-Newton

L. Boirin1, A. N. Parmar1, D. Barret2, S. Paltani3 and J. E. Grindlay4

1  Astrophysics Missions Division, Research and Scientific Support Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
2  Centre d'Etude Spatiale des Rayonnements, CNRS/UPS, 9 Av. du Colonel Roche, 31028 Toulouse Cedex 4, France
3  Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France
4  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street Cambridge, MA 02138, USA

(Received 21 October 2003 / Accepted 3 February 2004 )

Abstract
We report the discovery of narrow $\ion{Fe}{xxv}$ and $\ion{Fe}{xxvi}$ K $\alpha$ X-ray absorption lines at 6.65 +0.05-0.02 and 6.95 +0.05-0.04 keV in the persistent emission of the dipping low-mass X-ray binary (LMXB) XB 1916-053 during an XMM-Newton observation performed in September 2002. In addition, there is marginal evidence for absorption features at 1.48 keV, 2.67 keV, 7.82 keV and 8.29 keV consistent with $\ion{Mg}{xii}$, $\ion{S}{xvi}$, $\ion{Ni}{xxvii}$ K $\alpha$ and $\ion{Fe}{xxvi}$ K $\beta$ transitions, respectively. Such absorption lines from highly ionized ions are now observed in a number of high inclination (i.e. close to edge-on) LMXBs, such as XB 1916-053, where the inclination is estimated to be between 60-80 $^{\circ}$. This, together with the lack of any orbital phase dependence of the features (except during dips), suggests that the highly ionized plasma responsible for the absorption lines is located in a cylindrical geometry around the compact object. Using the ratio of $\ion{Fe}{xxv}$ and $\ion{Fe}{xxvi}$ column densities, we estimate the photo-ionization parameter of the absorbing material, $\xi$, to be  103.92 . Only the $\ion{Fe}{xxv}$ line is observed during dipping intervals and the upper-limits to the $\ion{Fe}{xxvi}$ column density are consistent with a decrease in the amount of ionization during dipping intervals. This implies the presence of cooler material in the line of sight during dipping. We also report the discovery of a 0.98 keV absorption edge in the persistent emission spectrum. The edge energy decreases to 0.87 keV during deep dipping intervals. The detected feature may result from edges of moderately ionized Ne and/or Fe with the average ionization level decreasing from persistent emission to deep dipping. This is again consistent with the presence of cooler material in the line of sight during dipping.


Key words: accretion, accretion disks -- stars: individual: XB 1916-053 -- X-rays: binaries

Offprint request: L. Boirin, L.Boirin@sron.nl

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© ESO 2004


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