A highly-ionized absorber in the X-ray binary 4U 1323-62: A new explanation for the dipping phenomenon

¹ The SRON, the National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
² Observatoire Astronomique de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France e-mail: boirin@astro.u-strasbg.fr
³ Astrophysics Missions Division, Research and Scientific Support Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands

Received: 2 September 2004
Accepted: 15 February 2005

Abstract

We report the detection of narrow  and   X-ray absorption lines at  keV and  keV in the persistent emission of the dipping low-mass X-ray binary 4U 1323-62 during a 2003 January XMM-Newton observation. These features are superposed on a broad emission feature centered on  keV. During dipping intervals the equivalent width of the  feature increases while that of the  feature decreases, consistent with the presence of less strongly ionized material in the line-of-sight. As observed previously, the changes in the 1.0–10 keV spectrum during dips are inconsistent with a simple increase in absorption by cool material. However, the changes in both the absorption features and the continuum can be modeled by variations in the properties of an ionized absorber. No partial covering of any component of the spectrum, and hence no extended corona, are required. From persistent to deep dipping, the photo-ionization parameter, ξ, expressed in , decreases from  of to  of , while the equivalent hydrogen column density of the ionized absorber increases from () 10²² atoms cm^-2 to () 10²² atoms cm^-2. Since highly-ionized absorption features are seen from many other dip sources, this mechanism may also explain the overall changes in X-ray spectrum observed during dipping intervals from these systems.