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

We report the detection of narrow Fe XXV and Fe XXVI X-ray absorption lines at 6.68 +/- 0.04 keV and 6.97 +/- 0.05 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 6.6 {+0.1}{-0.2} keV. During dipping intervals the equivalent width of the Fe XXV feature increases while that of the Fe XXVI 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, Xi, expressed in erg cm s^{-1}, decreases from log(Xi) of 3.9 +/- 0.1 to log(Xi) of 3.13 +/- 0.07, while the equivalent hydrogen column density of the ionized absorber increases from (3.8 +/- 0.4) 10^{22} atoms cm^{-2} to (37 +/- 2) 10^{22} 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.