A highly-ionized absorber as a new explanation for the spectral changes during dips from X-ray binaries

We report the XMM-Newton detection of narrow Fe XXV and Fe XXVI X-ray absorption lines at 7 keV in the persistent emission of the dipping low-mass X-ray binary (LMXB) 4U 1323-62. Such features have now been reported in a growing number of LMXBs seen almost edge-on, indicating that the highly-ionized plasma probably originates in an accretion disk atmosphere or wind. During dipping intervals of 4U 1323-62, the strength 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 X-ray spectrum during dips are inconsistent with a simple increase in absorption by cool material. However, we show that the changes in both the narrow absorption features and the continuum can be modeled self-consistently by variations in the properties of an ionized absorber. From persistent to deep dipping the photo-ionization parameter decreases while the equivalent hydrogen column density of the ionized absorber increases. No partial covering of any component of the spectrum, and hence no extended corona, are required. 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.