Resolving the Fe xxv Triplet with Chandra in Cen X-3

Abstract

We present the results of a 45 ks Chandra observation of the high-mass X-ray binary Cen X--3 at orbital phases between 0.13 and 0.40 (in the eclipse post-egress phases). Here we concentrate on the study of discrete features in the energy spectrum at energies between 6 and 7 keV, i.e. on the iron K$_\alpha$ line region, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. We clearly see a K$_\alpha$ neutral iron line at $\sim 6.40$ keV and were able to distinguish the three lines of the \ion{Fe}{25} triplet at 6.61 keV, 6.67 keV, and 6.72 keV, with an equivalent width of 6 eV, 9 eV, and 5 eV, respectively. The equivalent width of the K$_\alpha$ neutral iron line is 13 eV, an order of magnitude lower than previous measures. We discuss the possibility that the small equivalent width is due to a decrease of the solid angle subtended by the reflector.