Resolving the Fe XXV triplet with Chandra in Centaurus 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, that is, on the iron K alpha line region, using the High Energy Transmission Grating Spectrometer (HETGS) on board the Chandra satellite. We clearly see a K alpha neutral iron line at similar to 6.40 keV and were able to distinguish the three lines of the Fe xxv triplet at 6.61, 6.67, and 6.72 keV, with equivalent widths of 6, 9, and 5 eV, respectively. The equivalent width of the Ka 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.