X-Ray Spectroscopy of Centaurus X-3 with ASCA over an Eclipse

The X-ray binary pulsar Cen X-3 was observed with the ASCA satellite over an entire binary eclipse. With unprecedented energy resolution at the iron K-energy band (FWHM ∼ 2%), three iron emission lines at 6.4 keV, 6.7 keV, and 6.97 keV were clearly resolved. At lower energies, lines from hydrogenic ions of Ne, Mg, Si, and S were also observed. Spectral variations over the eclipse were studied, in particular variations of the iron emission lines. Although the intensity of the 6.4 keV line, which is due to fluorescence from low ionized iron, decreased about one order of magnitude during the eclipse, the intensity drop of the 6.7 and 6.97 keV lines, which are respectively from Fe XXV and Fe XXVI, was at most a factor of three. This, confirming the conjecture by Nagase et al. (1992, AAA56.117.175), indicates that the 6.4 keV fluorescent line is emitted from the region close to the neutron star while the highly photoionized plasma which emits 6.7 and 6.97 keV lines is more extended than the size of the companion star (∼ 12 R⊙). The 6.4 keV line equivalent-width did not show a large variation over the eclipse, and was 75–145 eV. The dense Alfvén shell or an optically thick accretion disk is a likely candidate for reprocessing the fluorescent iron line. From the intensity ratio of the 6.7 and 6.97 keV lines during the eclipse, the ionization parameter of the photoionized plasma is estimated as ξ ∼ 103.4. The size and the density of the plasma are estimated as ∼ 16 R⊙ and ∼ 1.3 × 1011 cm−3, respectively. It is suggested that a lower ionization state is dominated in the pre-eclipse phase, since the 6.7 and 6.97 keV lines were not clearly seen and other K-lines from lighter elements were strong.