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 o ver an entire binary eclipse. With unprecedented energy resolution at the iron K-energy band (FWHM similar to 2%), three iron emission lines at 6.4 keV, 6.7 keV, and 6.97 keV were clearly resolved. At lower ene rgies, lines from hydrogenic ions of Ne, Mg, Si, and S were also obser ved. 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, decre ased about one order of magnitude during the eclipse, the intensity dr op of the 6.7 and 6.97 keV lines, which are respectively from Fe XXV a nd Fe XXVI, was at most a factor of three. This, confirming the conjec ture by Nagase et al. (1992, AAA56.117.175), indicates that the 6.4 ke V fluorescent line is emitted from the region close to the neutron sta r while the highly photoionized plasma which emits 6.7 and 6.97 keV li nes is more extended than the size of the companion star (similar to 1 2 R.). The 6.4 keV line equivalent-width did not show a large variatio n over the eclipse, and was 75-145 eV. The dense Alfven shell or an op tically thick accretion disk is a likely candidate for reprocessing th e fluorescent iron line. From the intensity ratio of the 6.7 and 6.97 keV lines during the eclipse, the ionization parameter of the photoion ized plasma is estimated as xi similar to 10(3.4). The size and the de nsity of the plasma are estimated as similar to 16 R. and similar to 1 .3 X 10(11) cm(-3), respectively. It is suggested that a lower ionizat ion state is dominated in the pre-eclipse phase, since the 6.7 and 6.9 7 keV lines were not clearly seen and other K-lines from lighter eleme nts were strong.