GINGA OBSERVATIONS OF CENTAURUS X-3

A series of observations of Centaurus X-3 was made with Ginga on 1989 March 22-24, over a complete orbital cycle. The data set includes a pre-eclipse dip, the eclipse ingress and egress, the eclipse itself, and a phase of high, steady emission after egress. Pulse timing analysis of these data has provided a set of orbital parameters which confirms the change in the orbital period known previously and yields an improved rate of change of orbital period: P(orb)/P(orb) = -1.738 x 10(-6) yr-1. We also detected a double-peaked structure in the pulse profile at low energies which is rarely observed from Cen X-3. The spectrum after egress was found to be the cutoff power law typical of pulsars in general. At mid-eclipse, scattering by optically thin matter can account for the observed spectrum with the addition of a soft thermal component, while during the pre-eclipse dip, the spectrum has three components: a highly absorbed, pulsating one; a scattered non-pulsating one; and the same soft thermal component as seen at mid-eclipse. An iron emission line was detected at all binary phases. After egress, when the source was brightest, the energy of the iron emission line was predominantly at 6.4 keV due to fluorescence by cold circumstellar matter. However, during eclipse the iron emission was more complex: both 6.4 and 6.7 keV iron lines were observed from this binary system, with some evidence of a separate line at approximately 8.5 keV. It was found from the orbital phase dependence of the line intensities that the fluorescent 6.4 keV iron line is produced by cold matter located relatively close to the neutron star, whereas the 6.7 keV line is attributed to the hot highly ionized plasma spread out over the surface of the companion. The present observations give an estimate for the size of such a hot plasma zone formed by the X-ray irradiation: D6.7 > 8 x 10(11) cm.