1. Orbital Properties

Centaurus X-3 was discovered in May 1967 during a survey of the southern sky with a sounding rocket carrying proportional counters (Chodil et al. (1967)). Observations with Uhuru later showed that the X-ray flux was modulated by regular pulsations (Giacconi et al. (1971)) with a 4.8 s period. On the basis of Doppler variations in its pulse period observed with Uhuru, Cen X-3 was the first of the X-ray sources to be established as a binary (Schreier et al. (1972)). The X-ray eclipses which occur every 2.1 days suggest a high inclination angle i. Assuming an eclipse half angle of 40^o, Conti (1978) estimated the radius of the companion R_c to be 12.5 R_$\odot$ and i > 80^o. Clark et al. (1988) estimated the eclipse half angle as 32.9^o and derived a mass of (1.23$\pm$0.60)M_$\odot$ for the neutron star. After reviewing the available data van Kerkwijk et al. (1995) derived the 95% confidence values M_x = 1.09^+0.57_-0.52M_$\odot$, M_c = 18.9^+4.0_-1.8M_$\odot$, R_c = 11.1^+1.8_-1.1R_$\odot$, and i > 66^o. The companion is a highly reddened O-type star known as V779 Cen or Krzeminski's star. From the optical reddening a distance of $\sim$ 8 kpc has been inferred, with a lower limit of 6.2 kpc . Because Cen X-3 has not been detected in radio there is no dispersion measure to compare with this and the distance is uncertain. Day and Tennant (1991) used the EXOSAT ME to observe soft emission during two eclipses. They interpreted this as a dust-scattered halo and calculated the distance to the source as 5.4$\pm$0.3  kpc. The absolute distance to the source does not affect any of the results of this work except the calculated luminosities. For convenience the distance to Cen X-3 will be taken to be 8 kpc.

The binary parameters of Cen X-3 have been well determined (Joss and Rappaport (1984); Nagase et al. (1992) and references therein). The pulsar is spinning up with $\dot{P}$/P $\approx$ 2.3 x 10^-4 yr^-1 (Nagase et al. (1992)). This is due to the accretion of matter with a high specific angular momentum (Ghosh and Lamb (1979a,b); Ghosh et al. (1977)). The orbital period is decreasing with $\dot{P}_{\rm orb}^{}$/P_orb $\approx$ - 1.74 x 10⁶ yr^-1 (Nagase et al. (1992)) due to unstable tidal dissipation (Kelley et al. (1983)).