THE X-RAY NOVA CENTAURUS X-4 - COMPARISONS WITH A0620-00

Results are presented for the first simultaneous spectroscopic and photometric study of Cen X-4 in its quiescent state. The spectrum of this X-ray nova consists of a K5-K7V stellar part plus an accretion disk component. We confirm earlier determinations of the 0.629 day orbital period, and the ∼150 km s-1 K-velocity of the absorption lines; however, we find a system center-of-mass velocity of 137 ± 17 km s-1, which is substantially less than reported previously. We discuss and extend one of the models proposed by Chevalier et al. in which the secondary is a peculiar low-mass (∼0.1 M⊙) star viewed at a moderately low inclination angle (i ≈ 35°-40°). Within the context of this model, our I and V-I light curves can be explained by the tidal distortion of the secondary plus some "excess light" from the splash point, the region where the accretion stream strikes the edge of the disk. Support for this interpretation is provided by the phase of the Hβ (emission-line) velocity curve. Cen X-4 is compared to the black-hole X-ray nova A0620-00. It is suggested that some differences may be due to the presence of a disk/star boundary layer in Cen X-4 and its absence in A0620-00.