Anomalous Minima in Cen X-3

SCHREIER et al.¹ have presented convincing evidence that the highly variable pulsing X-ray source Cen X-3 is a member of a binary system. The UHURU satellite observations have shown that the intensity of the source varies with a 2.09 day period, and the short term pulsations show a periodically varying Doppler shift also compatible with this period to a high degree of accuracy. But a curious feature of the UHURU observations is that on several occasions the source remained at intensity minimum during periods when maxima were predicted. This anomalous behaviour must be explained before a detailed model can be formulated for Cen X-3. As pointed out by Schreier et al, the fact that the suggested optical counterpart of Cyg X-1 is a spectroscopic binary increases the importance of understanding the X-ray binary phenomenon.