Timing Analysis of the Light Curve of the Dipping - Bursting X - ray Binary X 1916 - 053

We present the timing analysis results for our observations of the x-ray dip source X1916-053 conducted with RXTE between February and October of 1996. Our goal was to finally measure the binary period-as either the x-ray dip period or ∼1% longer optical modulation period, thereby establishing if the binary has a precessing disk (SU UMa model) or a third star (triple model). Combined with historical data (1979-96), the x-ray dip period is measured to be 3000.6508 ± 0.0009 sec with a 2σ upper limit | ˙ P | ≤ 2.06×10 −11. From our quasi-simultaneous optical observations (May 14-23, 1996) and historical data (1987-96), we measure the optical modulation period to be 3027.5510 ± 0.0052 sec with a 2σ upper limit | ˙ P | ≤ 2.28 × 10 −10. The two periods are therefore each stable (over all recorded data) and require a 3.9087 ± 0.0008d beat period. This beat period, and several of its harmonics is also observed as variations in the dip shape. Phase modulation of x-ray dips, observed in a 10 consecutive day observation, is highly correlated with the ∼3.9d dip shape modulation. The 1987-1996 optical observations show that the optical phase fluctuations are a factor of 3 larger than those in the x-ray. We discuss SU UMa vs. triple models to describe the X1916-053 light curve behavior and conclude that the x-ray dip period, with smaller phase jitter, is probably the binary period so that the required precession is most likely similar to that observed in SU UMa and x-ray nova systems. However the " precession " period stability and especially the fact that the times of x-ray bursts may partially cluster to occur just after x-ray dips, continue to suggest that this system may be a hierarchical triple.