Timing Analysis of the Light Curve of the Dipping-Bursting X-ray Binary X1916-053

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 $\pm$ 0.0009 sec with a 2$σ$ upper limit $|\dot P| \leq 2.06 \times 10^{-11}$. 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 $\pm$ 0.0052 sec with a 2$σ$ upper limit $|\dot P| \leq 2.28 \times 10^{-10}$. The two periods are therefore each stable (over all recorded data) and require a $3.9087 \pm 0.0008$d 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 $\sim$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.