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 dipsource X1916-053 conducted with RXTE between February and October of 1996. Ourgoal was to finally measure the binary period - as either the x-ray dip periodor ~1% longer optical modulation period, thereby establishing if the binary hasa precessing disk (SU UMa model) or a third star (triple model). Combined withhistorical data (1979-96), the x-ray dip period is measured to be 3000.6508$\pm$ 0.0009 sec with a 2$\sigma$ upper limit $|\dot P| \leq 2.06 \times10^{-11}$. From our quasi-simultaneous optical observations (May 14-23, 1996)and historical data (1987-96), we measure the optical modulation period to be3027.5510 $\pm$ 0.0052 sec with a 2$\sigma$ upper limit $|\dot P| \leq 2.28\times 10^{-10}$. The two periods are therefore each stable (over all recordeddata) and require a $3.9087 \pm 0.0008$d beat period. This beat period, andseveral of its harmonics is also observed as variations in the dip shape. Phasemodulation of x-ray dips, observed in a 10 consecutive day observation, ishighly correlated with the $\sim$3.9d dip shape modulation. The 1987-1996optical observations show that the optical phase fluctuations are a factor of 3larger than those in the x-ray. We discuss SU UMa vs. triple models to describethe X1916-053 light curve behavior and conclude that the x-ray dip period, withsmaller phase jitter, is probably the binary period so that the requiredprecession is most likely similar to that observed in SU UMa and x-ray novasystems. However the ``precession'' period stability and especially the factthat the times of x-ray bursts may partially cluster to occur just after x-raydips, continue to suggest that this system may be a hierarchical triple.Comment: 33 pages, 13 figures, accepted by Ap