Swift reveals a 5.7 day super-orbital period in the M31 globular cluster X-ray binary XB158

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The M31 globular cluster X-ray binary XB158 (a.k.a. Bo 158) exhibits intensity dips on a 2.78 hr period in some observations, but not others. The short period suggests a low mass ratio, and an asymmetric, precessing disk due to additional tidal torques from the donor star since the disk crosses the 3:1 resonance. Previous theoretical 3D smoothed particle hydrodynamical modeling suggested a super-orbital disk precession period 29±1 times the orbital period, i.e. 81±3 hr. We conducted a Swift monitoring campaign of 30 observations over 1 month in order to search for evidence of such a super-orbital period. Fitting the 0.3--10 keV Swift XRT luminosity lightcurve with a sinusoid yielded a period of 5.65+/-0.05 days, and a >5σ improvement in χ² over the best fit constant intensity model. A Lomb-Scargle periodogram revealed that periods 5.4--5.8 days were detected at a >3σ level, with a peak at 5.6 days. We consider this strong evidence for a 5.65 day super-orbital period, 70% longer than the predicted period. The 0.3--10 keV luminosity varied by a factor 5, consistent with variations seen in long-term monitoring from Chandra. We conclude that other X-ray binaries exhibiting similar long-term behaviour are likely to also be X-ray binaries with low mass ratios and super-orbital periods.