Swift Reveals a ~5.7 Day Super-orbital Period in the M31 Globular Cluster X-Ray Binary XB158
Abstract
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 three-dimensional 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 X-Ray Telescope luminosity light curve 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 of 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 of ~5, consistent with variations seen in long-term monitoring from Chandra. We conclude that other X-ray binaries exhibiting similar long-term behavior are likely to also be X-ray binaries with low mass ratios and super-orbital periods.
- Publication:
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The Astrophysical Journal
- Pub Date:
- March 2015
- DOI:
- 10.1088/0004-637X/801/1/65
- arXiv:
- arXiv:1501.01978
- Bibcode:
- 2015ApJ...801...65B
- Keywords:
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- X-rays: binaries;
- X-rays: general;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Accepted for publication in ApJ