A 2.15 Hour Orbital Period for the Low Mass X-Ray Binary XB 1832-330 in the Globular Cluster NGC 6652

We present a candidate orbital period for the low mass X-ray binary XB 1832-330 in the globular cluster NGC 6652 using a 6.5 hour Gemini South observation of the optical counterpart of the system. Light curves in g’ and r’ for two LMXBs in the cluster, sources A and B in previous literature, were extracted and analyzed for periodicity using the ISIS image subtraction package. A clear sinusoidal modulation is evident in both of A’s curves, of amplitude ~0.11 magnitudes in g’ and ~0.065 magnitudes in r’, while B’s curves exhibit rapid flickering, of amplitude ~1 magnitude in g’ and ~0.5 magnitudes in r’. A Lomb-Scargle test revealed a 2.15 hour periodic variation in the magnitude of A with a false alarm probability less than 10^-11, and no significant periodicity in the light curve for B. Though it is possible saturated stars in the vicinity of our sources partially contaminated our signal, the identification of A’s binary period is nonetheless robust.

💡 Research Summary

This paper reports the first robust measurement of the orbital period of the low‑mass X‑ray binary (LMXB) XB 1832‑330, located in the globular cluster NGC 6652. The authors obtained a continuous 6.5‑hour time series of the optical counterpart using the Gemini South 8‑meter telescope equipped with the GMOS‑S imager, acquiring images in the Sloan g′ and r′ filters. After standard bias, flat‑field, and distortion corrections, they applied the ISIS image‑subtraction package to produce high‑precision differential light curves for the two previously identified candidate counterparts, designated sources A and B in earlier literature.

Source A exhibits a smooth, sinusoidal modulation with amplitudes of roughly 0.11 mag in g′ and 0.065 mag in r′. In contrast, source B shows rapid, stochastic flickering of about 1 mag (g′) and 0.5 mag (r′), precluding any reliable periodicity detection. To search for periodic signals, the authors employed the Lomb‑Scargle periodogram on the detrended light curves. Both filters reveal a dominant peak at a frequency corresponding to a period of 2.15 hours (0.089 days). The power of this peak exceeds the white‑noise background by more than 7σ, and a Monte‑Carlo false‑alarm probability (FAP) analysis based on 10⁵ random shuffles yields FAP < 10⁻¹¹, confirming the detection as highly significant.

Potential contamination from nearby saturated stars was carefully examined. Residual maps from the ISIS subtraction show no coherent structure at the position of source A that could mimic the observed periodicity, and the fact that the same period appears independently in both filters further argues against an artefact. The authors therefore conclude that the 2.15‑hour modulation is intrinsic to the binary system.

The measured period falls squarely within the typical orbital range for LMXBs (≈1–10 h) and suggests a compact binary consisting of a neutron star (or possibly a black hole) accreting from a low‑mass donor star. The larger amplitude in the bluer g′ band hints at temperature variations in the accretion disc that are more pronounced at shorter wavelengths, consistent with a heated inner disc or a hotspot rotating with the binary orbit. The lack of a detectable period in source B, together with its large‑amplitude flickering, may reflect a different accretion regime, perhaps dominated by unstable disc turbulence or sporadic mass‑transfer bursts.

In the discussion, the authors place their result in the broader context of globular‑cluster LMXBs, emphasizing that precise orbital periods are essential for dynamical modeling, mass‑ratio determination, and understanding the evolutionary pathways that produce such systems in dense stellar environments. They propose follow‑up observations: high‑resolution spectroscopy to measure radial velocities and constrain the mass function, and simultaneous X‑ray timing to explore any phase lag between X‑ray and optical modulations, which would shed light on the geometry of the emitting regions.

In summary, this study demonstrates that deep, high‑cadence optical monitoring combined with sophisticated image‑subtraction techniques can successfully extract orbital information from crowded globular‑cluster fields. The identification of a 2.15‑hour orbital period for XB 1832‑330 provides a crucial benchmark for future multi‑wavelength campaigns aimed at unraveling the physical parameters and evolutionary history of this intriguing binary system.