Probing of Hierarchical Triples with Multi-Wavelength Observations of Globular Cluster Ultra-luminous X-ray Source RZ2109

The globular cluster ultraluminous X-ray source (GCULX) RZ2109 is a complex and unique source which has now been detected at X-ray, ultra-violet and optical wavelengths. The X-ray luminosity shows order-of-magnitude variability on the timescale of a few hours, with a ~1.3 day periodicity. One possible model for this behaviour is the Kozai-Lidov mechanism resulting from a hierarchical triple, where the outer companion can regulate the orbit of the mass transferring donor by trading its eccentricity and inclination. The source also demonstrates broad [OIII] 5007 A emission which has been observed for the past 18 years, suggesting a white dwarf donor and ultra-compact X-ray binary. Meanwhile, the globular cluster counterpart is unusally bright in the NUV band. The UV emission may be due to X-ray/UV reprocessing in the source's accretion disc, or a signature of multiple stellar populations in the globular cluster impacting the evolution of the X-ray binary. I will discuss multi-wavelength observations of this rare source and their implications for understanding accretion physics in extreme cases, as well as the evolution of ultra-compact X-ray binaries in globular clusters, gravitational wave progenitors and fast radio burst progenitors.