Program: GS-2023A-FT-112

We request high resolution spectroscopy of the [OIII] 4959, 5007 emission line in the globular cluster ultraluminous X-ray source RZ2109. This source is the only known ultra-compact X-ray binary which is accreting at a rapid rate, consistent with being in an early phase of ultra-compact X-ray binary evolution phase. Recent observations from April 19, 2023, on the 4.1 metre SOAR telescope suggest that the flux in the [OIII] emission line has undertaken a sharp decline. If so, this would place valuable constraints on the age of the system, and allow for a benchmark to compare observations to theory for the first time. However, the April 19 observation night was plagued with issues, and we are requesting an independent confirmation of the flux in the system to verify the the sharp decrease in the flux.