Program: GS-2017A-Q-79

Our group previosly discovered the near-infrared (NIR) counterpart to the Galactic Center Chandra X-ray source CXO 174528.7-290942, and we used NIR photometry and spectroscopy to confirm that the counterpart is a red giant mass donor in an accreting system (Dewitt et al. 2013). Our more recent work has found that the system not only varies by up to ~1 mag on long timescales, but that it also varies by ~0.5 mag on timescales of a few hours in the WISE 1 (3.6 micron) band. The red giant star - which should dominate the NIR emission in this system - should not be capable of varying on short timescales at this amplitude. While other X-ray binaries can and do produce large-amplitude rapid variability, this is usually driven by X-ray-emitting accretion processes - but here we find that the X-ray luminosity is consistently too low to power the NIR variations (by a factor of ~100-1,000). We propose to use FLAMINGOS-2 on Gemini South to obtain time-resolved NIR spectra of the variability in order to see which spectral features (the continuum, the red giant CO absorption region, or the accretion disk emission lines) participate in this large-amplitude rapid features, and thus constrain the nature of this unique X-ray binary. NOTE: this proposal is for PI Guaranteed Time.