Searching for compact objects in the OGLE dataset of ellipsoidal variable stars by exploring optical activity in X-ray quiescence

Studying the population of black holes in the Milky Way is of key importance to modern astrophysics, with implications across the study of stellar evolution to supernova mechanics. However, the arsenal of techniques available to discover and study black holes in our galaxy is highly limited. Stellar-mass black holes are most often discovered in X-ray binaries (XRBs), a configuration in which the black hole accretes matter from a close-orbiting companion star. This process is extremely violent, releasing a large proportion of the latent rest mass of the accreting matter as light, primarily in the form of X-rays. If the accretion process cycles between different states of activity, the XRB may not emit X-rays for extended periods of time, rendering it undiscoverable. However, work by Cantrell et al. (2008) and MacDonald et al. (2014) have demonstrated that XRBs can vary aperiodically in optical brightness and color while remaining X-ray quiescent. We apply this principle to the OGLE-IV dataset of 25904 short-period ellipsoidal binary stars in the galactic bulge. After applying observability heuristics to reduce the dataset to 776 binaries, we examine their optical time-series and identify 16 candidates with likely aperiodic variability. Follow-up radial velocity measurements were taken using the Next Generation Palomar Spectrograph in June and July 2025. We present preliminary results for some of the objects, including radial velocity curves and basic physical models computed using PHOEBE and XRBinary. Extracting the mass of the ellipsoidal secondary from the RV curve, a measurement greater than 1.5 solar masses would indicate a neutron star candidate, and a measurement greater than 3 solar masses would indicate a black hole candidate.