High Tide: A Systematic Search for Ellipsoidal Variables

The majority of non-merging stellar mass black holes are discovered by observing high energy emission from accretion processes. These interacting systems have masses of 5—10 Msun with a low mass gap from ~ 2—5 Msun that separates the neutron stars and stellar mass black hole populations. Since only a small fraction of potential binary configurations are expected to result in an interacting system, we aim to identify non-interacting black holes by searching for the tidally-induced ellipsoidal variability of their stellar companions. We start from a sample of about 200,000 rotational variables, semi-regular variables, and eclipsing binary stars from the All-Sky Automated Survey for Supernova (ASAS-SN). We use a χ^2 ratio test followed by visual inspection of TESS and ASAS-SN light curves to identify 369 candidates for ellipsoidal variability. We then combine the amplitude of the variability with mass and radius estimates for observed stars to calculate a minimum companion mass and identify the most promising candidates for high mass companions.