Princeton University Thunch Talk

Characterizing the mass distribution of compact objects is essential for understanding the late-stage evolution of massive stars. The majority of black hole mass measurements come from X-ray binaries and gravitational wave mergers, but the majority of black holes in the Milky Way are expected to instead be either isolated, free-floating systems or in non-interacting binaries. The recent Gaia Focused Product Release contains radial velocity time-series for more than 9,000 long-period photometric variables. We search for binary systems with large radial velocity amplitudes to identify candidates with massive, unseen companions. Eight targets have large binary mass functions f(M) > 1 Msun, three of which are eclipsing binaries. The remaining five show evidence of ellipsoidal modulations. We fit spectroscopic orbit models to the Gaia radial velocities and fit the spectral energy distributions of three targets. For the two systems most likely to host dark companions, J0946 and J1640, we use PHOEBE to fit the ASAS-SN light curves and Gaia radial velocities. The derived companion masses are >3 Msun, but the high Galactic dust extinctions towards these objects limit our ability to rule out main sequence companions or subgiants hotter than the photometric primaries. These systems are similar to other stellar-mass black hole impostors, notably the Unicorn (V723 Mon) and the Giraffe (2M04123153+6738486). While it is possible that J1640 and J0946 are similar examples of stripped giant star binaries, high-resolution spectra can be used to determine the nature of their companions.