A Hidden Friend for the Galactic Center Black Hole, Sgr A*

The hierarchical nature of galaxy formation suggests that a supermassive black hole binary could exist in our galactic center. We propose a new approach to constraining the possible orbital configuration of such a binary companion to the galactic center black hole Sagittarius A* (Sgr A*) through the measurement of stellar orbits. Focusing on the star S0–2, we show that requiring its orbital stability in the presence of a companion to Sgr A* yields stringent constraints on the possible configurations of such a companion. Furthermore, we show that precise measurements of time variations in the orbital parameters of S0–2 could yield stronger constraints. Using existing data on S0–2 we derive upper limits on the binary black hole separation as a function of the companion mass. For the case of a circular orbit, we can rule out a 105 M⊙ companion with a semimajor axis greater than 170 au or 0.8 mpc. This is already more stringent than bounds obtained from studies of the proper motion of Sgr A*. Including other stars orbiting the galactic center should yield stronger constraints that could help uncover the presence of a companion to Sgr A*. We show that a companion can also affect the accretion process, resulting in a variability that may be consistent with the measured infrared flaring timescales and amplitudes. if such a companion exists, it will emit gravitational wave radiation, potentially detectable with the Laser Interferometer Space Antenna (LISA).