A hidden friend for the galactic center black hole, Sgr A*

TL;DR

This paper proposes a method to constrain the existence and properties of a potential binary companion to the galactic center black hole Sgr A* by analyzing stellar orbits, especially S0-2, and discusses implications for gravitational wave detection.

Contribution

It introduces a novel approach using stellar orbit measurements to set constraints on a possible black hole binary at the galactic center, improving previous bounds and exploring observational signatures.

Findings

Upper limits on companion mass and separation derived from stellar orbit data.

Potential for stronger constraints using additional stellar observations.

Implications for gravitational wave detection with LISA.

Abstract

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 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 {\it 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 10^5 M_sun companion with a semimajor axis greater than 170 astronomical units 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 which may be consistent with the measured infrared flaring timescales and amplitudes. Finally, if such a companion exists, it will emit gravitational wave radiation, potentially detectable with LISA.