2014ApJ...780..148A

Massive black holes (MBHs) in galactic nuclei are believed to be surrounded by a high-density stellar cluster, whose mass is mostly in hard-to-detect faint stars and compact remnants. Such dark cusps dominate the dynamics near the MBH: a dark cusp in the Galactic center (GC) of the Milky Way would strongly affect orbital tests of general relativity there; on cosmic scales, dark cusps set the rates of gravitational wave emission events from compact remnants that spiral into MBHs, and they modify the rates of tidal disruption events, to list only some implications. A recently discovered long-period massive young binary (with period P₁₂ ≲ 1 yr, total mass M₁₂ ∼ O(100M_☉), and age T₁₂∼ 6x10⁶ yr), only ∼0.1 pc from the Galactic MBH, sets a lower bound on the stellar two-body relaxation timescale there, min t_rlx∝(P₁₂/M₁₂)^2/3 T₁₂∼ 10⁷ yr, and, correspondingly, an upper bound on the stellar number density, max n_* ∼ few x 10⁸/(M_*²)/pc3 (M_*²)^1/2 is the rms stellar mass), based on the binary's survival against evaporation by the dark cusp. However, a conservative dynamical estimate, the drain limit, implies it_rlx > O(10⁸yr). Such massive binaries are thus too short-lived and tightly bound to constrain a dense relaxed dark cusp. We explore here in detail the use of longer-period, less massive, and longer-lived binaries (P₁₂∼ few yr, M₁₂∼ 2-4 M_☉, T₁₂∼ 10⁸-10¹⁰ yr), presently just below the detection threshold, for probing the dark cusp and develop the framework for translating their future detections among the giants in the GC into dynamical constraints.