The two tightest constraints on OC's IMBH mass are irreconcilable at face value. This workflow walks through each measurement, quantifies the tension, cross-checks with accretion physics, and identifies which upcoming observation closes the gap first.
Häberle et al. 2024 (Nature 631:285) identified seven fast-moving stars in OC's innermost 3 arcseconds using more than 500 HST images over two decades. The minimum enclosed mass required to keep those stars bound establishes a firm lower limit on any central compact object: ≥ 8,200 M☉. Their best-fit mass for a point source is 39,000–47,000 M☉, but the lower limit at the stated confidence level is 8,200 M☉.
Bañares-Hernández et al. 2025 (A&A 693:A104) combined VLT stellar kinematics with pulsar timing accelerations for OC's millisecond pulsars. Their joint analysis favors an extended central mass distribution (~2–3 × 10⁵ M☉) over a single IMBH, and places a 3σ upper limit of ≤ 6,000 M☉ on any central point mass. This is formally in tension with the Häberle lower bound.
The accretion state of an IMBH depends on its mass through the Eddington luminosity and Bondi radius. González Prieto et al. 2025 (arXiv:2512.09649) found no radio or X-ray counterpart, constraining the radiative efficiency to η ≲ 8 × 10⁻⁵. Crucially, this constraint is stronger at higher mass (larger Bondi radius → more predicted accretion → tighter efficiency limit). The accretion non-detection thus provides an independent cross-check on the allowed mass range.
Four instruments will independently constrain the IMBH mass in the next decade. The tension will be resolved when two methods agree. Set your confidence level for each instrument and see which combination closes the gap first.
The IMBH mass tension between Häberle et al. 2024 (≥8,200 M☉ lower bound from stellar kinematics) and Bañares-Hernández et al. 2025 (≤6,000 M☉ upper bound from pulsar timing) represents the sharpest open question in OC science. Both measurements are methodologically sound; the tension is real. Accretion non-detections (González Prieto 2025) are consistent with both mass scenarios at ADAF efficiencies. The tension window of 6,000–8,200 M☉ will be resolved by: (1) Gaia DR4 IMBH proper-motion astrometry (2026), (2) new MeerKAT/TRAPUM pulsars deepening the timing constraint, or (3) Roman Space Telescope microlensing (2027+). LISA will provide a definitive measurement via IMRI after 2035.