Four observational channels compared: which instrument resolves the 6,000–8,200 M☉ tension between Häberle et al. 2024 and Bañares-Hernández et al. 2025 — and when?
The Häberle et al. 2024 proper-motion result establishes a 3σ lower limit of ≥8,200 M☉. Bañares-Hernández et al. 2025 pulsar timing sets a 3σ upper limit of ≤6,000 M☉. These measurements are in tension at the 3σ level — both cannot be simultaneously correct under standard assumptions. Stage 2–4 show what each instrument needs to resolve this.
The Bañares-Hernández et al. 2025 constraint comes from 5 pulsars in OC's core. Adding more pulsars reduces the uncertainty on the gravitational potential. The MeerKAT timing sensitivity on the upper mass limit scales as 1/√N_psr × 1/√baseline. The key insight: to drop the upper limit below 8,200 M☉ requires √(N×B) > 3.66, so at N=5 pulsars a baseline of only ~2.7 years is sufficient — yet the tension persists, highlighting a genuine systematic conflict.
Chen et al. 2025 placed an upper limit on IMBH accretion luminosity from JWST NIRCam/MIRI observations, which translates to a mass limit only under assumptions about the accretion efficiency ε. The constraint weakens for low ε — at ε = 10⁻³ the limit is ~6,000 M☉, consistent with Bañares, but at ε = 10⁻⁵ (radiatively inefficient flow) the limit weakens to ~60,000 M☉, compatible with any proposed mass. The ambient gas density sets the Bondi rate.
If a stellar-mass black hole in OC spirals into the IMBH, LISA will measure the chirp mass to ~0.1% accuracy from the inspiral gravitational waveform. This translates to an IMBH mass posterior width that depends on the mass ratio q = m_stellar/M_IMBH and the signal-to-noise ratio. A single IMRI detection definitively closes the constraint window. The expected IMRI rate for a ~10,000 M☉ IMBH in a dense globular cluster is ~10⁻⁸ yr⁻¹ per cluster.
| Instrument | Upper limit / σ_M (M☉) | vs. Häberle 8,200 | Status |
|---|---|---|---|
| MeerKAT/SKA Pulsar | — | — | — |
| JWST Accretion | — | — | — |
| LISA IMRI | — | — | — |
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