The IMBH mass tension will be resolved by whichever instrument first delivers a measurement that is internally consistent and contradicts the other. This workflow maps the race — identifying each instrument's sensitivity, timeline, and probability of resolving the 6,000–8,200 M☉ window.
Two constraints bracket the IMBH mass from opposite sides. Häberle et al. 2024: kinematic lower bound ≥ 8,200 M☉. Bañares-Hernández et al. 2025: pulsar timing upper bound ≤ 6,000 M☉. These are formally inconsistent. Three additional constraints are consistent with both: JWST accretion non-detection, VLA/MeerKAT radio non-detection, and N-body models that prefer higher masses (~50,000 M☉).
Gaia DR4 will provide proper motions for stars within OC's innermost regions at ~15 µas/yr precision. The IMBH's Brownian wander signature differs between the 8,200 M☉ and 6,000 M☉ scenarios (heavier = less wander). The photocentric shift of the cluster center should be detectable if the IMBH mass exceeds ~5,000 M☉.
Expected outcome: Gaia DR4 will constrain the IMBH wander amplitude to ~10–20 µas/yr precision. This is marginally sensitive to the 6,000 vs. 8,200 M☉ difference (predicted wander: ~8 µas/yr vs. ~10 µas/yr). A clear non-detection of wander would weakly favor the lower-mass scenario.
TRAPUM (TRAnsients and PUlsars with MeerKAT) has already discovered PSR J1326-4728S (2026). Each new pulsar with good timing precision improves the mass upper bound as ~1/sqrt(N_pulsars × T_baseline²). Discovering 3–5 more pulsars within 60 arcsec of the center would move the upper bound from ≤6,000 M☉ toward ≤4,000 M☉ — potentially confirming or ruling out the Häberle lower bound.
If the projected upper bound reaches ≤4,000 M☉ with high confidence, and the Häberle lower bound of 8,200 M☉ is upheld, then the models must be reconciled — most likely by the stellar cusp alternative (no IMBH, or a lower-mass IMBH within an extended black-hole cluster).
Roman's ~1 µas/yr narrow-field astrometric precision will be transformative for OC. It will directly measure the IMBH's proper motion offset from the cluster photocenter, and resolve stellar proper motions in the innermost arcseconds that Hubble could only partially probe. At 1 µas/yr precision, the 8,200 vs. 6,000 M☉ scenarios predict different Brownian wander signatures at the 3–5σ level.
Roman is the most likely instrument to definitively resolve the tension before LISA. It can directly test both the kinematic (fast-star) and astrometric (wander) signatures simultaneously with the same dataset.
An intermediate mass ratio inspiral (IMRI) — a neutron star or stellar BH spiraling into OC's IMBH — would produce gravitational waves in LISA's 0.1–100 mHz band. The waveform encodes both the IMBH mass and spin with exquisite precision: uncertainty δM/M ~ 10⁻³ and δa★/a★ ~ 10⁻⁴ after a few months of observation. LISA is the definitive resolver, but requires an IMRI event in the right frequency window.
The mass precision from a single IMRI detection far exceeds all electromagnetic constraints combined. LISA will definitively answer both the mass question and the spin question — the latter being critical for MTH viability.
| Instrument | Timeline | Mass precision | Resolves tension? | Spin info? |
|---|---|---|---|---|
| Gaia DR4 | 2026 | Indirect (wander ~15 µas/yr) | Marginal — same order | No |
| MeerKAT TRAPUM | 2026–2030 | Upper bound improves to ~3,000–4,000 M☉ | Yes, if upper bound drops below 8,200 | No |
| Roman (narrow-field) | 2027+ | ~1 µas/yr astrometry → mass at ~20% level | ✓ Most likely — distinguishes 6,000 vs. 8,200 | Indirect |
| LISA IMRI | 2035+ | δM/M ~ 10⁻³ (definitive) | ✓ Definitive — but requires IMRI event | ✓ Definitive spin |
The Roman Space Telescope (2027+) is the most likely instrument to resolve the 6,000–8,200 M☉ tension before LISA, because its ~1 µas/yr astrometric precision in narrow-field mode directly measures the IMBH's Brownian wander amplitude — which differs by ~25% between the two mass scenarios. MeerKAT TRAPUM remains critical: discovering 5+ new inner-core pulsars would push the timing upper bound toward ≤4,000 M☉, making the Häberle lower bound formally inconsistent and forcing a reanalysis of the fast-star sample. LISA remains the definitive instrument, but the 2035 timeline makes it a decade away. The most informative near-term measurement is a single new MSP within 20 arcseconds of the OC core via MeerKAT.