OCS / Tools / Astrometric Microlensing Predictor
Gravity-only single-lens calculator: the centroid shift, Einstein timescale, and expected event count for a compact lens in front of an explicitly chosen background source population. The default background is Ω Cen's actual halo sightline, and it is sparse; the verdict reports exactly that.
When a compact mass passes near our line-of-sight to a background star, the star's light is gravitationally deflected. The star's apparent position shifts on the sky by a fraction of the Einstein radius — at peak alignment, δθ_max ≈ θ_E / (2√2). This is the astrometric signal. It is distinct from the photometric magnification (the brightness bump that OGLE, MOA, and KMTNet hunt for in bulge surveys). Astrometric shifts persist on time scales of years and survive even for "dark" lenses (isolated stellar-mass black holes, neutron stars, IMBHs) that produce no light themselves.
Gaia DR4 (planned December 2026) releases the full astrometric time series for each Gaia source, including the per-epoch positions beyond the 5-parameter fit, and pipelines like Wyrzykowski et al. 2025 (A&A) are built to search it for transient astrometric microlensing from isolated compact objects. But a lens only lenses sources behind it on the same sightline. Ω Centauri sits at (l, b) ≈ (309.1°, +15.0°), about 53° from the Galactic centre: its sightline points into the Galactic halo, not through the bulge, so the background population behind the cluster is sparse. The default Σ_bg in the sliders reflects that, and the expected event count, not the centroid-shift amplitude, is what decides the test. A dense-field configuration stays on the slider as a labelled hypothetical for comparison with published bulge-survey predictions.
Roman launched 2026-08-30 on a Falcon Heavy from LC-39A; the Wide Field Instrument was activated, NASA reported commissioning underway on 2026-09-15, and first images are expected by early 2027. Its Galactic Bulge Time Domain Survey, designed to find isolated stellar-mass black holes by astrometric microlensing across millions of bulge stars (Penny et al. 2019, ApJS 241:3), observes the Galactic bulge at (l ≈ 0.5°, b ≈ −1.4°) plus a Galactic-Centre field. That footprint does not contain ω Cen: the cluster's sightline lies ≈53° away, and Roman's on-orbit calibration "touchstone" fields are confined to the continuous-viewing zone at the ecliptic poles (Williams et al. 2025, NTRS 20250003012), also nowhere near the cluster. Ω Cen is a historic HST calibration field, the likely source of the earlier confusion on this page. The tool therefore treats Roman as a sensitivity reference for a bulge-field configuration that Ω Cen's own sky position does not provide.
A single IMBH produces rare but large-amplitude events: one mass × big θ_E × moderate event rate. A dark cluster of many smaller masses (stellar-mass BHs, neutron stars) produces frequent but smaller-amplitude events: many masses × small θ_E × high event rate. The two scenarios give numerically distinct distributions of (δθ_max, t_E) across detected events — which Gaia DR4 and Roman together will be able to discriminate. Compare to the alternative-hypothesis tool to see the signature difference.
Compare with Dark Cluster Alternative, IMBH Constraint Stacker, Pulsar Timing, and JWST Accretion. Astrometric microlensing is the only test in this set that depends solely on gravity — no assumptions about gas, radiative efficiency, or pulsar populations enter.
The event rate estimate Γ ≈ 2 θ_E σ_bg μ_rel is a back-of-envelope cross-section calculation; it assumes a uniform background density and ignores blending, finite-source effects, parallax, and binary-lens complications. The "detectable" verdicts use simple step-function thresholds (30 μas for Gaia, 10 μas for Roman) — real pipelines use full likelihood fits and can squeeze 2–3× more out of marginal events. The Einstein radius formula assumes a point lens; an IMBH is effectively a point at these scales, but a dark cluster is not.
Gaia DR4 release schedule: ESA/DPAC, planned December 2026 (cosmos.esa.int/gaia/release). Roman launch and commissioning: NASA news release, launch 2026-08-30, WFI activation announced 2026-09-15, first images early 2027 (nasa.gov; roman.gsfc.nasa.gov). Roman GBTDS footprint: STScI Roman Community Defined Surveys documentation (roman-docs.stsci.edu), five bulge fields near (l, b) ≈ (0.5°, −1.4°) plus a Galactic-Centre field. Calibration touchstone fields: Williams et al. 2025 (NTRS 20250003012), ecliptic-pole continuous-viewing zone. Ω Cen position: (l, b) ≈ (309.1°, +15.0°), ≈53° from the bulge fields. Penny et al. 2019 (ApJS 241:3), Roman microlensing survey simulations. Wyrzykowski et al. 2025 (A&A), astrometric-only microlensing pipeline targeting DR4. Dominik & Sahu 2000 (ApJ 534:213), the δθ(u) = u θ_E / (u² + 2) result used here. Paczyński 1986 (ApJ 304:1), the A(u) magnification formula used in this tool.