Roman
Microlensing

Peer-reviewed

Computes the Einstein radius and crossing timescale for a gravitational microlensing event observable by the Nancy Grace Roman Space Telescope. Uses the Gould 2000 formalism (kappa = 8.144 mas/M-sun). Also scales the Penny et al. 2019 stellar event yield by a survey fraction.

Pre-launch forecast Roman Space Telescope launches 2026-08-30. All outputs are forecasts until that date. The "released" toggle below switches the regime label only — it does not imply actual Roman data. Roman astrometric microlensing uniquely detects the lens mass directly via proper motion, enabling IMBH searches (t_E ~ years for an IMBH lens).
Lens and geometry parameters
1.0 M-sun
Lens mass in solar masses. Log scale. Try 8200 for the Haberle 2024 IMBH lower bound.
4.0 kpc
Lens distance. Omega Centauri: ~5.3 kpc. Must be less than D_S.
8.0 kpc
Source distance. Galactic bulge: ~8 kpc. Must exceed D_L.
5.0 mas/yr
Relative proper motion of lens and source. Typical bulge survey: 3-8 mas/yr.
100%
Scales the Penny et al. 2019 yield estimate proportionally.
Toggle to "live" after Roman launches (2026-08-30). Changes regime label only.
Results FORECAST
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theta_E (Einstein radius, mas)
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t_E (Einstein crossing, days)
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What this tool computes

      
Einstein radius and crossing time using Gould 2000 (ApJ 542, 785):

      
pi_rel = 1/D_L_kpc - 1/D_S_kpc (mas)

      
theta_E = sqrt(kappa * M * pi_rel) (mas),  kappa = 8.144 mas/M-sun

      
t_E = theta_E / mu * 365.25 (days)

      
For an IMBH lens at Omega Centauri distance (5.3 kpc) with galactic-bulge sources (8.2 kpc) and mass 8,200 M-sun (Haberle 2024 lower bound): theta_E ~ 67 mas, t_E ~ 22 years. Roman's astrometric precision can directly resolve the lens mass from the lens-source separation trajectory.

      
Yield estimate

      
Penny et al. 2019 (arXiv:1808.02490) estimates ~27,000 stellar microlensing events for the Roman 6-field 2-season Galactic Bulge Time Domain Survey. The yield_scaled_events field scales this by survey_fraction. This is for stellar-mass lenses; IMBH-induced events would be a small, model-dependent subset. Updated estimates: Johnson et al. 2024 (arXiv:2512.05182).

      
Pre-launch status

      
Roman launches 2026-08-30 (NASA target). Until launch, all outputs are pre-launch forecasts. The "released" toggle changes the regime label only and does not imply any actual data.

      
IMBH context

      
    
        
  • Kinematics (peer-reviewed): Haberle et al. 2024 — IMBH ≥ 8,200 M-sun
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  • Pulsar timing (peer-reviewed): Banares-Hernandez et al. 2025 — IMBH ≤ 6,000 M-sun
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  • These bounds are in tension. Roman microlensing offers an independent astrometric probe.
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Citations

      
    
        
  • Gould 2000, ApJ 542, 785 (DOI 10.1086/308744) — microlensing formalism
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  • Penny et al. 2019, ApJS 241, 3 (arXiv:1808.02490) — Roman yield
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  • Johnson et al. 2024, arXiv:2512.05182 — updated yield estimates
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  • Haberle et al. 2024, Nature 631, 285 (DOI 10.1038/s41586-024-07511-z)
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  • Banares-Hernandez et al. 2025, A&A 693 A104 (DOI 10.1051/0004-6361/202450240)
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Tool v1.2.0 · mandate me.omegacentauri/roman_microlensing · register: peer-reviewed · OCG artifact v0.4.0