A second acceleration channel for the fast stars, independent of proper-motion curvature: repeat NIRSpec radial-velocity visits open a drift-detection window free of frame-tie and geometric-distortion systematics. Compare the recoverable drift uncertainty against the line-of-sight acceleration a central mass would impose.
A central mass M at projected radius r imposes a line-of-sight acceleration a_los = GM/r² on a star at that projection. Repeat spectroscopic visits open an independent test of this acceleration: fit a linear drift to n radial-velocity measurements of per-epoch precision σ_RV, spread over a baseline T. For white noise and uniform cadence, the recoverable drift-rate uncertainty is
σ_v̇ = σ_RV · √12 / (√n · T)
This channel shares none of the proper-motion channel's systematics (HST frame tie, geometric distortion) and inherits the full r⁻² gain for any inner star: the same acceleration law drives both channels, so a discovery-case star closer to the centre helps this one just as much.
At the fiducial parameters (M = 2×10⁴ M☉, r = 0.08 pc, σ_RV = 3 km/s, annual cadence, T = 5 yr): a_los ≈ 4.4×10⁻⁷ m/s² ≈ 0.014 km/s/yr, and σ_v̇ ≈ 0.9 km/s/yr, a factor ~65–70 above the nominal signal, the same verdict as the proper-motion channel. At the MICADO-class discovery radius (r = 0.01 pc, same mass) the signal itself reaches ≈0.9 km/s/yr, comparable to σ_v̇ at this baseline: detectable within a five-year GO 5137 extension, and improving rapidly with either more visits or a longer baseline (σ_v̇ ∝ n⁻¹ᐟ²T⁻¹).
Orbital-phase projection factors (a randomly oriented star sees only a fraction of the full a_los on its line of sight), correlated instrumental systematics across epochs, crowded-field blending in crowded NIRSpec fields, and any prior on which stars are the best drift targets. The headline numbers are the bare white-noise floor.
This is the lower panel of the astrometric sensitivity figure in the Campaign Paper's Program 3 (Astrometry): the proper-motion curvature test is marginal on any current or near-future baseline (Pulsar Acceleration Mapper covers the timing side of the same mass tension), so the campaign routes its near-term decision weight through wander, reference-frame, and discovery measurements instead. This RV-drift channel is one of three worked measurements, and the one that most directly rewards finding an inner star.