Orateur
Description
Context. The orbits of S stars around the Galactic Center black hole, Sgr A, offer a unique opportunity to probe the relativistic regime of gravity and test the no-hair theorem. While the Schwarzschild precession has been detected with S2, the detection of spin-induced (Kerr) effects—Lense–Thirring and quadrupole moment terms—remains a major observational challenge.
Aims. We aim to quantify the time needed to detect the spin parameters of Sgr A—magnitude and orientation—by exploiting astrometric and spectroscopic monitoring of multiple S stars with current (GRAVITY+, ERIS) and future facilities (MICADO).
Methods. Using the post-Newtonian integrator, we simulate long-term astrometric observations of individual and combined S-star systems, including S2, S29, S38 and S55, and the recently discovered S301. We quantify spin detectability through χ2 minimization and forecast the evolution of spin-parameter uncertainties over the coming decades.
Results. We show that S301 has spin sensitivity comparable to a star identical to S2 but ten-times closer to Sgr A, and that combining its data with those of other stars of diverse orientations (S2, S29, S38, S55) reduces parameter degeneracies and accelerates spin
detection. Quantitative forecasts indicate that the spin magnitude can be constrained to σχ ≃ 0.1 by 2032, 0.05 by 2034, and 0.03 by 2036 when only fitting the spin magnitude χ, with comparable results when fitting all spin parameters. Astrometry provides the dominant leverage, while spectroscopy adds substantial value only if radial-velocity precision reaches 2 km/s, as expected with MICADO. Overall, spin detectability is robust given sufficient monitoring and knowledge of orbital parameters. The black hole orientation is a critical factor, along with multi-star, multi-instrument datasets, which together offer the most promising path forward.
Conclusions. The joint monitoring of multiple S stars with GRAVITY+ and MICADO will enable robust measurement of the spin of Sgr A within the next decade. Astrometry remains indispensable for achieving this goal, while high-precision spectroscopy will serve as a critical complement once the required velocity accuracy is reached.
