Future prospects for measuring 1PPN parameters using observations of S2 and S62 at the Galactic Center

Abstract

The Parameterized Post-Newtonian (PPN) formalism offers an agnostic framework for evaluating theories of gravity that extend beyond General Relativity. Departures from General Relativity are represented by a set of dimensionless parameters that, at the first order in the expansion, reduce to $β$ and $γ$, which describe deviations in spatial curvature and non-linear superposition effects of gravity, respectively. We exploit future observations of stars at the Galactic Center, orbiting the supermassive black hole Sagittarius A*, to forecast the ability to constrain the first-order PPN parameters $γ$ and $β$. We have generated a mock catalog of astrometric and spectroscopic data for S2, based on the Schwarzschild metric, simulating observations over multiple orbital periods with the GRAVITY and SINFONI instruments. Our analysis includes the effects of relativistic orbital precession and line-of-sight (LOS) velocity gravitational redshift. Since future data for S2 can provide constraints only on a linear combination of the PPN parameters $β$ and $γ$ we also analyzed the impact of future observations of the gravitational lensing for stars that pass closer in the sky to Sgr A*, like the known star S62, which can potentially provide tight constraints on the parameter $γ$, that alone regulates the amplitude of the astrometric deviations due to lensing. When combining lensing observations for S62, and the precise orbital tracking of S2, one obtains independent constraints on both $γ$ (with a potential precision as good as $\sim 1\%$) and $β$ (with a corresponding precision of $\sim 2\%$), providing a precision test of General Relativity and its extensions.