The Galactic Center Reference Frame: A 30-Year Solution

Abstract

Stellar orbits probe the metric of and mass around the Sgr A* massive black hole. To enable study of the black hole, the surrounding mass, and tests of gravity, a precise reference frame must be established and maintained. We present an updated stellar maser-based reference frame for the inner parsec of the Galactic Center, referenced to the radio continuum of Sgr A*. This work synthesizes 30 years of astrometric observations of 15 SiO stellar masers and the Sgr A* continuum at 43 GHz. The uncertainty in the mean offset of the maser ensemble from Sgr A* is 0.08 mas in the current epoch and will decay to 0.10 mas by 2030. At a distance of 8.2 kpc, these correspond to position uncertainties of 0.6 AU and 0.8 AU, respectively, which is similar to or smaller than the stellar SiO-emitting region. The mean maser proper motion uncertainties are 0.05 mas yr${}^{-1}$ in RA and 0.08 mas yr${}^{-1}$ in Dec, corresponding to 1.8 km s${}^{-1}$ and 3.2 km s${}^{-1}$. The reference frame stability between IR and maser astrometry is 10~$\mu$as yr${}^{-1}$ (0.40 km s${}^{-1}$) in RA and 15 $\mu$as yr${}^{-1}$ (0.57 km s${}^{-1}$) in Dec. The masers show substantial variability in flux density and velocity structure, most notably IRS~7, which shows a 30-fold brightening in a blueshifted component since 2022. This variability may set a floor on future astrometry improvements and especially measurements of radial acceleration.

arXiv comment20 pages, 8 figures, 3 tables. Accepted by ApJ