# The Galactic Center: Improved Relative Astrometry for Velocities, Accelerations, and Orbits near the Supermassive Black Hole

## TL;DR

This paper presents enhanced astrometric measurements of stars near the Galactic Center, improving precision and detecting more accelerations, which aids in understanding the black hole's properties and testing General Relativity.

## Contribution

The study introduces new methods for astrometric correction and uncertainty estimation, significantly increasing the number of detected accelerating stars and reducing systematic errors.

## Key findings

- Detected 24 accelerating stars, 2.6 times more than previous studies.
- Reduced systematic error of SMBH measurements by a factor of 2.
- Identified a potential astrometric binary candidate S0-27.

## Abstract

We present improved relative astrometry for stars within the central half parsec of our Galactic Center based on data obtained with the 10 m W. M. Keck Observatory from 1995 to 2017. The new methods used to improve the astrometric precision and accuracy include correcting for local astrometric distortions, applying a magnitude dependent additive error, and more carefully removing instances of stellar confusion. Additionally, we adopt jackknife methods to calculate velocity and acceleration uncertainties. The resulting median proper motion uncertainty is 0.05 mas/yr for our complete sample of 1184 stars in the central 10'' (0.4 pc). We have detected 24 accelerating sources, 2.6 times more than the number of previously published accelerating sources, which extend out to 4'' (0.16 pc) from the black hole. Based on S0-2's orbit, our new astrometric analysis has reduced the systematic error of the supermassive black hole (SMBH) by a factor of 2. The linear drift in our astrometric reference frame is also reduced in the North-South direction by a factor of 4. We also find the first potential astrometric binary candidate S0-27 in the Galactic center. These astrometric improvements provide a foundation for future studies of the origin and dynamics of the young stars around the SMBH, the structure and dynamics of the old nuclear star cluster, the SMBH's properties derived from orbits, and tests of General Relativity (GR) in a strong gravitational field.