## Mid-Term Report Summary - PROGRESO (Probing General Relativity with Stellar Orbits)

This grant proposal, PROGRESO, aims at detecting relativistic effects in the stellar orbits around the massive black hole in the Galactic Center, and in the motions of hot spots close to the event horizon. The main data source will be the next-generation instrument GRAVITY for ESO's Very Large Telescope Interferometer. The data will be used in conjunction with a long-term data set collected at ESO telescope since 1992.

Scientifically, this will test gravity in a so far unprobed regime of mass and space-time curvature. The Galactic Center is a uniquely accessible laboratory for gravitational physics, being the location of the closest massive black hole.

Given the mass ratio of 10^−5 between the stars and the black hole, the stars are ideal test particles for the potential in which they move. Thus the keys to directly probing the gravitational potential are ultra-precise astrometric measurements of stellar positions and accurately determined radial velocities of the stars.

All data to date can be described by purely Keplerian orbits around a single central mass. However, relativistic effects occur and lead to post-Newtonian orbits.

Detection of these effects will soon be possible with the combined data set from GRAVITY, and previous spectroscopic and astrometric observations.

So far, our team has achieved the following key improvements for the data analysis of the existing data set:

- We have refined the coordinate system definition by applying an advanced distortion model to our astrometric data. This allows pinpointing the mass of the massive black hole and thus leads to more precisely measured orbits - a key when searching for deviations from Keplerian motion.

- We have improved the point spread function extraction, allowing to measure positions of fainter stars and at greater precision.

- Currently we are working on a paper draft, presenting an update on the overall stellar monitoring program, including the technical improvements. This paper draft will present refined values for the mass of the massive black in the Galactic Center, and for its distance.

- A major result is the design and implementation of a novel concept of the GRAVITY metrology system. The purpose of the metrology system is to measure the internal optical paths to nanometer level precision. This is the key to astrometry at the 10 microarcsecond level.

Scientifically, this will test gravity in a so far unprobed regime of mass and space-time curvature. The Galactic Center is a uniquely accessible laboratory for gravitational physics, being the location of the closest massive black hole.

Given the mass ratio of 10^−5 between the stars and the black hole, the stars are ideal test particles for the potential in which they move. Thus the keys to directly probing the gravitational potential are ultra-precise astrometric measurements of stellar positions and accurately determined radial velocities of the stars.

All data to date can be described by purely Keplerian orbits around a single central mass. However, relativistic effects occur and lead to post-Newtonian orbits.

Detection of these effects will soon be possible with the combined data set from GRAVITY, and previous spectroscopic and astrometric observations.

So far, our team has achieved the following key improvements for the data analysis of the existing data set:

- We have refined the coordinate system definition by applying an advanced distortion model to our astrometric data. This allows pinpointing the mass of the massive black hole and thus leads to more precisely measured orbits - a key when searching for deviations from Keplerian motion.

- We have improved the point spread function extraction, allowing to measure positions of fainter stars and at greater precision.

- Currently we are working on a paper draft, presenting an update on the overall stellar monitoring program, including the technical improvements. This paper draft will present refined values for the mass of the massive black in the Galactic Center, and for its distance.

- A major result is the design and implementation of a novel concept of the GRAVITY metrology system. The purpose of the metrology system is to measure the internal optical paths to nanometer level precision. This is the key to astrometry at the 10 microarcsecond level.