Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole

TL;DR

This study detects the gravitational redshift and relativistic effects in the orbit of star S2 near the Galactic centre's black hole, providing strong evidence for General Relativity's predictions in a strong gravitational field.

Contribution

The paper presents the first robust detection of gravitational redshift and relativistic effects in the orbit of S2, confirming predictions of General Relativity in the Galactic centre.

Findings

Detection of gravitational redshift and transverse Doppler effect in S2's orbit

Quantitative measurement of relativistic effects with f ≈ 0.90

Data inconsistent with pure Newtonian dynamics

Abstract

The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A* is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU, ~1400 Schwarzschild radii, the star has an orbital speed of ~7650 km/s, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z ~ 200 km/s / c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f, with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 +/- 0.09 (stat) +\- 0.15 (sys). The S2 data are inconsistent with pure Newtonian dynamics.