Letter to the Editor

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

¹ Max Planck Institute for extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany
² LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ 1 st Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
⁵ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
⁷ Faculdade de Engenharia, Universidade do Porto, rua Dr. Roberto Frias, 4200-465 Porto, Portugal
⁸ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁹ European Southern Observatory, Casilla, 19001 Santiago 19, Chile
¹⁰ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
¹¹ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
¹² Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
¹³ Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA, 94720 USA
¹⁴ CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
¹⁵ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
¹⁶ European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 26 June 2018
Accepted: 29 June 2018

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⁻¹, 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 = Δλ / λ ≈ 200 km s⁻¹/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.