The star S2 orbiting the compact radio source Sgr A* is a precision probe of the gravitational field around the closest massive black hole (candidate). Over the last 2.7 decades we have monitored the star’s radial velocity and motion on the sky, mainly with the SINFONI and NACO adaptive optics (AO) instruments on the ESO VLT, and since 2017, with the four-telescope interferometric beam combiner instrument GRAVITY. In this Letter we report the first detection of the General Relativity (GR) Schwarzschild Precession (SP) in S2’s orbit. Owing to its highly elliptical orbit (e = 0.88), S2’s SP is mainly a kink between the pre-and post-pericentre directions of motion ≈±1 year around pericentre passage, relative to the corresponding Kepler orbit. The superb 2017−2019 astrometry of GRAVITY defines the pericentre passage and outgoing direction. The incoming direction is anchored by 118 NACO-AO measurements of S2’s position in the infrared reference frame, with an additional 75 direct measurements of the S2-Sgr A* separation during bright states (“flares”) of Sgr A*. Our 14-parameter model fits for the distance, central mass, the position and motion of the reference frame of the AO astrometry relative to the mass, the six parameters of the orbit, as well as a dimensionless parameter fSP for the SP (fSP = 0 for Newton and 1 for GR). From data up to the end of 2019 we robustly detect the SP of S2, δϕ ≈ 12′ per orbital period. From posterior fitting and MCMC Bayesian analysis with different weighting schemes and bootstrapping we find fSP = 1.10 ± 0.19. The S2 data are fully consistent with GR. Any extended mass inside S2’s orbit cannot exceed ≈0.1% of the central mass. Any compact third mass inside the central arcsecond must be less than about 1000 M⊙.

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.

Context. There are a number of faint compact infrared excess sources in the central stellar cluster of the Milky Way. Their nature and origin is unclear. In addition to several isolated objects of this kind there is a small but dense cluster of comoving sources (IRS13N) located ∼3 west of SgrA* just 0.5 north of the bright IRS13E cluster of Wolf-Rayet and O-type stars. Based on the analysis of their color and brightness, there are two main possibilities: (1) they may be dust-embedded stars older than a few Myr; or (2) very young, dusty stars with ages younger than 1 Myr. Aims. We present a first K s -band identification and proper motions of the IRS13N members, the high-velocity dusty S-cluster object (DSO, also referred to as G2), and other infrared excess sources in the central field. Goal is to constrain the nature of these source. Methods. The L -(3.8 μm) K s -(2.2 μm) and H-band (1.65 μm) observations were carried out using the NACO adaptive optics system at the ESO VLT. Proper motions were obtained by linear fitting of the stellar positions extracted by StarFinder as a function of time, weighted by positional uncertainties, and by Gaussian fitting from high-pass filtered and deconvolved images. We also present results of near-infrared (NIR) H-and K s -band ESO-SINFONI integral field spectroscopy of the Galactic center cluster ISR13N. Results. We show that within the uncertainties, the positions and proper motions of the IRS13N sources in K s -and L -band are identical. The HK−sL colors then indicate that the bright L -band IRS13N sources are indeed dust-enshrouded stars rather than core-less dust clouds. The proper motions also show that the IRS13N sources are not strongly gravitationally bound to each other. Combined with their NIR colors, this implies that they have been formed recently. For the DSO we obtain proper motions and a K s -L -color. Conclusions. Most of the compact L -band excess emission sources have a compact H-or K s -band counterpart and therefore are likely stars with dust shells or disks. Our new results and orbital analysis from our previous work favor the hypothesis that the infrared excess IRS13N members and other dusty sources close to SgrA* are young dusty stars and that star formation at the Galactic center (GC) is a continuously ongoing process. For the DSO the color information indicates that it may be a dust cloud or a dust-embedded star.

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We present a 0.16% precise and 0.27% accurate determination of R0, the distance to the Galactic center. Our measurement uses the star S2 on its 16-year orbit around the massive black hole Sgr A* that we followed astrometrically and spectroscopically for 27 years. Since 2017, we added near-infrared interferometry with the VLTI beam combiner GRAVITY, yielding a direct measurement of the separation vector between S2 and Sgr A* with an accuracy as good as 20 μas in the best cases. S2 passed the pericenter of its highly eccentric orbit in May 2018, and we followed the passage with dense sampling throughout the year. Together with our spectroscopy, in the best cases with an error of 7 km s−1, this yields a geometric distance estimate of R0 = 8178 ± 13stat. ± 22sys. pc. This work updates our previous publication, in which we reported the first detection of the gravitational redshift in the S2 data. The redshift term is now detected with a significance level of 20σ with fredshift = 1.04 ± 0.05.

We report the detection of continuous positional and polarization changes of the compact source SgrA* in high states (“flares”) of its variable near-infrared emission with the near-infrared GRAVITY-Very Large Telescope Interferometer (VLTI) beam-combining instrument. In three prominent bright flares, the position centroids exhibit clockwise looped motion on the sky, on scales of typically 150 μas over a few tens of minutes, corresponding to about 30% the speed of light. At the same time, the flares exhibit continuous rotation of the polarization angle, with about the same 45(±15) min period as that of the centroid motions. Modelling with relativistic ray tracing shows that these findings are all consistent with a near face-on, circular orbit of a compact polarized “hot spot” of infrared synchrotron emission at approximately six to ten times the gravitational radius of a black hole of 4 million solar masses. This corresponds to the region just outside the innermost, stable, prograde circular orbit (ISCO) of a Schwarzschild–Kerr black hole, or near the retrograde ISCO of a highly spun-up Kerr hole. The polarization signature is consistent with orbital motion in a strong poloidal magnetic field.

GRAVITY is a new instrument to coherently combine the light of the European Southern Observatory Very Large Telescope Interferometer to form a telescope with an equivalent 130 m diameter angular resolution and a collecting area of 200 m 2 . The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phasetracking, dual-beam operation, and laser metrology. GRAVITY opens up to optical/infrared interferometry the techniques of phase referenced imaging and narrow angle astrometry, in many aspects following the concepts of radio interferometry. This article gives an overview of GRAVITY and reports on the performance and the first astronomical observations during commissioning in 2015/16. We demonstrate phase-tracking on stars as faint as m K ≈ 10 mag, phase-referenced interferometry of objects fainter than m K ≈ 15 mag with a limiting magnitude of m K ≈ 17 mag, minute long coherent integrations, a visibility accuracy of better than 0.25%, and spectro-differential phase and closure phase accuracy better than 0.5• , corresponding to a differential astrometric precision of better than ten microarcseconds (µas). The dual-beam astrometry, measuring the phase difference of two objects with laser metrology, is still under commissioning. First observations show residuals as low as 50 µas when following objects over several months. We illustrate the instrument performance with the observations of archetypical objects for the different instrument modes. Examples include the Galactic center supermassive black hole and its fast orbiting star S2 for phase referenced dual-beam observations and infrared wavefront sensing, the high mass X-ray binary BP Cru and the active galactic nucleus of PDS 456 for a few µas spectro-differential astrometry, the T Tauri star S CrA for a spectro-differential visibility analysis, ξ Tel and 24 Cap for high accuracy visibility observations, and η Car for interferometric imaging with GRAVITY.

Sagittarius A*, the super-massive black hole at the center of the Milky Way, is surrounded by a small cluster of high velocity stars, known as the S-stars. We aim to constrain the amount and nature of stellar and dark mass associated with the cluster in the immediate vicinity of Sagittarius A*. We use near-infrared imaging to determine the K s -band luminosity function of the S-star cluster members, and the distribution of the diffuse background emission and the stellar number density counts around the central black hole. This allows us to determine the stellar light and mass contribution expected from the faint members of the cluster. We then use post-Newtonian N-body techniques to investigate the effect of stellar perturbations on the motion of S2, as a means of detecting the number and masses of the perturbers. We find that the stellar mass derived from the K s -band luminosity extrapolation is much smaller than the amount of mass that might be present considering the uncertainties in the orbital motion of the star S2. Also the amount of light from the fainter S-cluster members is below the amount of residual light at the position of the S-star cluster after removing the bright cluster members. If the distribution of stars and stellar remnants is strongly enough peaked near Sagittarius A*, observed changes in the orbital elements of S2 can be used to constrain both their masses and numbers. Based on simulations of the cluster of high velocity stars we find that at a wavelength of 2.2 μm close to the confusion level for 8 m class telescopes blend stars will occur (preferentially near the position of Sagittarius A*) that last for typically 3 years before they dissolve due to proper motions.