Frank Eisenhauer

More than a century ago, Albert Einstein presented his general theory of gravitation (GR) to the Prussian Academy of Sciences. One of the predictions of the theory is that not only particles and objects with mass, but also the quanta of light, photons, are tied to the curvature of space-time, and thus to gravity. There must be a critical compactness, above which photons cannot escape. These are black holes (henceforth BH). It took fifty years after the theory was announced before possible candidate objects were identified by observational astronomy. And another fifty years have passed, until we finally have in hand detailed and credible experimental evidence that BHs of 10 to 10^10 times the mass of the Sun exist in the Universe. Three very different experimental techniques, but all based on Michelson interferometry or Fourier-inversion spatial interferometry have enabled the critical experimental breakthroughs. It has now become possible to investigate the space-time structure in the vicinity of the event horizons of BHs. We briefly summarize these interferometric techniques, and discuss the spectacular recent improvements achieved with all three techniques. Finally, we sketch where the path of exploration and inquiry may go on in the next decades.

After decades of fast-paced technical advances, optical/infrared (O/IR) interferometry has seen a revolution in recent years: ▪The GRAVITY instrument at the Very Large Telescope Interferometer (VLTI) with four 8-m telescopes reaches thousand-times-fainter objects than possible with earlier interferometers, and the Center for High Angular Resolution Astronomy array (CHARA) routinely offers up to 330-m baselines and aperture synthesis with six 1-m telescopes.▪The observed objects are fainter than 19 mag, the images have submilliarcsecond resolution, and the astrometry reaches microarcsecond precision.▪This led to breakthrough results on the Galactic Center, exoplanets, active galactic nuclei, young stellar objects, and stellar physics. Following a primer in interferometry, we summarize the advances that led to the performance boost of modern interferometers: ▪Single-mode beam combiners now combine up to six telescopes, and image reconstruction software has advanced over earlier developments for radio interferometry.▪With a combination of large telescopes, adaptive optics (AO), fringe tracking, and especially dual-beam interferometry, GRAVITY has boosted the sensitivity by many orders of magnitude. Another order-of-magnitude improvement will come from laser guide star AO. In combination with large separation fringe tracking, O/IR interferometry will then provide complete sky coverage for observations in the Galactic plane and substantial coverage for extragalactic targets.

This article describes the operation of the near-infrared wavefront sensing based Adaptive Optics (AO) system CIAO. The Coudé Infrared Adaptive Optics (CIAO) system is a central auxiliary component of the Very Large Telescope (VLT) interferometer (VLTI). It enables in particular the observations of the Galactic Center (GC) using the GRAVITY instrument. GRAVITY is a highly specialized beam combiner, a device that coherently combines the light of the four 8-m telescopes and finally records interferometric measurements in the K-band on 6 baselines simultaneously. CIAO compensates for phase disturbances caused by atmospheric turbulence, which all four 8 m Unit Telescopes (UT) experience during observation. Each of the four CIAO units generates an almost diffraction-limited image quality at its UT, which ensures that maximum flux of the observed stellar object enters the fibers of the GRAVITY beam combiner. We present CIAO performance data obtained in the first 3 years of operation as a function of weather conditions. We describe how CIAO is configured and used for observations with GRAVITY. In addition, we focus on the outstanding features of the near-infrared sensitive Saphira detector, which is used for the first time on Paranal, and show how it works as a wavefront sensor detector.

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 ofz= Δλ/λ≈ 200 km s⁻¹/cwith different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factorf, withf= 0 andf= 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemesf= 0.90 ± 0.09|_stat± 0.15|_sys. The S2 data are inconsistent with pure Newtonian dynamics.

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 m². The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phase-tracking, 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 asm_K≈ 10 mag, phase-referenced interferometry of objects fainter thanm_K≈ 15 mag with a limiting magnitude ofm_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 50μas 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 fewμas 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, andηCar for interferometric imaging with GRAVITY.

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, phase-tracking, dual beam operation and laser metrology [...]. 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{\deg}, corresponding to a differential astrometric precision of better than 10 microarcseconds ({\mu}as). 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 {\mu}as 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 few {\mu}as spectro-differential astrometry, the T Tauri star S CrA for a spectro-differential visibility analysis, {\xi} Tel and 24 Cap for high accuracy visibility observations, and {\eta} Car for interferometric imaging with GRAVITY.

The Galactic Center constitutes the best astrophysical evidence for the existence of black holes, and it is the ideal laboratory for studying physics in the vicinity of such objects. The combination of infrared observations of three dimensional orbits of stars within the central light days and the extreme compactness and motionlessness of the radio-counterpart of the gravitational center have shown beyond any reasonable doubt that the Galactic Center harbors a supermassive black hole. The flaring activity from the black hole gives first insights to the physical processes close to the last stable orbit. Here I review the current state of observations and theory of the Galactic Center black hole and give an update on the latest results. I also outline the next steps towards even higher angular resolution observations, which give promise to directly probe the physics and space-time curvature just outside the event horizon.

We present the adaptive optics assisted, near-infrared VLTI instrument GRAVITY for precision narrow-angle astrometry and interferometric phase referenced imaging of faint objects. With its two fibers per telescope beam, its internal wavefront sensors and fringe tracker, and a novel metrology concept, GRAVITY will not only push the sensitivity far beyond what is offered today, but will also advance the astrometric accuracy for UTs to 10 μas. GRAVITY is designed to work with four telescopes, thus providing phase referenced imaging and astrometry for 6 baselines simultaneously. Its unique capabilities and sensitivity will open a new window for the observation of a wide range of objects, and — amongst others — will allow the study of motion within a few times the event horizon size of the Galactic Center black hole.