First light for GRAVITY: Phase referencing optical interferometry for the Very Large Telescope Interferometer

Abstract

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.

Weak deflection gravitational lensing for photons coupled to Weyl tensor in a Schwarzschild black hole

“…When the weak deflection lensing is considered that b m • , P tot,min even for a source with d * LS ∼ 10 −3 (and resulting ε ∼ 7.16 × 10 −3 ) is still larger than the current resolution as low as 50 microarcsecond (μas) achieved by phase referencing optical/infrared interferometry [60]. It means that P tot , ΔP, F tot , ΔF and Δτ can be obtained according to the positions, flux and timing of two separated images.…”

Weak and strong deflection gravitational lensings by a charged Horndeski black hole

“…It will be theoretically and practically interesting to know their observational signatures that might be helpful to distinguish them. In this work, we focus on their gravitational lensings, especially by a supermassive black hole, because ground-based infrared and radio interferometry [141,142] have continuously improved capabilities to probe lensing signals of Sgr A* and M87 and physical processes nearby them. Detection of gravitational waves from supermassive black hole binaries may have to wait for next generation space-borne detectors.…”

GRAVITY Spectro-interferometric Study of the Massive Multiple Stellar System HD 93206 A

“…With this configuration, GRAVITY equally splits the incoming light of the science target between the fringe tracker and the science beam combiners to produce interference fringes simultaneously in each of them. While the science beam combiner disperses the light at the desired spectral resolution, the fringe tracker beam combiner always works with a lowspectral resolution of R∼22 (Gillessen et al 2010) but at a high-frequency sampling (∼1 kHz). This allows for the atmospheric piston to be corrected, stabilizing the fringes of the science beam combiner for up to several tens of seconds.…”