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

TLDR

GRAVITY is a new instrument that coherently combines the light of the ESO Very Large Telescope Interferometer to create a telescope with an equivalent 130 m diameter angular resolution and a collecting area of 200 m². This article provides an overview of GRAVITY and reports on its performance and the first astronomical observations during commissioning in 2015/16. GRAVITY employs fiber‑fed integrated‑optics beam combination, high‑resolution spectroscopy, near‑infrared wavefront sensing, phase‑tracking, dual‑beam operation, and laser metrology, with dual‑beam astrometry still under commissioning, and its performance is illustrated by observations of the Galactic Center, BP Cru, PDS 456, S CrA, ξ Tel, 24 Cap, and η Car. The instrument achieves phase tracking on stars as faint as K≈10 mag, phase‑referenced interferometry down to K≈15 mag with a limiting magnitude of K≈17 mag, minute‑long coherent integrations, visibility accuracy <0.25 %, spectro‑differential phase and closure phase accuracy <0.5°, yielding differential astrometric precision <10 μas, and first observations show residuals as low as 50 μas over several months.

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, 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.