First Light for GRAVITY: Phase Referencing Optical Interferometry for the Very Large Telescope Interferometer (1705.02345v1)

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.

• The paper details GRAVITY's main contribution: implementing phase referencing to achieve astrometric precision down to 10 microarcseconds.
• It outlines advanced methodologies including dual-beam operation, integrated optics, and high-resolution spectroscopy for superior imaging.
• Observational results, such as precise orbital measurements near the Galactic Center, validate its capability to probe intricate astrophysical phenomena.

An Expert Overview of GRAVITY's First Light: Enhancing Optical Interferometry

The academic paper entitled "First Light for GRAVITY: Phase Referencing Optical Interferometry for the Very Large Telescope Interferometer" outlines the groundbreaking capabilities of the GRAVITY instrument, a state-of-the-art optical interferometer situated at the European Southern Observatory's Very Large Telescope Interferometer (VLTI). The GRAVITY instrument leverages phase referencing to achieve unprecedented precision in optical/infrared interferometry, offering transformative opportunities to investigate astronomical phenomena with extraordinary detail.

Instrument Design and Capabilities

GRAVITY is designed to synthesize the light from the VLTI's telescopes to achieve the angular resolution equivalent of a single telescope with a 130-meter diameter. The instrument employs several advanced technologies, including fiber-fed integrated optics for beam combination, high-resolution spectroscopy, near-infrared wavefront sensing, and dual-beam operation for phase referencing. Moreover, its laser metrology system enables it to achieve astrometric precision down to 10 microarcseconds (μas).

The GRAVITY instrument's fundamental advantages lie in its capability to perform phase-referenced imaging and narrow-angle astrometry, akin to radio interferometry methodologies. Theoretical advancements and technical innovations have enabled GRAVITY to facilitate observations of weak objects by improving sensitivity through coherent integration.

Significant Observational Results

The instrument's efficacy is demonstrated through multiple high-stakes observations:

• Galactic Center Observations: GRAVITY successfully observed the supermassive black hole at the center of the Milky Way, providing precise measurements of the star S2's orbit, which is pivotal for probing general relativistic effects and black hole characteristics.
• Other Astrophysical Applications: GRAVITY has been employed for high-accuracy visibility observations on resolved stars, providing insights into stellar surfaces with unprecedented precision. Additionally, GRAVITY's spectro-differential capabilities have revealed detailed structures in T Tauri stars and high-mass X-ray binaries (HMXBs), such as testing theories concerning the circumstellar environments and accretion processes in these systems.

Implications and Future Directions

The observations of the Galactic Center underscore GRAVITY's revolutionary potential for testing fundamental physics by enabling astrometric measurements of orbital dynamics around black holes. The ability to interrogate faint astrophysical halls such as quasars and explore stellar environments will enhance our understanding of galaxy formation, stellar evolution, and accretion physics.

The dual-beam astrometry capability opens new horizons for precisely measuring the position of astronomical objects at an exceptional level of detail, paving the way for detecting minute companions within complex stellar systems. The instrument sets a new benchmark in long-baseline interferometry, enabling deeper insights into highly dynamic and faint astronomical phenomena.

Conclusion

GRAVITY demonstrates an exceptional leap forward in optical interferometry, marrying theoretical breakthroughs with technological prowess to offer unprecedented observational capabilities. The implications of GRAVITY's expansive range of potential applications are broad, promising to significantly enhance our empirical grasp of diverse astrophysical processes in the near universe. The integration of astrometry, high-resolution spectroscopy, and adaptive optics within a unified platform reflects a holistic approach to tackling some of the most pressing questions in modern astrophysics.