GRAVITY: getting to the event horizon of Sgr A*

TL;DR

GRAVITY is a planned advanced VLTI instrument designed to observe relativistic phenomena near Sgr A*'s event horizon, enabling high-precision measurements of matter dynamics in this extreme environment.

Contribution

It introduces a novel instrument concept with integrated optics, adaptive optics, fringe tracking, and metrology to achieve microarcsecond astrometry near Sgr A*.

Findings

Simulations confirm feasibility of 10 microarcsecond astrometry for K=15 sources.

Key design features enable observation of relativistic motions near black hole.

Instrument concept meets scientific requirements for studying Sgr A*.

Abstract

We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase. GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches the scientific needs; in particular that 10 microarcsecond astrometry is feasible for a source with a magnitude of K=15 like Sgr A*, given the availability of suitable phase reference sources.