Performance of astrometric detection of a hotspot orbiting on the innermost stable circular orbit of the galactic centre black hole

TL;DR

This paper evaluates the astrometric capabilities of the GRAVITY instrument in imaging mode to detect and analyze a hotspot orbiting at the innermost stable circular orbit of the galactic center black hole, Sgr A*, providing insights into strong gravity and black hole spin.

Contribution

The study demonstrates that GRAVITY can measure the orbit of a hotspot near the ISCO of Sgr A*, enabling tests of general relativity and black hole spin estimation.

Findings

GRAVITY's astrometric precision is below the Schwarzschild radius of Sgr A*.

The instrument can confirm the orbital motion of a hotspot at the ISCO.

It can estimate the orbit size for sources as bright as magnitude 14 in K-band.

Abstract

The galactic central black hole Sgr A* exhibits outbursts of radiation in the near infrared (so-called IR flares). One model of these events consists in a hotspot orbiting on the innermost stable circular orbit (ISCO) of the hole. These outbursts can be used as a probe of the central gravitational potential. One main scientific goal of the second generation VLTI instrument GRAVITY is to observe these flares astrometrically. Here, the astrometric precision of GRAVITY is investigated in imaging mode, which consists in analysing the image computed from the interferometric data. The capability of the instrument to put in light the motion of a hotspot orbiting on the ISCO of our central black hole is then discussed. We find that GRAVITY's astrometric precision for a single star in imaging mode is smaller than the Schwarzschild radius of Sgr A*. The instrument can also demonstrate that a body orbiting on the last stable orbit of the black hole is indeed moving. It yields a typical size of the orbit, if the source is as bright as m_K=14. These results show that GRAVITY allows one to study the close environment of Sgr A*. Having access to the ISCO of the central massive black hole probably allows constraining general relativity in its strong regime. Moreover, if the hotspot model is appropriate, the black hole spin can be constrained.