Prospects for testing the nature of Sgr A*'s NIR flares on the basis of current VLT- and future VLTI-observations

TL;DR

This paper explores how current and future infrared interferometry can test the nature of Sgr A*'s near-infrared flares by modeling hot spots near the black hole and predicting observable relativistic effects.

Contribution

It develops a hot spot model combined with general relativistic ray tracing to predict astrometric signatures of flares for upcoming interferometric observations.

Findings

Astrometric measurements can reveal relativistic effects near Sgr A*.

Future VLTI observations may detect hot spot proper motions.

Relativistic effects are marginal in light curves but prominent in centroid tracks.

Abstract

Sagittarius A*, the supermassive compact object at the center of the Galaxy, exhibits outbursts in the near infrared and X-ray domains. These flares are likely due to energetic events very close to the central object, on a scale of a few Schwarzschild radii. Optical interferometry will soon be able to provide astrometry with an accuracy of this order (~10 muas). In this article we use recent photometric near infrared data observed with the adaptive optics system NACO at the Very Large Telescope combined with simulations in order to deploy a method to test the nature of the flares and to predict the possible outcome of observations with the Very Large Telescope Interferometer. To accomplish this we implement a hot spot model and investigate its appearance for a remote observer in terms of light curves and centroid tracks, based on general relativistic ray tracing methods. First, we use a simplified model of a small steady source in order to investigate the relativistic effects qualitatively. A more realistic scenario is then being developed by fitting our model to existing flare data. While indications for the spin of the black hole and multiple images due to lensing effects are marginal in the light curves, astrometric measurements offer the possibility to reveal these high-order general relativistic effects. This study makes predictions on these astrometric measurements and leads us to the conclusion that future infrared interferometers will be able to detect proper motion of hot spots in the vicinity of Sagittarius A*.