Comparing timelike geodesics around a Kerr black hole and a boson star

TL;DR

This paper compares the trajectories of stars orbiting a Kerr black hole and a boson star, highlighting potential observational differences that could help determine the true nature of the Galactic center's compact object.

Contribution

It provides a detailed analysis of stellar orbits around boson stars versus Kerr black holes, emphasizing the potential for observational tests with current instruments.

Findings

Significant orbital differences for stars with close pericenters (<30 M)

Closer stars than S2 could distinguish between models

Potential to constrain the nature of the Galactic center object

Abstract

The second-generation beam combiner at the Very Large Telescope (VLT), GRAVITY, observes the stars orbiting the compact object located at the center of our galaxy, with an unprecedented astrometric accuracy of 10 $\mu$as. The nature of this compact source is still unknown since black holes are not the only candidates explaining the four million solar masses at the Galactic center. Boson stars are such an alternative model to black holes. This paper focuses on the study of trajectories of stars orbiting a boson star and a Kerr black hole. We put in light strong differences between orbits obtained in both metrics when considering stars with sufficiently close pericenters to the compact object, typically $\lesssim 30~M$. Discovery of closer stars to the Galactic center than the S2 star by the GRAVITY instrument would thus be a powerful tool to possibly constrain the nature of the central source.