Imaging a boson star at the Galactic center

TL;DR

This paper explores whether images of a boson star at the Galactic center could mimic black hole signatures, demonstrating that relativistic boson stars can produce black hole-like shadows, complicating observational distinctions.

Contribution

It provides the first detailed imaging predictions of boson stars at the Galactic center, showing they can produce black hole-like shadows in VLBI observations.

Findings

Relativistic boson stars can produce shadow-like features similar to black holes.

Images of boson stars are nearly indistinguishable from Kerr black holes in strong-field regimes.

Unambiguous detection of event horizons remains challenging with current imaging techniques.

Abstract

Millimeter very long baseline interferometry will soon produce accurate images of the closest surroundings of the supermassive compact object at the center of the Galaxy, Sgr A*. These images may reveal the existence of a central faint region, the so-called shadow, which is often interpreted as the observable consequence of the event horizon of a black hole. In this paper, we compute images of an accretion torus around Sgr A* assuming this compact object is a boson star, i.e. an alternative to black holes within general relativity, with no event horizon and no hard surface. We show that very relativistic rotating boson stars produce images extremely similar to Kerr black holes, showing in particular shadow-like and photon-ring-like structures. This result highlights the extreme difficulty of unambiguously telling the existence of an event horizon from strong-field images.