How to tell an accreting boson star from a black hole

TL;DR

This paper demonstrates that the Event Horizon Telescope can distinguish boson stars from black holes by their different observational signatures at 230 GHz, due to the absence of an event horizon and matter accumulation effects.

Contribution

The study provides the first detailed simulated images of non-rotating boson stars at horizon scales, showing observable differences from black holes using EHT data.

Findings

Boson stars exhibit matter accumulation in their interior unlike black holes.

Differences in images are due to the absence of an event horizon and related dynamical effects.

EHT can potentially identify horizonless objects through these observational signatures.

Abstract

The capability of the Event Horizon Telescope (EHT) to image the nearest supermassive black hole candidates at horizon-scale resolutions offers a novel means to study gravity in its strongest regimes and to test different models for these objects. Here, we study the observational appearance at 230 GHz of a surfaceless black hole mimicker, namely a non-rotating boson star, in a scenario consistent with the properties of the accretion flow onto Sgr A*. To this end, we perform general relativistic magnetohydrodynamic simulations followed by general relativistic radiative transfer calculations in the boson star space-time. Synthetic reconstructed images considering realistic astronomical observing conditions show that, despite qualitative similarities, the differences in the appearance of a black hole -- either rotating or not -- and a boson star of the type considered here are large enough to be detectable. These differences arise from dynamical effects directly related to the absence of an event horizon, in particular, the accumulation of matter in the form of a small torus or a spheroidal cloud in the interior of the boson star, and the absence of an evacuated high-magnetization funnel in the polar regions. The mechanism behind these effects is general enough to apply to other horizonless and surfaceless black hole mimickers, strengthening confidence in the ability of the EHT to identify such objects via radio observations.