How to tell an accreting boson star from a black hole

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.

• The paper demonstrates that GRMHD simulations reveal distinct image structures of boson stars, such as mini tori and altered luminosity profiles, compared to black holes.
• The paper finds that boson stars exhibit minimal magnetization and lack jet formation, contrasting sharply with the behavior of classical black holes.
• The paper identifies quasi-periodic oscillations in mass accretion rates for boson stars, offering a clear observational marker to differentiate them from black holes.

Distinguishing Accreting Boson Stars from Black Holes

The paper "How to Tell an Accreting Boson Star from a Black Hole" by H. Olivares et al. explores the complexities associated with differentiating between black holes and boson stars using observational data. This research represents a detailed inquiry into the practical capabilities of the Event Horizon Telescope (EHT) in distinguishing between these two types of astrophysical objects at 230 GHz. The investigation utilizes general relativistic magnetohydrodynamic (GRMHD) simulations to analyze accretion scenarios that mimic the environment around Sagittarius A* (Sgr A*), which is recognized as the supermassive compact object at the center of our galaxy.

Research Context and Methodology

In astronomical studies, the identification and characterization of black holes are pivotal. These objects are typically specified by their mass, angular momentum, and charge. However, the existence of black hole mimickers, such as boson stars, has been a topic of theoretical interest as alternative explanations for phenomena typically attributed to black holes. Boson stars, unlike classical black holes, do not feature an event horizon or surface. The primary challenge is differentiating boson stars from black holes observationally, considering they can both satisfy the compactness criteria essential for explaining certain gravitational observations.

The authors utilize existing data and theoretical models to study the appearance of non-rotating boson stars through 3D GRMHD simulations combined with general relativistic radiative transfer, with the goal of producing synthetic images resembling those captured by the EHT. By comparing these images, particularly looking at Schwarzschild and Kerr black holes alongside boson stars, stark observational distinctions are identified.

Key Findings

1. Image Structure and Dynamics: Accretion onto boson stars results in specific dynamic processes due to the absence of an event horizon. This lacks the gravitational capture cross-section typical of black holes, leading to the formation of characteristic mini tori or clouds of matter. These structures could potentially modify the observed luminosity profiles and source sizes compared to black holes, which often exhibit crescent-shaped images dominated by Doppler boosting effects.
2. Magnetohydrodynamic Effects: The absence of significant magnetization in the polar regions of boson stars results in no substantial jet production, a feature often associated with rotating black holes. This characteristic is observable in the synthetic images, which are particularly distinct from those of black holes.
3. Variability and Accretion Rates: The mass accretion rates for boson stars show noticeable quasi-periodic oscillations linked to the epicyclic frequencies of the plasma dynamics within the star. These variations, especially for boson stars with stalled accretion structures, provide a distinct signal useful for distinguishing boson stars from black holes.

Implications and Future Work

The implications of these findings are profound for astrophysics and cosmology. Distinguishing between black holes and their mimickers allows for more accurate models of galaxy cores and compact object formation. Additionally, these results could impact our understanding of scalar field theories, with potential repercussions in both general relativity and alternative gravity models.

Future observations by the EHT or similar VLBI arrays could utilize time-resolved imaging to explore these variability signals further. Moreover, extending this research to incorporate rotating boson star models or considering different scalar field potentials might reveal new dynamical features that could enhance our discriminating power between these compact objects.

The study contributes significantly to gravitational astrophysics by advancing methodologies that leverage state-of-the-art simulations and observations. This work not only enhances our comprehension of accretion physics surrounding supermassive black holes but also provides a foundation for future theoretical explorations into the nature of compact objects within the strong-field gravity regime.