Abstract
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 on to 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.
Highlights
Observations of the Galactic Centre have confirmed the existence of a supermassive compact object at the radio source Sgr A∗
The M87 observations are consistent with the expectations for a Kerr black hole (Event Horizon Telescope Collaboration 2019a, e, f), namely, a ‘crescent’ or ring-like feature, consisting of a dark region obscuring the lensed image of a bright accretion flow (Cunningham & Bardeen 1973; Falcke, Melia & Agol 2000; Grenzebach 2016). The shape of this dark region can be exploited either to determine the properties of the black hole within the Kerr assumption (Event Horizon Telescope Collaboration 2019e, f), or to perform tests of general relativity (Abdujabbarov, Rezzolla & Ahmedov 2015; Psaltis et al 2015b; Psaltis, Wex & Kramer 2016; Younsi et al 2016), a possibility assessed for Sgr A∗ by Mizuno et al (2018) in a realistic scenario for the 2017 EHTC campaign and for near-future observations
We have carried out the first 3D general relativistic ideal magnetohydrodynamic (GRMHD) simulations of disc accretion on to boson stars and combined them with general relativistic radiative transfer calculations, with the goal of determining whether, under realistic observing conditions such as those of the EHTC, an accreting non-rotating boson star can be distinguished from a black hole of the same mass
Summary
Observations of the Galactic Centre have confirmed the existence of a supermassive compact object at the radio source Sgr A∗. Motivated by the forthcoming observations of the EHTC, Vincent et al (2016) reached similar conclusions by comparing strong-field images of stationary tori in equilibrium around a Kerr black hole and several boson stars They found that a central dark region that mimics the shape and size of a black hole shadow may appear for boson stars as a result of lensing of the empty space around which the torus orbits. We revisit the question of the observational appearance at 230 GHz of a boson star at the Galactic Centre, and of its distinguishability from an SMBH To this end, we produce strong-field synthetic EHTC images of accreting black holes and of an accreting boson star, modelling the accretion flow by means of fully dynamic general relativistic ideal magnetohydrodynamic (GRMHD) simulations. On the basis of these considerations it is possible to state quite generically that horizonless and surfaceless objects can form dark regions that are qualitatively similar to the shadow of a black hole, these will be smaller than that expected size of the shadow of a black hole of the same mass under very general circumstances
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