Toward very large baseline interferometry observations of black hole structure

Abstract

Black holes hold a tremendous discovery potential. In this paper the extent to which the Event Horizon Telescope and its next generation upgrade can resolve their structure is quantified. Black holes are characterized by a perfectly absorptive boundary, with a specific area determined by intrinsic parameters of the black hole. We use a general parametrization of spherically symmetric spacetimes describing deviations from this behavior, with parameters controlling the size of the central object and its interaction with light, in particular through a specular reflection coefficient Γ and an intrinsic luminosity measured as a fraction η of that of the accretion disc. This enables us to study exotic compact objects and compare them with black holes in a model-independent manner. We determine the image features associated with the existence of a surface in the presence of a geometrically thin and optically thick accretion disc, identifying requirements for very large baseline interferometry observations to be able to cast meaningful constraints on these parameters, in particular regarding angular resolution and image dynamic range. For face-on observations, constraints of order η ≲ 10 −4 , Γ ≲ 10 −1 are possible with an enhanced Event Horizon Telescope array, imposing strong constraints on the nature of the central object.

Shadows and photon rings of regular black holes and geonic horizonless compact objects

“…If such singularities are to solved by an upgraded description of the gravitational field at certain scales, metric-affine gravities could potentially live from that job. This work can thus be seen as the first one towards the understanding of what modifications can be expected in shadow and photon ring images of singularity-free geometries of metric-affine type, which can be potentially searched for using very long-baseline interferometry [112]. In implementing this programme we have already faced the widespread and well known problem that the role played by the background geometry and the geometrical, optical, and emission properties of the accretion disk are highly entangled in determining the properties of such images.…”

Scale-invariance at the core of quantum black holes

“…The resulting RG-improved spacetime would thus mimick classical black holes in observations which do not explicitly probe the horizon as a perfect absorber. On the one hand, such quantum modifications might lead to observational signatures, such as gravitational wave echos [18][19][20][21][22] or to excess emission in the central brightness depression of a black-hole shadow [23][24][25]. On the other hand, a common view point in QG suggests that quantum-gravitational effects should be negligible at the scale of the horizon of macroscopic black holes.…”

“…Nevertheless, there is room in current experimental data for exotic compact objects as mimickers of black holes, see for instance gravastars [6,7], various versions of wormholes [8,9], fuzzballs and firewalls [10][11][12], or, more recently, 2:2 holes [13][14][15][16][17]. Potential observable signatures of horizon-like structures with a non-zero reflectivity include gravitational wave echoes associated with time-delayed signals following the main merger ringdown [18][19][20][21][22] and excess emission in the central brightness depression of a black-hole shadow [23][24][25].…”

Key Science Goals for the Next-Generation Event Horizon Telescope

“…To leading order, the image dynamic range of the ngEHT will determine the luminosity of the features that can be studied, while the angular resolution will determine the size of the features that can be studied. Hence, quantitative statements about the existence of horizons will be primarily influenced by the dynamic range that can be achieved, while quantitative properties of the spacetime will be determined by the angular resolution [70,71]. Figure 6 shows an example of the improvement in both quantities that is possible using the ngEHT, enabling new studies of image signatures of the horizon.…”