Context. It has been proposed that Very Long Baseline Interferometry (VLBI) at sub-millimeter waves will allow us to image the shadow of the black hole in the center of our Milky Way, Sagittarius A* (Sgr A*), and thereby test basic predictions of the theory of general relativity. Aims. This paper presents imaging simulations of a new Space VLBI (SVLBI) mission concept. An initial design study of the concept has been presented in the form of the Event Horizon Imager (EHI). The EHI may be suitable for imaging Sgr A* at high frequencies (up to ∼ 690 GHz), which has significant advantages over performing ground-based VLBI at 230 GHz. The concept EHI design consists of two or three satellites in polar or equatorial circular Medium-Earth Orbits (MEOs) with slightly different radii. Due to the relative drift of the satellites along the individual orbits over the course of several weeks, this setup will result in a dense spiral-shaped uv-coverage with long baselines (up to ∼ 60 Gλ), allowing for extremely high-resolution and high-fidelity imaging of radio sources. Methods. We simulate observations of general relativistic magnetohydrodynamics (GRMHD) models of Sgr A* for the proposed configuration and calculate the expected noise based on preliminary system parameters. On long baselines, where the signal-to-noise ratio may be low, fringes could be detected if the system is sufficiently phase stable and the satellite orbits can be reconstructed with sufficient accuracy. Averaging visibilities accumulated over multiple epochs of observations could then help improving the image quality. With three satellites instead of two, closure phases could be used for imaging. Results. Our simulations show that the EHI could be capable of imaging the black hole shadow of Sgr A* with a resolution of 4 µas (about 8 % of the shadow diameter) within several months of observing time. Conclusions. Our preliminary study of the EHI concept shows that it is potentially of high scientific value, as it could be used to measure black hole shadows much more precisely than with ground-based VLBI, allowing for stronger tests of General Relativity and accretion models.

High-resolution imaging of supermassive black hole shadows is a direct way to verify the theory of general relativity under extreme gravity conditions. Very Long Baseline Interferometry (VLBI) observations at millimetre/submillimetre wavelengths can provide such angular resolution for the supermassive black holes located in Sgr A* and M87. Recent VLBI observations of M87 with the Event Horizon Telescope (EHT) have shown such capabilities. The maximum obtainable spatial resolution of the EHT is limited by the Earth's diameter and atmospheric phase variations. In order to improve the image resolution, longer baselines are required. The Radioastron space mission successfully demonstrated the capabilities of space–Earth VLBI with baselines much longer than the Earth's diameter. Millimetron is the next space mission of the Russian Space Agency and will operate at millimetre wavelengths. The nominal orbit of the observatory will be located around the Lagrangian L2 point of the Sun–Earth system. In order to optimize the VLBI mode, we consider a possible second stage of the mission that could use a near-Earth high elliptical orbit (HEO). In this paper, a set of near-Earth orbits is used for synthetic space–Earth VLBI observations of Sgr A* and M87 in a joint Millimetron and EHT configuration. General relativistic magnetohydrodynamic models for the supermassive black hole environments of Sgr A* and M87 are used for static and dynamic imaging simulations at 230 GHz. A comparison preformed between ground and space–Earth baselines demonstrates that joint observations with Millimetron and EHT significantly improve the image resolution and allow the EHT + Millimetron to obtain snapshot images of Sgr A*, probing the dynamics at fast time-scales.

Submillimeter interferometry has the potential to image supermassive black holes on event horizon scales, providing tests of the theory of general relativity and increasing our understanding of black hole accretion processes. The Event Horizon Telescope (EHT) performs these observations from the ground, and its main imaging targets are Sagittarius A* in the Galactic Center and the black hole at the center of the M87 galaxy. However, the EHT is fundamentally limited in its performance by atmospheric effects and sparse terrestrial (u, v)-coverage (Fourier sampling of the image). The scientific interest in quantitative studies of the horizon size and shape of these black holes has motivated studies into using space interferometry which is free of these limitations. Angular resolution considerations and interstellar scattering effects push the desired observing frequency to bands above 500 GHz.This paper presents the requirements for meeting these science goals, describes the concept of interferometry from Polar or Equatorial Medium Earth Orbits (PECMEO) which we dub the Event Horizon Imager (EHI), and utilizes suitable space technology heritage. In this concept, two or three satellites orbit at slightly different orbital radii, resulting in a dense and uniform spiral-shaped (u, v)-coverage over time. The local oscillator signals are shared via an inter-satellite link, and the data streams are correlated on-board before final processing on the ground. Inter-satellite metrology and satellite positioning are extensively employed to facilitate the knowledge of the instrument position vector, and its time derivative. The European space heritage usable for both the front ends and the antenna technology of such an instrument is investigated. Current and future sensors for the required inter-satellite metrology are listed. Intended performance estimates and * The research work reported in the paper was partly supported by the Project NPI-552 "Space-to-space Interferometer System to Image the Event Horizon of the Super Massive Black Hole in the Center of Our Galaxy" co-funded by the European Space Agency (ESA) and the Radboud University of Nijmegen (ESA contract 4000122812), and by the NWO project PIPP "Breakthrough Technologies for Interferometry in Space".

Black hole imaging challenges the third-generation space VLBI, the Very Long Baseline Interferometry, to operate on a 500[Formula: see text]GHz band. The coherent integration time needed here is 450[Formula: see text]s though the available space oscillators cannot offer more than 10[Formula: see text]s. Self-calibration methods might solve this issue in an interferometer formed by three antenna/satellite systems, but the need for the third satellite increases the mission costs. A frequency transfer is of special interest to alleviate both performance and cost issues. A concept of two-way optical frequency transfer is examined to investigate its suitability to enable space-to-space interferometry, in particular, to image the “shadows” of black holes from space. The concept, promising on paper, has been demonstrated by tests. The laboratory test set-up is presented and the verification of the temporal stability using standard analysis tool as TimePod has been passed. The resulting Allan Deviation is dominated by the 1/[Formula: see text] phase noise trend since the frequency transfer timescale of interest is shorter than 0.2[Formula: see text]s. This trend continues into longer integration times, as proven by the longest tests spanning over a few hours. The Allan Deviation between derived 103.2[Formula: see text]GHz oscillators is [Formula: see text]/[Formula: see text] within 10[Formula: see text][Formula: see text][Formula: see text]s that degrades twice towards the longest delay of 0.2[Formula: see text]s. The worst case satisfies the requirement with a margin of 11 times. The obtained coherence in the range of 0.997[Formula: see text]0.9998 is beneficial for space VLBI at 557[Formula: see text]GHz. The result is of special interest to future science missions for black hole imaging from space.

Very Long Baseline Interferometry (VLBI) at sub-millimeter waves has the potential to image the shadow of the black hole in the Galactic Center, Sagittarius A* (Sgr A*), and thereby test basic predictions of the theory of general relativity. We investigate the imaging prospects of a new Space VLBI mission concept. The setup consists of two satellites in polar or equatorial circular Medium-Earth Orbits with slightly different radii, resulting in a dense spiral-shaped uv-coverage with long baselines, allowing for extremely high-resolution and high-fidelity imaging of radio sources. We simulate observations of a general relativistic magnetohydrodynamics model of Sgr A* for this configuration with noise calculated from model system parameters. After gridding the uv-plane and averaging visibilities accumulated over multiple months of integration, images of Sgr A* with a resolution of up to 4 μ as could be reconstructed, allowing for stronger tests of general relativity and accretion models than with ground-based VLBI.