Metrics and Motivations for Earth-Space VLBI: Time-Resolving Sgr A* with the Event Horizon Telescope

Abstract

Very-long-baseline interferometry (VLBI) at frequencies above 230 GHz with Earth-diameter baselines gives spatial resolution finer than the $\sim 50\mu$as "shadow" of the supermassive black hole at the Galactic Center, Sagittarius A* (Sgr A*). Imaging static and dynamical structure near the "shadow" provides a test of general relativity and may allow measurement of black hole parameters. However, traditional Earth-rotation synthesis is inapplicable for sources (such as Sgr A*) with intra-day variability. Expansions of ground-based arrays to include space-VLBI stations may enable imaging capability on time scales comparable to the prograde innermost stable circular orbit (ISCO) of Sgr A*, which is predicted to be 4-30 minutes, depending on black hole spin. We examine the basic requirements for space-VLBI, and we develop tools for simulating observations with orbiting stations. We also develop a metric to quantify the imaging capabilities of an array irrespective of detailed image morphology or reconstruction method. We validate this metric on example reconstructions of simulations of Sgr A* at 230 and 345 GHz, and use these results to motivate expanding the Event Horizon Telescope (EHT) to include small dishes in Low Earth Orbit (LEO). We demonstrate that high-sensitivity sites such as the Atacama Large Millimeter/Submillimeter Array (ALMA) make it viable to add small orbiters to existing ground arrays, as space-ALMA baselines would have sensitivity comparable to ground-based non-ALMA baselines. We show that LEO-enhanced arrays sample half of the diffraction-limited Fourier plane of Sgr A* in less than 30 minutes, enabling reconstructions of near-horizon structure with normalized root-mean-square error $\lesssim 0.3$ on sub-ISCO timescales.

16 pages, 9 figures, accepted for publication in ApJ

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• Concerns regarding the use of black hole shadows as standard rulers
• Modelling the Sgr A* Black Hole Immersed in a Dark Matter Spike
• Evaluation of New Submillimeter VLBI Sites for the Event Horizon Telescope
• Observational signatures of disk and jet misalignment in images of accreting black holes
• Photon Ring Autocorrelations
• Discriminating Accretion States via Rotational Symmetry in Simulated Polarimetric Images of M87
• Reference Array and Design Consideration for the next-generation Event Horizon Telescope
• TeraHertz Exploration and Zooming-in for Astrophysics (THEZA): ESA Voyage 2050 White Paper
• Studying Black Holes on Horizon Scales with VLBI Ground Arrays
• Simulations of M87 and Sgr A* Imaging with the Millimetron Space Observatory on near-Earth orbits
• Silhouettes of invisible black holes
• Observational Signatures of Frame Dragging in Strong Gravity
• Selective Dynamical Imaging of Interferometric Data
• Using space-VLBI to probe gravity around Sgr A*
• Constraining Wormhole Geometries using the Orbit of S2 Star and the Event Horizon Telescope
• The Galactic Center as a laboratory for theories of gravity and dark matter
• Black hole parameter estimation with synthetic Very Long Baseline Interferometry data from the ground and from space
• Enabling Transformational ngEHT Science via the Inclusion of 86 GHz Capabilities
• Analysis of Orbital Configurations for Millimetron Space Observatory
• Dynamic and polarimetric VLBI imaging with a multiscalar approach
• An Event Horizon Imager (EHI) Mission Concept Utilizing Medium Earth Orbit Sub-mm Interferometry
• Orbital configurations of spaceborne interferometers for studying photon rings of supermassive black holes
• First Space-VLBI Observations of Sagittarius A*
• OmniUV: A Multi-Purpose Simulation Toolkit for VLBI Observation
• Black Hole Shadow Observations with Space-Ground Interferometers
• Expanding Sgr A* dynamical imaging capabilities with an African extension to the Event Horizon Telescope
• Imaging the event horizon of M87* from space on different timescales
• Climbing out of the shadows: building the distance ladder with black hole images
• Impact of the nonthermal electron radiation effects on the horizon scale image structure of Sagittarius A*
• A proof-of-concept neural network for inferring parameters of a black hole from partial interferometric images of its shadow
• Studying black holes on horizon scales with space-VLBI
• Learning a Probabilistic Strategy for Computational Imaging Sensor Selection
• Effect of Earth's Oblateness on Black Hole Imaging Through Earth-Space and Space-Space VLBI
• Orbit design for mitigating interstellar scattering effects in Earth-space VLBI observations of Sgr A*
• Visible shapes of black holes M87* and SgrA*