Simulations of imaging the event horizon of Sagittarius A* from space

Abstract

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.

Imaging black holes and jets with a VLBI array including multiple space-based telescopes

“…A logarithmic transfer function, with the color range spanning three orders of magnitude in dynamic range, is used to highlight weak features in the jet. vertical offset, as in the Event Horizon Imager (EHI) concept (Roelofs et al, 2018) could easily be accommodated in a space-ground architecture by the simple addition of a second satellite in MEO. The reverse statement is not true; it would be extremely difficult, if not impossible, to incorporate a number of additional satellites (and ground stations) into the EHI architecture.…”

“…For instance, Palumbo (2018) explored an array consisting of four telescopes in LEO, optimized for dynamical imaging. Roelofs et al (2018) explored a minimal configuration of two MEO satellites in a configuration such that the orbits slowly evolve over the course of six months to image Sgr A*. The uniting theme in these studies is that they have focused on a single scientific case and identified a single type of array to address that specific case.…”

Space VLBI: from first ideas to operational missions

“…That said, the presence of the H-maser on board the RadioAstron spacecraft made it possible to conduct the ad hoc Gravitational Redshift Experiment (Litvinov et al, 2018;Nunes et al, 2019). Future advanced SVLBI systems operating at millimetre and sub-millimetre wavelengths (Andrianov et al, 2019;Fish et al, 2019;Kudrishov et al, 2019;Linz et al, 2019;Roelofs et al, 2019) might require new approaches to space-borne telescope heterodyning.…”

Testing the Schwarzschild metric in a strong field region with the Event Horizon Telescope

“…A more realistic simulation is given by [11], which calculated the image of a Schwarzschild black hole with the standard thin accretion disk. [12][13][14] simulated the image for the Kerr black hole and other accretion disk models. In order to compare observations with theories, the EHT collaboration [5,6,15] simulated the accretion matters based on the general relativistic magnetohydrodynamic (GRMHD, see [16] for pedagogical resources) simulations and produced the images by ray tracing [17].…”