Michael D. Johnson

The Black Hole Explorer (BHEX) is a space-VLBI mission that will extend the Event Horizon Telescope into space. The cryogenic receivers must be cooled down to 4.5K. A cryogenic system consisting of two Stirling cryocoolers and a Joule-Thomson cooler has been explored which consists of a 20K and a 4.5K cold stage in order to cool a combined heat load of approximately 250mW. The integration challenges of the cryocooling system with the receivers and broader instrument are explored, where power, mass, and thermal challenges require careful considerations and trade-off. This study presents a feasible cryocooling design that reaches the cold temperature requirements of the BHEX instrument.

The Black Hole Explorer (BHEX), is an orbiting, multi-band, millimeter radio-telescope, in hybrid combination with millimeter terrestrial radio-telescopes, designed to discover and measure the thin photon ring around the supermassive black holes M87* and Sgr A*. In order to guide the mission design for the BHEX instruments, this paper explores various aspects of the photon ring, like the spin-induced changes to its shape, or the intricate flow of light around a spinning black hole, by tracking, through visual simulations, photons as they course along geodesics. Ultimately, the aim of these visualizations is to advance the foundational aims of the EHE instrument, and through this experiment to articulate spacetime geometry via the photon ring.

The photon ring is a narrow ring-shaped feature (predicted by general relativity, but not yet observed) that appears in black hole images. It is caused by the extreme bending of light within a few Schwarzschild radii of the event horizon and provides a direct probe of the unstable bound photon orbits of the Kerr black hole geometry. The precise shape of the observable photon ring is remarkably insensitive to the details of the astronomical source and can therefore be used as a precise probe of strong-field gravity. The Black Hole Explorer (BHEX) is a proposed space-based experiment targeting the supermassive black holes M87* and Sgr A* with radio-interferometric observations at frequencies of 86 through 345 GHz and from an orbital distance of ~40,000km. We forecast that its design will enable a measurement of the photon rings around M87* and Sgr A* and confirm the Kerr nature of these two sources.

We present a baseline science operations plan for the Black Hole Explorer (BHEX), a space mission concept aiming to confirm the existence of the predicted sharp “photon ring” resulting from strongly lensed photon trajectories around black holes, as predicted by general relativity, and to measure its size and shape to determine the black hole's spin. The BHEX radio antenna will co-observe with a ground-based very long baseline interferometric (VLBI) array, providing unprecedented high resolution with the extension to space that will enable photon ring detection and studies of active galactic nuclei. Here we outline the concept of operations for the hybrid observatory coordinating both a VLBI network and an optical downlink terminal network, the available observing modes, the proposal and observation planning process, and data delivery to achieve the BHEX mission goals and meet mission requirements.

We describe the baseline design of the science instrument for the Black Hole Explorer (BHEX), a space very long baseline interferometry (VLBI) mission concept currently in the formulation phase. BHEX will study supermassive black holes to understand fundamental physics, black hole jets, and the growth of black holes in galaxies. By co-observing with ground radio telescopes, BHEX will achieve 3-5 micro-arcsecond resolution from a distance of ~40,000 km. Observations will be conducted in two simultaneous bands between 80-350GHz, using an on-board low-power, low-mass ultra-stable oscillator as the master frequency reference, and the digitized data will be transmitted to the ground through an ultra-wide bandwidth laser downlink.

The Black Hole Explorer: (BHEX) is a space mission to image radio emissions of black holes by expanding both the baseline and time resolution of Very Long Baseline Interferometry (VLBI). This involves integrating a space telescope into an array of ground telescopes, such as the Event Horizon Telescope (EHT). Ultimately, the EHE will enable transformative science and the mission goals are well aligned with the Astro 2020 Decadal Survey. The EHE mission concept study was designed with three major stages: (1) a Science Study which articulates plausible goals and objectives (2) an Engineering Study which articulates overall feasi- bility and technological readiness and (3) a Mission Architecture Study which combines the results of the previous studies to match achievable science goals and objectives with feasible engineering to yield a plausible mission architecture. This paper review the initial steps taken under stage two of the EHE project.

The Black Hole Explorer (BHEX) is a next-generation space very long baseline interferometry (VLBI) mission concept that will extend the ground-based Event Horizon Telescope into space. The Japanese community is poised to make major contributions to the mission, ranging from science to mission-critical instrumentation. Here we present the Japanese vision for the mission. A potential major technical contribution is providing key components for its sensitive tri-band receiving system, including SIS mixers at 230 and 345 GHz and a space-qualified multi-stage 4.5K cryocooler similar to that on JAXA’s Hitomi and XRISM satellites. The Japanese community envisions broad science cases spanning from various black hole physics/astrophysics explored with VLBI to molecular universe explored by the potential single-dish observing mode at radio frequencies to be explored for the first time with the BHEX mission.

We present the motivation and vision for the Black Hole Explorer (BHEX), a mission that will extend submillimeter Very Long Baseline Interferometry (VLBI) to space. BHEX, currently under formulation for a NASA Small Explorer mission, will discover and measure the bright and narrow “photon ring” that is predicted to exist in images of black holes, will reveal the processes that drive supermassive black hole creation and growth, and will connect supermassive black holes to their relativistic jets.

In a companion paper, we present the first spatially resolved polarized image of Sagittarius A* on event horizon scales, captured using the Event Horizon Telescope, a global very long baseline interferometric array operating at a wavelength of 1.3 mm. Here we interpret this image using both simple analytic models and numerical general relativistic magnetohydrodynamic (GRMHD) simulations. The large spatially resolved linear polarization fraction (24%–28%, peaking at ∼40%) is the most stringent constraint on parameter space, disfavoring models that are too Faraday depolarized. Similar to our studies of M87*, polarimetric constraints reinforce a preference for GRMHD models with dynamically important magnetic fields. Although the spiral morphology of the polarization pattern is known to constrain the spin and inclination angle, the time-variable rotation measure (RM) of Sgr A* (equivalent to ≈46° ± 12° rotation at 228 GHz) limits its present utility as a constraint. If we attribute the RM to internal Faraday rotation, then the motion of accreting material is inferred to be counterclockwise, contrary to inferences based on historical polarized flares, and no model satisfies all polarimetric and total intensity constraints. On the other hand, if we attribute the mean RM to an external Faraday screen, then the motion of accreting material is inferred to be clockwise, and one model passes all applied total intensity and polarimetric constraints: a model with strong magnetic fields, a spin parameter of 0.94, and an inclination of 150°. We discuss how future 345 GHz and dynamical imaging will mitigate our present uncertainties and provide additional constraints on the black hole and its accretion flow.

The Event Horizon Telescope observed the horizon-scale synchrotron emission region around the Galactic center supermassive black hole, Sagittarius A* (Sgr A*), in 2017. These observations revealed a bright, thick ring morphology with a diameter of 51.8 ± 2.3 μas and modest azimuthal brightness asymmetry, consistent with the expected appearance of a black hole with mass M ≈ 4 × 106 M ⊙. From these observations, we present the first resolved linear and circular polarimetric images of Sgr A*. The linear polarization images demonstrate that the emission ring is highly polarized, exhibiting a prominent spiral electric vector polarization angle pattern with a peak fractional polarization of ∼40% in the western portion of the ring. The circular polarization images feature a modestly (∼5%–10%) polarized dipole structure along the emission ring, with negative circular polarization in the western region and positive circular polarization in the eastern region, although our methods exhibit stronger disagreement than for linear polarization. We analyze the data using multiple independent imaging and modeling methods, each of which is validated using a standardized suite of synthetic data sets. While the detailed spatial distribution of the linear polarization along the ring remains uncertain owing to the intrinsic variability of the source, the spiraling polarization structure is robust to methodological choices. The degree and orientation of the linear polarization provide stringent constraints for the black hole and its surrounding magnetic fields, which we discuss in an accompanying publication.

Around the horizon of a black hole, an edge of the universe, light is captured, spun into orbit by the black hole’s powerful gravitational pull. Lying within the orange donut in the famous first image of a black hole, this “photon ring” would be a prize to measure—it would reveal the nature of spacetime itself, directly, near the horizon. Indeed, the shape of this pure ring of light tells everything about the black hole. With the stakes this high, a new collaboration—physicists, astronomers, engineers from around the world—has formed to loft a spacecraft that capture the photon ring. We are at the beginning of what is probably a ten-year effort—this is a film about the start of that adventure.

A film by Peter Galison, Michael Johnson, and Chyld King

In April 2019, the Event Horizon Telescope (EHT) Collaboration reported the first-ever event-horizon-scale images of a black hole, resolving the central compact radio source in the giant elliptical galaxy M 87. These images reveal a ring with a southerly brightness distribution and a diameter of ∼42 μas, consistent with the predicted size and shape of a shadow produced by the gravitationally lensed emission around a supermassive black hole. These results were obtained as part of the April 2017 EHT observation campaign, using a global very long baseline interferometric radio array operating at a wavelength of 1.3 mm. Here, we present results based on the second EHT observing campaign, taking place in April 2018 with an improved array, wider frequency coverage, and increased bandwidth. In particular, the additional baselines provided by the Greenland telescope improved the coverage of the array. Multiyear EHT observations provide independent snapshots of the horizon-scale emission, allowing us to confirm the persistence, size, and shape of the black hole shadow, and constrain the intrinsic structural variability of the accretion flow. We have confirmed the presence of an asymmetric ring structure, brighter in the southwest, with a median diameter of as. The diameter of the 2018 ring is remarkably consistent with the diameter obtained from the previous 2017 observations. On the other hand, the position angle of the brightness asymmetry in 2018 is shifted by about 30° relative to 2017. The perennial persistence of the ring and its diameter robustly support the interpretation that the ring is formed by lensed emission surrounding a Kerr black hole with a mass ∼6.5 × 10<sup>9<sup/> <i>M<i/><sub>⊙<sub/>. The significant change in the ring brightness asymmetry implies a spin axis that is more consistent with the position angle of the large-scale jet.

Event Horizon Telescope (EHT) observations have revealed a bright ring of emission around the supermassive black hole at the center of the M87 galaxy. EHT images in linear polarization have further identified a coherent spiral pattern around the black hole, produced from ordered magnetic fields threading the emitting plasma. Here we present the first analysis of circular polarization using EHT data, acquired in 2017, which can potentially provide additional insights into the magnetic fields and plasma composition near the black hole. Interferometric closure quantities provide convincing evidence for the presence of circularly polarized emission on event-horizon scales. We produce images of the circular polarization using both traditional and newly developed methods. All methods find a moderate level of resolved circular polarization across the image (divided by v divided by > < 3.7%), consistent with the low image-integrated circular polarization fraction measured by the Atacama Large Millimeter/submillimeter Array (divided by v(int)divided by < 1%). Despite this broad agreement, the methods show substantial variation in the morphology of the circularly polarized emission, indicating that our conclusions are strongly dependent on the imaging assumptions because of the limited baseline coverage, uncertain telescope gain calibration, and weakly polarized signal. We include this upper limit in an updated comparison to general relativistic magnetohydrodynamic simulation models. This analysis reinforces the previously reported preference for magnetically arrested accretion flow models. We find that most simulations naturally produce a low level of circular polarization consistent with our upper limit and that Faraday conversion is likely the dominant production mechanism for circular polarization at 230 GHz in M87*.

The Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply improve the angular resolution, dynamic range, and temporal coverage of the existing EHT observations. These improvements will uniquely enable a wealth of transformative new discoveries related to black hole science, extending from event-horizon-scale studies of strong gravity to studies of explosive transients to the cosmological growth and influence of supermassive black holes. Here, we present the key science goals for the ngEHT and their associated instrument requirements, both of which have been formulated through a multi-year international effort involving hundreds of scientists worldwide.