Publications

The Event Horizon Telescope (EHT) Collaboration recently published the first images of the supermassive black holes in the cores of the Messier 87 and Milky Way galaxies. These observations have provided a new means to study supermassive black holes and probe physical processes occurring in the strong-field regime. We review the prospects of future observations and theoretical studies of supermassive black hole systems with the next-generation Event Horizon Telescope (ngEHT), which will greatly enhance the capabilities of the existing EHT array. These enhancements will open up several previously inaccessible avenues of investigation, thereby providing important new insights into the properties of supermassive black holes and their environments. This review describes the current state of knowledge for five key science cases, summarising the unique challenges and opportunities for fundamental physics investigations that the ngEHT will enable.

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.

Context. 3C 84 is a nearby radio source with a complex total intensity structure, showing linear polarisation and spectral patterns. A detailed investigation of the central engine region necessitates the use of very-long-baseline interferometry (VLBI) above the hitherto available maximum frequency of 86 GHz.

Aims. Using ultrahigh resolution VLBI observations at the currently highest available frequency of 228 GHz, we aim to perform a direct detection of compact structures and understand the physical conditions in the compact region of 3C 84.

Methods. We used Event Horizon Telescope (EHT) 228 GHz observations and, given the limited (u, v)-coverage, applied geometric model fitting to the data. Furthermore, we employed quasi-simultaneously observed, ancillary multi-frequency VLBI data for the source in order to carry out a comprehensive analysis of the core structure.

Results. We report the detection of a highly ordered, strong magnetic field around the central, supermassive black hole of 3C 84. The brightness temperature analysis suggests that the system is in equipartition. We also determined a turnover frequency of ν_m = (113 ± 4) GHz, a corresponding synchrotron self-absorbed magnetic field of B_SSA = (2.9 ± 1.6) G, and an equipartition magnetic field of B_eq = (5.2 ± 0.6) G. Three components are resolved with the highest fractional polarisation detected for this object (m_net = (17.0 ± 3.9)%). The positions of the components are compatible with those seen in low-frequency VLBI observations since 2017–2018. We report a steeply negative slope of the spectrum at 228 GHz. We used these findings to test existing models of jet formation, propagation, and Faraday rotation in 3C 84.

Conclusions. The findings of our investigation into different flow geometries and black hole spins support an advection-dominated accretion flow in a magnetically arrested state around a rapidly rotating supermassive black hole as a model of the jet-launching system in the core of 3C 84. However, systematic uncertainties due to the limited (u, v)-coverage, however, cannot be ignored. Our upcoming work using new EHT data, which offer full imaging capabilities, will shed more light on the compact region of 3C 84.

Key words: techniques: high angular resolution / techniques: interferometric / galaxies: active / galaxies: individual: NGC 1275 / galaxies: 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.

We report on the observations of the quasar NRAO 530 with the Event Horizon Telescope (EHT) on 2017 April 5-7, when NRAO 530 was used as a calibrator for the EHT observations of Sagittarius A*. At z = 0.902, this is the most distant object imaged by the EHT so far. We reconstruct the first images of the source at 230 GHz, at an unprecedented angular resolution of similar to 20 mu as, both in total intensity and in linear polarization (LP). We do not detect source variability, allowing us to represent the whole data set with static images. The images reveal a bright feature located on the southern end of the jet, which we associate with the core. The feature is linearly polarized, with a fractional polarization of similar to 5%-8%, and it has a substructure consisting of two components. Their observed brightness temperature suggests that the energy density of the jet is dominated by the magnetic field. The jet extends over 60 mu as along a position angle similar to -28 degrees. It includes two features with orthogonal directions of polarization (electric vector position angle), parallel and perpendicular to the jet axis, consistent with a helical structure of the magnetic field in the jet. The outermost feature has a particularly high degree of LP, suggestive of a nearly uniform magnetic field. Future EHT observations will probe the variability of the jet structure on microarcsecond scales, while simultaneous multiwavelength monitoring will provide insight into the high-energy emission origin.