Fundamental physics opportunities with future ground-based mm/sub-mm VLBI arrays

The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of ∼1.3 mm, EHT angular resolution (λ/D) is ∼25 μas, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit s−1, exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.

ABSTRACTThe Event Horizon Telescope (EHT), now with its first ever image of the photon ring around the supermassive black hole of M87, provides a unique opportunity to probe the physics of supermassive black holes through Very Long Baseline Interferometry (VLBI), such as the existence of the event horizon, the accretion processes as well as jet formation in low-luminosity AGNs (LLAGNs). We build a theoretical model that includes an advection dominated accretion flow (ADAF) with emission from thermal and non-thermal electrons in the flow and a simple radio jet outflow. The predicted spectral energy distribution (SED) of this model is compared to sub-arcsec resolution observations to get the best estimates of the model parameters. The model-predicted radial emission profiles at different frequency bands are used to predict whether the inflow can be resolved by the EHT or with telescopes such as the Global 3-mm VLBI array (GMVA). In this work the model is initially tested with high-resolution SED data of M87 and then applied to our sample of five galaxies (Cen A, M84, NGC 4594, NGC 3998, and NGC 4278). The model then allows us to predict if one can detect and resolve the inflow for any of these galaxies using the EHT or GMVA within an 8 h integration time.

The Event Horizon Telescope (EHT) collaboration has revealed the first-ever images of a black hole shadow at the heart of a giant elliptical galaxy Messier 87 (M87). The EHT links ground-based radio telescopes around the globe to form an Earth-sized virtual telescope with an unprecedented highest angular resolution using very long baseline interferometry (VLBI) at millimeter wavelengths. Images visually reveal the strongest evidence of an existence of a black hole in the universe. The bright compact radio source with a diameter of 42±3 micro-arcsecond (μas) suggests a supermassive black hole (SMBH) of (6.5±0.7)× 109 M⨀ (solar mass). An asymmetric ring-like morphology strongly suggests that we see gravitationally lensed emission from plasma rotating around the very vicinity of the SMBH event horizon. The image also supports the longstanding hypothesis that a SMBH powers an active galactic nucleus (AGN). The EHT collaboration demonstrates that VLBI at millimeter/sub-millimeter bands offers a powerful method to explorer gravity in its most extreme limit and at a previously inaccessible mass scale.

Based on a deduced 2153±5 s orbiting period of the extreme central binary of the supermassive black hole (ECB-SMBH), at Sgr A*,that is confirmed being based on the decameter radio wave pulse observations compared with 1.3 mm wavelength very long baseline interferometry (VLBI) data from the event horizon telescope (EHT-Data), we depict moving images of Sgr A* from EHT-Data, confirming further the existence of ECB-SMBH. The results show that radio wave images of the two members of ECB-SMBH are radio-bright objects due to emissions caused by source agents accreting from a thick disk toward ECB-SMBH. We extend the confirmed concept of ECB-SMBH to the interpretation of M87*, based on two images released by Miyoshi et al. in 2022 using EHT-Data and by Lu et al. in 2023 using the 3.5 mm wavelength data from the Global Millimeter VLBI Arrays as core telescopes. For the double brilliant spots displayed in two different images of M87*, we consider them the same two objects that shift locations during the 369-day interval of the observation periods of the two images. It is concluded that M87* A and M87* B (temporarily named) are orbiting with a period of 168.8 d at velocities of (6.28±0.54)% and (16.7±1.5)% of the speed of light, respectively. For the existence of ECB-SMBH, the condition of “no gravitational wave radiation” from the SMBH is essential. Regarding the secondary BH of blazar OJ 287, we find around 58~60-year period oscillation of the orbiting period, which is recently in a shortening phase. By applying the concept of ECB-SMBH to the OJ287 primary BH, whose orbiting period is approximately 1.6 yr, we interpret that the orbital period variation of the secondary BH is not due to the radiation of gravitational waves but is caused by the orbital motions of ECB-SMBH

Sagittaruis A* is the name given to the compact radio source at the center of the Milky Way. In 1994 Narayan et al. noted that the source exhibited relatively low luminosity for the large amount of mass it was accreting. They posited that this could be resolved with the black hole model for accretion in which most of the energy was absorbed by the black hole rather than being released as radiation. In 1998, using the Keck 10m telescope to measure the proper motion of stars around the galactic core, Ghez et al. were able to determine constraints for size and mass of the object that aligned with the characteristics of a massive black hole. With more refined calculations, these key observations amongst others provide strong evidence to suggest Sagittarius A* is a supermassive black hole. Given its relative proximity and mass, Sgr A* has the largest angular event horizon of any black hole, making it ideal for further analysis. Millimeter VLBI observations of Sgr A* performed with the Event Horizon Telescope (EHT) demonstrate the existence of structural variation on timescales that correspond to the Schwarzchild radius. The ability to resolve both the spatial and temporal structures on event horizon scales suggests many applications for testing General Relativity in very strong gravitation fields. In preparation for higher-resolution submillimeter VLBI data coming in the next 6 months, we explore closure amplitude and phase signatures of simulated GRMHD movies. These valuable plots allow us to determine important values such as Sag A*?s angle of orientation and the amount of accreted matter there is around. By synthesizing and analyzing data ahead of time, we will be more apt to understand what is going on when the real data comes through. This paper discusses synthesized observations and how to interpret the true VLBI data when it becomes available in early 2015. Research Supervisors: Sheperd Doeleman, Michael Johnson

The proposed next generation Event Horizon Telescope (ngEHT) concept envisions the imaging of various astronomical sources on scales of microarcseconds in unprecedented detail with at least two orders of magnitude improvement in the image dynamic ranges by extending the Event Horizon Telescope (EHT). A key technical component of ngEHT is the utilization of large aperture telescopes to anchor the entire array, allowing the connection of less sensitive stations through highly sensitive fringe detections to form a dense network across the planet. Here, we introduce two projects for planned next generation large radio telescopes in the 2030s on the Chajnantor Plateau in the Atacama desert in northern Chile, the Large Submillimeter Telescope (LST) and the Atacama Large Aperture Submillimeter Telescope (AtLAST). Both are designed to have a 50-meter diameter and operate at the planned ngEHT frequency bands of 86, 230 and 345 GHz. A large aperture of 50 m that is co-located with two existing EHT stations, the Atacama Large Millimeter/Submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX) Telescope in the excellent observing site of the Chajnantor Plateau, will offer excellent capabilities for highly sensitive, multi-frequency, and time-agile millimeter very long baseline interferometry (VLBI) observations with accurate data calibration relevant to key science cases of ngEHT. In addition to ngEHT, its unique location in Chile will substantially improve angular resolutions of the planned Next Generation Very Large Array in North America or any future global millimeter VLBI arrays if combined. LST and AtLAST will be a key element enabling transformative science cases with next-generation millimeter/submillimeter VLBI arrays.

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.

The Galactic center supermassive black hole Sagittarius A* (Sgr A*) is one of the most promising targets to study the dynamics of black hole accretion and outflow via direct imaging with very long baseline interferometry (VLBI). At 3.5 mm (86 GHz), the emission from Sgr A* is resolvable with the Global Millimeter VLBI Array (GMVA). We present the first observations of Sgr A* with the phased Atacama Large Millimeter/submillimeter Array (ALMA) joining the GMVA. Our observations achieve an angular resolution of ∼87 μas, improving upon previous experiments by a factor of two. We reconstruct a first image of the unscattered source structure of Sgr A* at 3.5 mm, mitigating the effects of interstellar scattering. The unscattered source has a major-axis size of 120 ± 34 μas (12 ± 3.4 Schwarzschild radii) and a symmetrical morphology (axial ratio of ), which is further supported by closure phases consistent with zero within 3σ. We show that multiple disk-dominated models of Sgr A* match our observational constraints, while the two jet-dominated models considered are constrained to small viewing angles. Our long-baseline detections to ALMA also provide new constraints on the scattering of Sgr A*, and we show that refractive scattering effects are likely to be weak for images of Sgr A* at 1.3 mm with the Event Horizon Telescope. Our results provide the most stringent constraints to date for the intrinsic morphology and refractive scattering of Sgr A*, demonstrating the exceptional contribution of ALMA to millimeter VLBI.

The Event Horizon Telescope (EHT) is a project to assemble a Very Long Baseline Interferometry (VLBI) network of mm wavelength dishes that can resolve strong field General Relativistic signatures near a supermassive black hole. As planned, the EHT will include enough dishes to enable imaging of the predicted black hole "shadow", a feature caused by severe light bending at the black hole boundary. The center of M87, a giant elliptical galaxy, presents one of the most interesting EHT targets as it exhibits a relativistic jet, offering the additional possibility of studying jet genesis on Schwarzschild radius scales. Fully relativistic models of the M87 jet that fit all existing observational constraints now allow horizon-scale images to be generated. We perform realistic VLBI simulations of M87 model images to examine detectability of the black shadow with the EHT, focusing on a sequence of model images with a changing jet mass load radius. When the jet is launched close to the black hole, the shadow is clearly visible both at 230 and 345 GHz. The EHT array with a resolution of 20-30$\mu$as resolution ($\sim$2-4 Schwarzschild radii) is able to image this feature independent of any theoretical models and we show that imaging methods used to process data from optical interferometers are applicable and effective for EHT data sets. We demonstrate that the EHT is also capable of tracing real-time structural changes on a few Schwarzschild radii scales, such as those implicated by VHE flaring activity of M87. While inclusion of ALMA in the EHT is critical for shadow imaging, generally the array is robust against loss of a station.

With its large collecting area, the NOrthern Extended Millimeter Array (NOEMA) is a prime candidate for a highly sensitive very long baseline interferometry (VLBI) station in the millimeter range. In this work, we describe the phasing system used for coherently adding the 12 antennas of the array. We developed and installed VLBI dedicated hardware and a new correlator firmware mode to achieve this goal. We also developed an active phasing software to compensate in real time for tropospheric phase variations across the array. This phasing system enabled the NOEMA array to achieve a level of sensitivity equivalent to a ∼ 50,m single dish antenna. Since 2021, phased NOEMA has been participating regularly in VLBI observations as part of the existing millimeter VLBI networks: the Global Millimeter VLBI Array (GMVA) and the Event Horizon Telescope (EHT).

We describe the latest development of the control and monitoring system of the Greenland Telescope (GLT). The GLT is a 12-m radio telescope aiming to carry out the sub-millimeter Very Long Baseline Interferometry (VLBI) observations through the Event Horizon Telescope (EHT) and the Global Millimeter VLBI Array (GMVA), to image the shadows of super massive black holes. The telescope is currently located at the Thule Air Base for commissioning before deployed to the Summit Station. The GLT participated in the VLBI observing campaigns in 2018 and 2019 and fringes were successfully detected at 86 and 230 GHz. Our antenna control software was adapted from the Submillimeter Array (SMA), and as a result for single-dish observations we added new routines to coordinate it with other instruments. We are exploring new communication interfaces; we utilized both in-memory and on-disk databases to be part of the interfaces not only for hardware monitoring but also for engineering event logging. We plan to incorporate the system of the James Clerk Maxwell Telescope for the full Linux-based receiver control. The current progress of integrating our receivers, spectrometers, sub-reflector, and continuum detector into control is presented, together with the implementation of the commissioning software for spectral line pointing. We also describe how we built the anti-collision protection and the recovery mechanism for the sub-reflector hexapod.

The compact and, with 4.3+-0.3 million solar masses, very massive object located at the center of the Milky Way is currently the very best candidate for a supermassive black hole (SMBH) in our immediate vicinity. The strongest evidence for this is provided by measurements of stellar orbits, variable X-ray emission, and strongly variable polarized near-infrared emission from the location of the radio source Sagittarius~A* (SgrA*) in the middle of the central stellar cluster. If SgrA* is indeed a SMBH it will, in projection onto the sky, have the largest event horizon and will certainly be the first and most important target of the Event Horizon Telescope (EHT) Very Long Baseline Interferometry (VLBI) observations currently being prepared. These observations in combination with the infrared interferometry experiment GRAVITY at the Very Large Telescope Interferometer (VLTI) and other experiments across the electromagnetic spectrum might yield proof for the presence of a black hole at the center of the Milky Way. It is, however, unclear when the ever mounting evidence for SgrA* being associated with a SMBH will suffice as a convincing proof. Additional compelling evidence may come from future gravitational wave observatories. This manuscript reviews the observational facts, theoretical grounds and conceptual aspects for the case of SgrA* being a black hole. We treat theory and observations in the framework of the philosophical discussions about "(Anti)Realism and Underdetermination", as this line of arguments allows us to describe the situation in observational astrophysics with respect to supermassive black holes. Questions concerning the existence of supermassive black holes and in particular SgrA* are discussed using causation as an indispensable element. We show that the results of our investigation are convincingly mapped out by this combination of concepts.

Detailed and long‐term VLBI (Very Long Baseline Interferometry) studies of the variable jets of supermassive black holes helps us to understand the emission processes of these fascinating phenomena. When observed and traced precisely, jet component kinematics reveals details about the potential motion of the jet base. Following this motion over decades with VLBI monitoring reveals – in some cases – the signatures of precession. While several processes can cause precession, the most likely cause seems to be a supermassive binary black hole in the central region of the AGN. We present examples of the analysis of high‐resolution VLBI observations which provides us with insight into the physics of these objects and reveals evidence for the presence of double black hole cores. EHT (Event Horizon Telescope) observations will probably soon tell us more about the jet origin and launching mechanism at the very centers of nearby active galactic nuclei. An important question to be addressed by the EHT and related observations will be whether Sgr A*, the supermassive black hole in the Galactic Center, has a jet as well. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Context. Due to the limited number of antennas and the limited observation time, an array of antennas in very long baseline interfer-ometry (VLBI) often samples the Fourier domain only very sparsely. Powerful deconvolution algorithms are needed to compute a final image. Multiscale imaging approaches such as DoG-HiT have recently been developed to solve the VLBI imaging problem and show promising performance: they are fast, accurate, unbiased, and automatic. Aims. We extend the multiscalar imaging approach to polarimetric imaging, to reconstructions of dynamically evolving sources, and finally to dynamic polarimetric reconstructions. Methods. These extensions (mr-support imaging) utilize a multiscalar approach. The time-averaged Stokes I image was decomposed by a wavelet transform into single subbands. We used the set of statistically significant wavelet coefficients, the multiresolution support (mr-support), computed by DoG-HiT as a prior in a constrained minimization manner; we fitted the single-frame (polarimetric) observables by only varying the coefficients in the multiresolution support. Results. The Event Horizon Telescope (EHT) is a VLBI array imaging supermassive black holes. We demonstrate on synthetic data that mr-support imaging offers ample regularization and is able to recover simple geometric dynamics at the horizon scale in a typical EHT setup. The approach is relatively lightweight, fast, and largely automatic and data driven. The ngEHT is a planned extension of the EHT designed to recover movies at the event horizon scales of a supermassive black hole. We benchmark the performance of mr-support imaging for the denser ngEHT configuration demonstrating the major improvements the additional ngEHT antennas will bring to dynamic polarimetric reconstructions. Conclusions. Current and upcoming instruments offer the observational possibility to do polarimetric imaging of dynamically evolving structural patterns with the highest spatial and temporal resolution. State-of-the-art dynamic reconstruction methods can capture this motion with a range of temporal regularizers and priors. With this work, we add an additional simpler regularizer to the list: constraining the reconstruction to the multiresolution support.

The Strategic Priority Research Program on Space Science of the Chinese Academy of Sciences, “Space Millimeter-wavelength VLBI Array (SMVA)”, aims to build the first space millimeter-wavelength Very Long Baseline Interferometry (VLBI) array in the world. The SMVA, which has the ultra-high spatial resolution with a best of 20 micro-arcsecond that a ground-based VLBI array can not attain, is a powerful tool in imaging the hyperfine emission structure surrounding the black holes and other compact celestial objects. The simulations, such as UV coverage simulation, numerical simulation and image simulation, play a crucial role in all phases of the project, so a space VLBI simulation software was designed and implemented. This software has all the necessary and auxiliary simulation functions. And it has many advantages compared to other similar software packages: its interface is more friendly, the software package is robust and independent of operation system, the code is much easier to expand and scalable.