Search

Many active galaxies harbor powerful relativistic jets, however, the detailed mechanisms of their formation and acceleration remain poorly understood. To investigate the area of jet acceleration and collimation with the highest available angular resolution, we study the innermost region of the bipolar jet in the nearby low-ionization nuclear emission-line region (LINER) galaxy NGC 1052. We combined observations of NGC 1052 taken with VLBA, GMVA, and EHT over one week in the spring of 2017. For the first time, NGC 1052 was detected with the EHT, providing a size of the central region in-between both jet bases of 250 RS (Schwarzschild radii) perpendicular to the jet axes. This size estimate supports previous studies of the jets expansion profile which suggest two breaks of the profile at around 300 RS and 10000 RS distances to the core. Furthermore, we estimated the magnetic field to be 1.25 Gauss at a distance of 22 {\mu}as from the central engine by fitting a synchrotron-self absorption spectrum to the innermost emission feature, which shows a spectral turn-over at about 130 GHz. Assuming a purely poloidal magnetic field, this implies an upper limit on the magnetic field strength at the event horizon of 26000 Gauss, which is consistent with previous measurements. The complex, low-brightness, double-sided jet structure in NGC 1052 makes it a challenge to detect the source at millimeter (mm) wavelengths. However, our first EHT observations have demonstrated that detection is possible up to at least 230 GHz. This study offers a glimpse through the dense surrounding torus and into the innermost central region, where the jets are formed. This has enabled us to finally resolve this region and provide improved constraints on its expansion and magnetic field strength., Comment: 22 pages, 10 figures, published in A&A

The 2017 observing campaign of the Event Horizon Telescope (EHT) delivered the first very long baseline interferometry (VLBI) images at the observing frequency of 230 GHz, leading to a number of unique studies on black holes and relativistic jets from active galactic nuclei (AGN). In total, eighteen sources were observed: the main science targets, Sgr A* and M87 along with various calibrators. We investigated the morphology of the sixteen AGN in the EHT 2017 data set, focusing on the properties of the VLBI cores: size, flux density, and brightness temperature. We studied their dependence on the observing frequency in order to compare it with the Blandford-K\"onigl (BK) jet model. We modeled the source structure of seven AGN in the EHT 2017 data set using linearly polarized circular Gaussian components and collected results for the other nine AGN from dedicated EHT publications, complemented by lower frequency data in the 2-86 GHz range. Then, we studied the dependences of the VLBI core flux density, size, and brightness temperature on the frequency measured in the AGN host frame. We compared the observations with the BK jet model and estimated the magnetic field strength dependence on the distance from the central black hole. Our results indicate a deviation from the standard BK model, particularly in the decrease of the brightness temperature with the observing frequency. Either bulk acceleration of the jet material, energy transfer from the magnetic field to the particles, or both are required to explain the observations.

Using the Event Horizon Telescope (EHT), the gravitationally lensed rings around the supermassive black holes (SMBHs) in Messier 87 (M87) and Sagittarius A* (Sgr A*) have now been successfully imaged at a resolution under 10 gravitational radii (R$_{\rm g}$ $ = \rm{GM/c^2}$). To expand studies beyond M87 and Sgr A*, we have constructed the Event Horizon and Environs (ETHER) sample, a comprehensive database encompassing approximately 3.15 million SMBH mass estimates, $\sim$ 20,000 Very-Long Baseline Interferometry (VLBI) radio flux densities, and $\sim$ 36,000 hard X-ray flux densities. This database is designed to identify and optimize target selection for the EHT and its upgrades on the ground and in space. We have identified a Gold Sample (GS) of nearby low-luminosity Active Galactic Nuclei (AGNs) within it that are ideal for studying jet bases and potentially imaging black hole shadows. We observed 27 of these AGNs using the EHT from 2022 to 2024, providing an opportunity to resolve and image accretion flows and jets at resolutions of $\leq$ 100 R$_{\rm g}$. Only a few SMBHs have sufficiently high enough flux density to be imaged at scales of $\leq$ 50 R$_{\rm g}$ with the present EHT. Among these are M87, Sgr A*, NGC4594 (Sombrero/M104), NGC4261, and NGC4374 (Messier 84/M84). Of these, NGC4261, Sombrero, and M84 have been observed and/or are scheduled for deep imaging with EHT+ALMA from 2023 to 2025. Sombrero, NGC4261, M84, NGC4278, and NGC5232 are clearly detected in our EHT+ALMA observations in 2022, indicating that the 230 GHz flux density from the accretion flows is significantly high. Ongoing imaging of the ETHER GS will enable measurements of black hole mass and spin, help constrain General Relativity, and enrich our understanding of jet launching and accretion inflows across a broad multi-parameter space, including black hole mass, spin, accretion rate, and orientation., Comment: 9 pages, 6 figures, 1 table, published in Proceedings of the 16th EVN Symposium, Ed. E. Ros, P. Benke, S.A. Dzib, I. Rottmann, & J.A. Zensus, Bonn: Max-Planck-Institut f\"ur Radioastronomie, 2024, pages 75-84, https://cloud.mpifr-bonn.mpg.de/index.php/s/BkX2CC2Xjn2aKR4

The $n=1$ photon ring is an important probe of black hole (BH) properties and will be resolved by the Black Hole Explorer (BHEX) for the first time. However, extraction of black hole parameters from observations of the $n=1$ subring is not trivial. Developing this capability can be achieved by building a sample of $n=1$ subring simulations, as well as by performing feature extraction on this high-volume sample to track changes in the geometry, which presents significant computational challenges. Here, we present a framework for the study of $n=1$ photon ring behavior and BH property measurement from BHEX images. We use KerrBAM to generate a grid of $\gtrsim10^6$ images of $n=1$ photon rings spanning the entire space of Kerr BH spins and inclinations. Intensity profiles are extracted from images using a novel feature extraction method developed specifically for BHEX. This novel method is highly optimized and outperforms existing EHT methods by a factor of ${\sim}3000$. Additionally, we propose a novel, minimal set of geometric measurables for characterizing the behavior of the $n=1$ subring geometry. We apply these measurables to our simulation grid and test spin recovery on simulated images using: (i) gradient boosting, a machine learning algorithm; and (ii) an extension of Deep Horizon, a deep learning framework. We find $\gtrsim90$\% correct recovery of BH properties using the machine/deep learning approaches, and characterize the space of resolution-dependent geometric degeneracies. Finally, we test both approaches on GRMHD simulations of black hole accretion flows, and report accurate recovery of spin at the expected inclination of M87*., Comment: 15 pages, 9 figures, Submitted for publication in ApJ

The first very long baseline interferometry (VLBI) detections at 870$\mu$m wavelength (345$\,$GHz frequency) are reported, achieving the highest diffraction-limited angular resolution yet obtained from the surface of the Earth, and the highest-frequency example of the VLBI technique to date. These include strong detections for multiple sources observed on inter-continental baselines between telescopes in Chile, Hawaii, and Spain, obtained during observations in October 2018. The longest-baseline detections approach 11$\,$G$\lambda$ corresponding to an angular resolution, or fringe spacing, of 19$\mu$as. The Allan deviation of the visibility phase at 870$\mu$m is comparable to that at 1.3$\,$mm on the relevant integration time scales between 2 and 100$\,$s. The detections confirm that the sensitivity and signal chain stability of stations in the Event Horizon Telescope (EHT) array are suitable for VLBI observations at 870$\mu$m. Operation at this short wavelength, combined with anticipated enhancements of the EHT, will lead to a unique high angular resolution instrument for black hole studies, capable of resolving the event horizons of supermassive black holes in both space and time., Comment: Corresponding author: S. Doeleman

Recent developments in very long baseline interferometry (VLBI) have made it possible for the Event Horizon Telescope (EHT) to resolve the innermost accretion flows of the largest supermassive black holes on the sky. The sparse nature of the EHT's $(u, v)$-coverage presents a challenge when attempting to resolve highly time-variable sources. We demonstrate that the changing (u, v)-coverage of the EHT can contain regions of time over the course of a single observation that facilitate dynamical imaging. These optimal time regions typically have projected baseline distributions that are approximately angularly isotropic and radially homogeneous. We derive a metric of coverage quality based on baseline isotropy and density that is capable of ranking array configurations by their ability to produce accurate dynamical reconstructions. We compare this metric to existing metrics in the literature and investigate their utility by performing dynamical reconstructions on synthetic data from simulated EHT observations of sources with simple orbital variability. We then use these results to make recommendations for imaging the 2017 EHT Sgr A* data set., Comment: 20 pages, 14 figures, published in ApJL (2022), uploaded for open access

Provenance data from astronomical pipelines are instrumental in establishing trust and reproducibility in the data processing and products. In addition, astronomers can query their provenance to answer questions routed in areas such as anomaly detection, recommendation, and prediction. The next generation of astronomical survey telescopes such as the Vera Rubin Observatory or Square Kilometre Array, are capable of producing peta to exabyte scale data, thereby amplifying the importance of even small improvements to the efficiency of provenance storage or querying. In order to determine how astronomers should store and query their provenance data, this paper reports on a comparison between the turtle and JSON provenance serialisations. The triple store Apache Jena Fuseki and the graph database system Neo4j were selected as representative database management systems (DBMS) for turtle and JSON, respectively. Simulated provenance data was uploaded to and queried over each DBMS and the metrics measured for comparison were the accuracy and timing of the queries as well as the data upload times. It was found that both serialisations are competent for this purpose, and both have similar query accuracy. The turtle provenance was found to be more efficient at storing and uploading the data. Regarding queries, for small datasets ($<$5MB) and simple information retrieval queries, the turtle serialisation was also found to be more efficient. However, queries for JSON serialised provenance were found to be more efficient for more complex queries which involved matching patterns across the DBMS, this effect scaled with the size of the queried provenance., Comment: 9 pages, 8 figures, to be published in the 16th International Workshop on Theory and Practice of Provenance

The Event Horizon Telescope has produced resolved images of the supermassive black holes Sgr A* and M87*, which present the largest shadows on the sky. In the next decade, technological improvements and extensions to the array will enable access to a greater number of sources, unlocking studies of a larger population of supermassive black holes through direct imaging. In this paper, we identify 12 of the most promising sources beyond Sgr A* and M87* based on their angular size and millimeter flux density. For each of these sources, we make theoretical predictions for their observable properties by ray tracing general relativistic magnetohydrodynamic models appropriately scaled to each target's mass, distance, and flux density. We predict that these sources would have somewhat higher Eddington ratios than M87*, which may result in larger optical and Faraday depths than previous EHT targets. Despite this, we find that visibility amplitude size constraints can plausibly recover masses within a factor of 2, although the unknown jet contribution remains a significant uncertainty. We find that the linearly polarized structure evolves substantially with Eddington ratio, with greater evolution at larger inclinations, complicating potential spin inferences for inclined sources. We discuss the importance of 345 GHz observations, milli-Jansky baseline sensitivity, and independent inclination constraints for future observations with upgrades to the Event Horizon Telescope (EHT) through ground updates with the next-generation EHT (ngEHT) program and extensions to space through the Black Hole Explorer (BHEX)., Comment: 21 pages, 9 figures, 1 table, submitted to ApJ

The Black Hole Explorer (BHEX), an orbiting, multi-band, millimeter radio-telescope, in hybrid combination with millimeter terrestrial radio-telescopes, is designed to discover and measure the thin photon ring around the supermassive black holes M87* and Sgr A*. As background to the BHEX instruments, this paper explores various aspects of the photon ring, focusing on the intricate flow of light around a spinning black hole, and tracking, through visual simulations, photons as they course along geodesics. Ultimately, the aim of these visualizations is to advance the foundational aims of the BHEX instrument, and through this experiment to articulate spacetime geometry via the photon ring., Comment: 7 pages, 6 figures. Submitted to SPIE Space Telescopes & Instrumentation

The Black Hole Explorer (BHEX) is a space very-long-baseline interferometry (VLBI) mission concept that is currently under development. BHEX will study supermassive black holes at unprecedented resolution, isolating the signature of the "photon ring" - light that has orbited the black hole before escaping - to probe physics at the edge of the observable universe. It will also measure black hole spins, study the energy extraction and acceleration mechanisms for black hole jets, and characterize the black hole mass distribution. BHEX achieves high angular resolution by joining with ground-based millimeter-wavelength VLBI arrays, extending the size, and therefore improving the angular resolution of the earthbound telescopes. Here we discuss the science instrument concept for BHEX. The science instrument for BHEX is a dual-band, coherent receiver system for 80-320 GHz, coupled to a 3.5-meter antenna. BHEX receiver front end will observe simultaneously with dual polarizations in two bands, one sampling 80-106 GHz and one sampling 240-320 GHz. An ultra-stable quartz oscillator provides the master frequency reference and ensures coherence for tens of seconds. To achieve the required sensitivity, the front end will instantaneously receive 32 GHz of frequency bandwidth, which will be digitized to 64 Gbits/sec of incompressible raw data. These data will be buffered and transmitted to the ground via laser data link, for correlation with data recorded simultaneously at radio telescopes on the ground. We describe the challenges associated with the instrument concept and the solutions that have been incorporated into the baseline design., Comment: SPIE Astronomical Telescopes + Instrumentation, 2024

This paper describes specification and early design of back end signal processing subsystems for the Black Hole Explorer (BHEX) Very Long Baseline Interferometry (VLBI) space telescope. The "back end" consists of two subsystems. First, the block downconverter (BDC) is a heterodyne system that performs a frequency translation of the analog signal from IF to baseband and amplifies and filters it for digitization. Second, the digital back end (DBE) samples the analog signal with an analog-to-digital converters (ADC) and digitally processes the data stream formatting them to the VLBI "VDIF" standard and converting to Ethernet packets for 100 gigabit-per-second (Gb/s) Ethernet transport to the optical downlink system. Both the BDC and the DBE for BHEX support eight channels of 4.096 GHz bandwidth each, for a total processed bandwidth of 32.768 GHz. The BHEX back end benefits from mature terrestrial back end heritage, described in some detail. The BHEX back end itself is in the early stages of design, with requirements, interface specifications, and component trade studies well advanced. The aim is to build a prototype using terrestrial grade parts which are available in functionally identical space grade equivalents, and to use this prototype to advance the back end Technology Readiness Level (TRL) preparing for a Small Explorer (SMEX) proposal in 2025., Comment: 13 pages including abstract and references, 6 figures

The Black Hole Explorer (BHEX) is a space-based very-long baseline interferometry (VLBI) mission aimed at precision black hole measurements, detecting the photon ring around black holes, exploring spacetime, spin, and mass properties, and validating predictions of General Relativity. These objectives are achieved using cryogenic receivers with quantum-limited sensitivities across a broad frequency range. Dual-band receivers at 80-106 GHz and 240-320 GHz require operating temperatures of 20 K and 4.5 K, respectively. A cryocooling system with two cold stages will be employed: a 20 K stage handling a 125 mW heat load and a 4.5 K stage handling a 10 mW heat load. To design the cryocooling system, the mission leverages existing space industry technology at high Technology Readiness Levels (TRLs), informed by missions such as Planck, JEM/SMILES, Hitomi, and XRISM, and advancements from the ACTDP/JWST program. Integrating the cryocooler with the receivers and broader instrument involves careful consideration of thermal challenges, including linking the cold ends of each cooling stage to minimize heat losses and ensuring adequate passive cooling for the cryocooler warm end heat rejection. Key challenges and trade-offs include sizing the mass and reducing power consumption while meeting the receiver cold temperature requirements, which impact the scientific objectives. This paper addresses efforts to balance the scientific requirements with the limitations of technical cryocooling capabilities within the framework of a small-class (SMEX) space mission, presenting an overview of cooling needs, initial design considerations, a survey of 4 K spaceflight cryocooler developments, and trade-offs.

We present the Black Hole Explorer (BHEX), a mission that will produce the sharpest images in the history of astronomy by extending submillimeter Very-Long-Baseline Interferometry (VLBI) to space. BHEX will discover and measure the bright and narrow "photon ring" that is predicted to exist in images of black holes, produced from light that has orbited the black hole before escaping. This discovery will expose universal features of a black hole's spacetime that are distinct from the complex astrophysics of the emitting plasma, allowing the first direct measurements of a supermassive black hole's spin. In addition to studying the properties of the nearby supermassive black holes M87* and Sgr A*, BHEX will measure the properties of dozens of additional supermassive black holes, providing crucial insights into the processes that drive their creation and growth. BHEX will also connect these supermassive black holes to their relativistic jets, elucidating the power source for the brightest and most efficient engines in the universe. BHEX will address fundamental open questions in the physics and astrophysics of black holes that cannot be answered without submillimeter space VLBI. The mission is enabled by recent technological breakthroughs, including the development of ultra-high-speed downlink using laser communications, and it leverages billions of dollars of existing ground infrastructure. We present the motivation for BHEX, its science goals and associated requirements, and the pathway to launch within the next decade., Comment: Proceedings for SPIE Astronomical Telescopes and Instrumentation

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. BHEX will co-observe with a ground-based very long baseline interferometric (VLBI) array at high-frequency radio wavelengths, providing unprecedented high resolution with the extension to space that will enable photon ring detection and studies of active galactic nuclei. Science operations require a simultaneous coordination between BHEX and a ground array of large and small radio apertures to provide opportunities for surveys and imaging of radio sources, while coordination with a growing network of optical downlink terminals provides the data rates necessary to build sensitivity on long baselines to space. Here we outline the concept of operations for the hybrid observatory, the available observing modes, the observation planning process, and data delivery to achieve the mission goals and meet mission requirements., Comment: 12 pages, 10 figures, submitted to SPIE Astronomical Telescopes + Instrumentation (AS24)

The Black Hole Explorer (BHEX) is a mission concept that can dramatically improve state-of-the-art astronomical very long baseline interferometry (VLBI) imaging resolution by extending baseline distances to space. To support these scientific goals, a high data rate downlink is required from space to ground. Laser communications is a promising option for realizing these high data rate, long-distance space-to-ground downlinks with smaller space/ground apertures. Here, we present a scalable laser communications downlink design and current lasercom mission results., Comment: Submitted to SPIE Astronomical Telescopes + Instrumentation, 2024

In this paper, we introduce the receiver architecture for the Black Hole Explorer (BHEX) Mission, designed to reveal the photon ring of black holes. The primary instrument is a dual-polarization receiver operating over the 240-320 GHz frequency range, utilizing a Superconductor-Insulator-Superconductor (SIS) mixer. This Double-Side-Band (DSB) receiver has an intermediate frequency (IF) range of 4-12 GHz and operates at a bath temperature of 4.5 K, for optimal performance, which necessitates the integration of a cryocooler. Complementing the primary receiver is a secondary unit covering the 80-106 GHz spectrum, featuring a cryogenic low noise amplifier. This secondary receiver, affixed to the 20 K stage of the cryocooler, serves to augment the SIS receiver performance by employing the Frequency Phase Transfer technique to boost the signal-to-noise ratio at the correlator output. Together, this sophisticated receiver duo is engineered to achieve the quantum-limited sensitivity required to detect the photon ring of black holes, marking a breakthrough in astrophysical observation., Comment: To be presented at the SPIE Space Telescopes and Instrumentation 2024: Optical, Infrared, and Millimeter Waves, June 2024, Yokohama, Japan

The Black Hole Explorer (BHEX) is a next-generation space very long baseline interferometry (VLBI) mission concept that will extend the ground-based millimeter/submillimeter arrays into space. The mission, closely aligned with the science priorities of the Japanese VLBI community, involves an active engagement of this community in the development of the mission, resulting in the formation of the Black Hole Explorer Japan Consortium. Here we present the current Japanese vision for the mission, ranging from scientific objectives to instrumentation. The Consortium anticipates a wide range of scientific investigations, from diverse black hole physics and astrophysics studied through the primary VLBI mode, to the molecular universe explored via a potential single-dish observation mode in the previously unexplored 50-70\,GHz band that would make BHEX the highest-sensitivity explorer ever of molecular oxygen. A potential major contribution for the onboard instrument involves supplying essential elements for its high-sensitivity dual-band receiving system, which includes a broadband 300\,GHz SIS mixer and a space-certified multi-stage 4.5K cryocooler akin to those used in the Hitomi and XRISM satellites by the Japan Aerospace Exploration Agency. Additionally, the Consortium explores enhancing and supporting BHEX operations through the use of millimeter/submillimeter facilities developed by the National Astronomical Observatory of Japan, coupled with a network of laser communication stations operated by the National Institute of Information and Communication Technology., Comment: Submitted to SPIE Conference Series, 17 pages for the main text, 33 pages for the references, 4 figures, SPIE Astronomical Telescopes + Instrumentation 2024

General relativity predicts that black hole images ought to display a bright, thin (and as-of-yet-unresolved) ring. This "photon ring" is produced by photons that explore the strong gravity of the black hole, flowing along trajectories that experience extreme light bending within a few Schwarzschild radii of the horizon before escaping. The shape of the photon ring is largely insensitive to the precise details of the emission from the astronomical source surrounding the black hole and therefore provides a direct probe of the Kerr geometry and its parameters. 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 100 GHz through 300 GHz and from an orbital distance of ~30,000 km. This design will enable measurements of the photon rings around both M87* and Sgr A*, confirming the Kerr nature of these sources and delivering sharp estimates of their masses and spins., Comment: Submitted to SPIE Space Telescopes & Instrumentation

Event Horizon Telescope (EHT) images of the horizon-scale emission around the Galactic Center supermassive black hole Sagittarius A* (Sgr A*) favor accretion flow models with a jet component. However, this jet has not been conclusively detected. Using the "best-bet" models of Sgr A* from the EHT collaboration, we assess whether this non-detection is expected for current facilities and explore the prospects of detecting a jet with VLBI at four frequencies: 86, 115, 230, and 345 GHz. We produce synthetic image reconstructions for current and next-generation VLBI arrays at these frequencies that include the effects of interstellar scattering, optical depth, and time variability. We find that no existing VLBI arrays are expected to detect the jet in these best-bet models, consistent with observations to-date. We show that next-generation VLBI arrays at 86 and 115 GHz -- in particular, the EHT after upgrades through the ngEHT program and the ngVLA -- successfully capture the jet in our tests due to improvements in instrument sensitivity and (u,v) coverage at spatial scales critical to jet detection. These results highlight the potential of enhanced VLBI capabilities in the coming decade to reveal the crucial properties of Sgr A* and its interaction with the Galactic Center environment., Comment: 17 pages, 12 figures, submitted to ApJ

We propose a simple, analytic dual-cone accretion model for horizon scale images of the cores of Low-Luminosity Active Galactic Nuclei (LLAGN), including those observed by the Event Horizon Telescope (EHT). Our underlying model is of synchrotron emission from an axisymmetric, magnetized plasma, which is constrained to flow within two oppositely oriented cones that are aligned with the black hole's spin axis. We show that this model can accurately reproduce images for a variety of time-averaged general relativistic magnetohydrodynamic (GRMHD) simulations, that it accurately recovers both the black hole and emission parameters from these simulations, and that it is sufficiently efficient to be used to measure these parameters in a Bayesian inference framework with radio interferometric data. We show that non-trivial topologies in the source image can result in non-trivial multi-modal solutions when applied to observations from a sparse array, such as the EHT 2017 observations of M87${}^*$. The presence of these degeneracies underscores the importance of employing Bayesian techniques that adequately sample the posterior space for the interpretation of EHT measurements. We fit our model to the EHT observations of M87${}^*$ and find a 95% Highest Posterior Density Interval (HPDI) for the mass-to-distance ratio of $\theta_g\in(2.84,3.75)\,\mu{\rm as}$, and give an inclination of $\theta_{\rm o}\in(11^\circ,24^\circ)$. These new measurements are consistent with mass measurements from the EHT and stellar dynamical estimates (e.g., Gebhardt et al. 2011; EHTC et al. 2019a,b; Liepold et al. 2023), and with the spin axis inclination inferred from properties of the M87${}^*$ jet (e.g., Walker et al. 2018).

Data volumes and rates of research infrastructures will continue to increase in the upcoming years and impact how we interact with their final data products. Little of the processed data can be directly investigated and most of it will be automatically processed with as little user interaction as possible. Capturing all necessary information of such processing ensures reproducibility of the final results and generates trust in the entire process. We present PRAETOR, a software suite that enables automated generation, modelling, and analysis of provenance information of Python pipelines. Furthermore, the evaluation of the pipeline performance, based upon a user defined quality matrix in the provenance, enables the first step of machine learning processes, where such information can be fed into dedicated optimisation procedures., Comment: 4 pages, 3 figures

Very long baseline interferometry (VLBI) provides the highest-resolution images in astronomy. The sharpest resolution is nominally achieved at the highest frequencies, but as the observing frequency increases so too does the atmospheric contribution to the system noise, degrading the sensitivity of the array and hampering detection. In this paper, we explore the limits of high-frequency VLBI observations using ngehtsim, a new tool for generating realistic synthetic data. ngehtsim uses detailed historical atmospheric models to simulate observing conditions, and it employs heuristic visibility detection criteria that emulate single- and multi-frequency VLBI calibration strategies. We demonstrate the fidelity of ngehtsim's predictions using a comparison with existing 230 GHz data taken by the Event Horizon Telescope (EHT), and we simulate the expected performance of EHT observations at 345 GHz. Though the EHT achieves a nearly 100% detection rate at 230 GHz, our simulations indicate that it should expect substantially poorer performance at 345 GHz; in particular, observations of M87 at 345 GHz are predicted to achieve detection rates of $\lesssim$20% that may preclude imaging. Increasing the array sensitivity through wider bandwidths and/or longer integration times -- as enabled through, e.g., the simultaneous multi-frequency upgrades envisioned for the next-generation EHT -- can improve the 345 GHz prospects and yield detection levels that are comparable to those at 230 GHz. M87 and Sgr A* observations carried out in the atmospheric window around 460 GHz could expect to regularly achieve multiple detections on long baselines, but analogous observations at 690 and 875 GHz consistently obtain almost no detections at all., Comment: 29 pages, 12 figures, accepted for publication in ApJ