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1. Multifrequency Synthesis via CHIBI: Colorful Hierarchical Interferometric Bayesian Imaging

   https://doi.org/10.3847/1538-4357/ae75a9  

   Chavez, Erandi; Tiede, Paul; Issaoun, Sara; Johnson, Michael D; Pesce, Dominic W; Tsunetoe, Yuh; Palumbo, Daniel_C M (July 2026, The Astrophysical Journal)

   Abstract From magnetized plasma of relativistic jets to dust grains within protoplanetary disks, we study the emission mechanisms of radio sources via their rich spectral structure. Multifrequency synthesis (MFS) is a technique in which interferometric data at multiple frequencies are imaged simultaneously, resulting in a denser sampling of spatial scales, higher imaging fidelity, and tighter constraints on the source’s spectral structure and evolution. We describe a new method of MFS imaging reconstruction in a hierarchical interferometric Bayesian inference framework, CHIBI. The model parameterization is based on the spectral behavior of synchrotron radiation, the emission mechanism dominating the radio emission observed from galactic nuclei. We show results of this method on observations of jet sources from the MOJAVE catalog with the Very Long Baseline Array, and showcase the prospects for MFS imaging of M87* with simulated data from the Event Horizon Telescope (EHT) and future expansions such as the next-generation EHT and the black hole Explorer. These demonstrations highlight the benefit of MFS to reconstruct higher-fidelity images and spectral index maps, producing scientifically richer results in a statistically grounded framework, implemented inComrade.jl. 

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2. The ngEHT Analysis Challenges

   https://doi.org/10.3390/galaxies11010012  

   Roelofs, Freek; Blackburn, Lindy; Lindahl, Greg; Doeleman, Sheperd S.; Johnson, Michael D.; Arras, Philipp; Chatterjee, Koushik; Emami, Razieh; Fromm, Christian; Fuentes, Antonio; et al (February 2023, Galaxies)

   The next-generation Event Horizon Telescope (ngEHT) will be a significant enhancement of the Event Horizon Telescope (EHT) array, with ∼10 new antennas and instrumental upgrades of existing antennas. The increased uv-coverage, sensitivity, and frequency coverage allow a wide range of new science opportunities to be explored. The ngEHT Analysis Challenges have been launched to inform the development of the ngEHT array design, science objectives, and analysis pathways. For each challenge, synthetic EHT and ngEHT datasets are generated from theoretical source models and released to the challenge participants, who analyze the datasets using image reconstruction and other methods. The submitted analysis results are evaluated with quantitative metrics. In this work, we report on the first two ngEHT Analysis Challenges. These have focused on static and dynamical models of M87* and Sgr A* and shown that high-quality movies of the extended jet structure of M87* and near-horizon hourly timescale variability of Sgr A* can be reconstructed by the reference ngEHT array in realistic observing conditions using current analysis algorithms. We identify areas where there is still room for improvement of these algorithms and analysis strategies. Other science cases and arrays will be explored in future challenges. 

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3. Theoretical Foundation of Black Hole Image Reconstruction Using PRIMO

   https://doi.org/10.3847/1538-4357/ada60f  

   Psaltis, Dimitrios; Özel, Feryal; Medeiros, Lia; Lauer, Tod R (April 2025, The Astrophysical Journal)

   Abstract A new image-reconstruction algorithm, Principal-component Interferometric Modeling (PRIMO), applied to the interferometric data of the M87 black hole collected with the Event Horizon Telescope (EHT), resulted in an image that reached the native resolution of the telescope array.PRIMOis based on learning a compact set of image building blocks obtained from a large library of high-fidelity, physics-based simulations of black hole images. It uses these building blocks to fill the sparse Fourier coverage of the data that results from the small number of telescopes in the array. In this paper, we show that this approach is readily justified. Since the angular extent of the image of the black hole and of its inner accretion flow is finite, the Fourier space domain is heavily smoothed, with a correlation scale that is at most comparable to the sizes of the data gaps in the coverage of Fourier space with the EHT. Consequently,PRIMOor other machine learning algorithms can faithfully reconstruct the images without the need to generate information that is unconstrained by the data within the resolution of the array. We also address the completeness of the eigenimages and the compactness of the resulting representation. We show thatPRIMOprovides a compact set of eigenimages that have sufficient complexity to recreate a broad set of images well beyond those in the training set. 

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4. Key Science Goals for the Next-Generation Event Horizon Telescope

   https://doi.org/10.3390/galaxies11030061  

   Johnson, Michael D.; Akiyama, Kazunori; Blackburn, Lindy; Bouman, Katherine L.; Broderick, Avery E.; Cardoso, Vitor; Fender, Rob P.; Fromm, Christian M.; Galison, Peter; Gómez, José L.; et al (June 2023, Galaxies)

   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. 

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5. Probing plasma physics with spectral index maps of accreting black holes on event horizon scales

   https://doi.org/10.1093/mnras/stac3796  

   Ricarte, Angelo; Gammie, Charles; Narayan, Ramesh; Prather, Ben S (January 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The Event Horizon Telescope (EHT) collaboration has produced the first resolved images of the supermassive black holes at the centre of our galaxy and at the centre of the elliptical galaxy M87. As both technology and analysis pipelines improve, it will soon become possible to produce spectral index maps of black hole accretion flows on event horizon scales. In this work, we predict spectral index maps of both M87* and Sgr A* by applying the general relativistic radiative transfer (GRRT) code ipole to a suite of general relativistic magnetohydrodynamic (GRMHD) simulations. We analytically show that the spectral index increases with increasing magnetic field strength, electron temperature, and optical depth. Consequently, spectral index maps grow more negative with increasing radius in almost all models, since all of these quantities tend to be maximized near the event horizon. Additionally, photon ring geodesics exhibit more positive spectral indices, since they sample the innermost regions of the accretion flow with the most extreme plasma conditions. Spectral index maps are sensitive to highly uncertain plasma heating prescriptions (the electron temperature and distribution function). However, if our understanding of these aspects of plasma physics can be tightened, even the spatially unresolved spectral index around 230 GHz can be used to discriminate between models. In particular, Standard and Normal Evolution (SANE) flows tend to exhibit more negative spectral indices than Magnetically Arrested Disc (MAD) flows due to differences in the characteristic magnetic field strength and temperature of emitting plasma. 

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