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1. Bayesian Accretion Modeling: Axisymmetric Equatorial Emission in the Kerr Spacetime

   https://doi.org/10.3847/1538-4357/ac9ab7  

   Palumbo, Daniel C.; Gelles, Zachary; Tiede, Paul; Chang, Dominic O.; Pesce, Dominic W.; Chael, Andrew; Johnson, Michael D. (November 2022, The Astrophysical Journal)

   Abstract The Event Horizon Telescope (EHT) has produced images of two supermassive black holes, Messier 87* (M 87*) and Sagittarius A* (Sgr A*). The EHT collaboration used these images to indirectly constrain black hole parameters by calibrating measurements of the sky-plane emission morphology to images of general relativistic magnetohydrodynamic (GRMHD) simulations. Here, we develop a model for directly constraining the black hole mass, spin, and inclination through signatures of lensing, redshift, and frame dragging, while simultaneously marginalizing over the unknown accretion and emission properties. By assuming optically thin, axisymmetric, equatorial emission near the black hole, our model gains orders of magnitude in speed over similar approaches that require radiative transfer. Using 2017 EHT M 87* baseline coverage, we use fits of the model to itself to show that the data are insufficient to demonstrate existence of the photon ring. We then survey time-averaged GRMHD simulations fitting EHT-like data, and find that our model is best-suited to fitting magnetically arrested disks, which are the favored class of simulations for both M 87* and Sgr A*. For these simulations, the best-fit model parameters are within ∼10% of the true mass and within ∼10° for inclination. With 2017 EHT coverage and 1% fractional uncertainty on amplitudes, spin is unconstrained. Accurate inference of spin axis position angle depends strongly on spin and electron temperature. Our results show the promise of directly constraining black hole spacetimes with interferometric data, but they also show that nearly identical images permit large differences in black hole properties, highlighting degeneracies between the plasma properties, spacetime, and, most crucially, the unknown emission geometry when studying lensed accretion flow images at a single frequency. 

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2. The ngEHT’s Role in Measuring Supermassive Black Hole Spins

   https://doi.org/10.3390/galaxies11010006  

   Ricarte, Angelo; Tiede, Paul; Emami, Razieh; Tamar, Aditya; Natarajan, Priyamvada (February 2023, Galaxies)

   While supermassive black-hole masses have been cataloged across cosmic time, only a few dozen of them have robust spin measurements. By extending and improving the existing Event Horizon Telescope (EHT) array, the next-generation Event Horizon Telescope (ngEHT) will enable multifrequency, polarimetric movies on event-horizon scales, which will place new constraints on the space-time and accretion flow. By combining this information, it is anticipated that the ngEHT may be able to measure tens of supermassive black-hole masses and spins. In this white paper, we discuss existing spin measurements and many proposed techniques with which the ngEHT could potentially measure spins of target supermassive black holes. Spins measured by the ngEHT would represent a completely new sample of sources that, unlike pre-existing samples, would not be biased towards objects with high accretion rates. Such a sample would provide new insights into the accretion, feedback, and cosmic assembly of supermassive black holes. 

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3. Ring Asymmetry and Spin in M87*

   https://doi.org/10.3847/1538-4357/ae34af  

   Bernshteyn, Vadim; Conroy, Nicholas S; Bauböck, Michi; Tiede, Paul; Joshi, Abhishek V; Prather, Ben S; Gammie, Charles F; Akiyama, Kazunori; Albentosa-Ruíz, Ezequiel; Alberdi, Antxon; et al (March 2026, The Astrophysical Journal)

   Abstract Event Horizon Telescope (EHT) images of the supermassive black hole M87* depict an asymmetric ring of emission. General relativistic magnetohydrodynamic (GRMHD) models of M87* and its accretion disk predict that the amplitude and location of the ring’s peak brightness asymmetry should fluctuate due to turbulence in the source plasma. We compare the observed distribution of brightness asymmetry amplitudes to the simulated distribution in GRMHD models, across varying black hole spina_*. We show that, for strongly magnetized (MAD) models, three epochs of EHT data marginally disfavor ∣a_*∣ ≲ 0.2. This is consistent with the Blandford–Znajek model for M87’s jet, which predicts that M87* should have nonzero spin. We show quantitatively how future observations could improve spin constraints and discuss how improved spin constraints could distinguish between differing jet-launching mechanisms and black hole growth scenarios. 

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4. Assessing the Role of Intrinsic Variability in Black Hole Parameter Inference Using Multiepoch EHT Data

   https://doi.org/10.3847/2041-8213/ade431  

   Chang, Dominic O; Johnson, Michael D; Tiede, Paul (August 2025, The Astrophysical Journal Letters)

   Abstract Event Horizon Telescope (EHT) observations of M87* provide a means of constraining the parameters of both the black hole and its surrounding plasma. However, the intrinsic variability of the emitting material introduces major sources of uncertainty, which complicates parameter inference. The precise nature of this variability remains uncertain, and previous studies have largely relied on general relativistic magnetohydrodynamic simulations to estimate its effects. Here, we fit a semianalytic, dual-cone model of the emitting plasma to multiple years of EHT observations to empirically assess the impact of intrinsic variability and improved array coverage on key measurements, including the black hole mass-to-distance ratio, spin, and viewing inclination. Despite substantial differences in the images of the two epochs, we find that the inferred mass-to-distance ratio remains stable and mutually consistent. The black hole spin is unconstrained for both observations, despite the improved baseline coverage in 2018. We show that intrinsic variability can contribute significantly to the inference error and that the inferred position angle and inclination of the black hole spin axis are discrepant between the two years. Our findings highlight both the promise and challenges of multiepoch EHT observations: while they can refine parameter constraints, they also reveal the limitations of simple parametric models in capturing the full source complexity. Our analysis—the first to fit semianalytic emission models to 2018 EHT observations—underscores the importance of quantifying data contributions from intrinsic variability in future high-resolution imaging studies of black hole environments and the role of repeated observations in quantifying these uncertainties. 

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5. Photon Ring Symmetries in Simulated Linear Polarization Images of Messier 87*

   https://doi.org/10.3847/1538-4357/ac59b4  

   Palumbo, Daniel C.; Wong, George N. (April 2022, The Astrophysical Journal)

   Abstract The Event Horizon Telescope (EHT) recently released the first linearly polarized images of the accretion flow around the supermassive black hole Messier 87*, hereafter M87*. The spiraling polarization pattern found in the EHT images favored magnetically arrested disks as the explanation for the EHT image. With next-generation improvements to very long baseline interferometry on the horizon, understanding similar polarized features in the highly lensed structure known as the “photon ring,” where photons make multiple half orbits about the black hole before reaching the observer, will be critical to the analysis of future images. Recent work has indicated that this image region may be depolarized relative to more direct emission. We expand this observation by decomposing photon half orbits in the EHT library of simulated images of the M 87* accretion system and find that images of magnetically arrested disk simulations show a relative depolarization of the photon ring attributable to destructive interference of oppositely spiraling electric field vectors; this antisymmetry, which arises purely from strong gravitational lensing, can produce up to ∼50% depolarization in the photon ring region with respect to the direct image. In systems that are not magnetically arrested and with the exception of systems with high spin and ions and electrons of equal temperature, we find that highly lensed indirect subimages are almost completely depolarized, causing a modest depolarization of the photon ring region in the complete image. We predict that next-generation EHT observations of M 87* polarization should jointly constrain the black hole spin and the underlying emission and magnetic field geometry. 

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