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1. The Event Horizon Science Gateway

   https://doi.org/10.1145/3626203.3670585  

   Quick, Rob; Gokpinar_Shelton, Esen; Wang, Jun; West, Andrew (July 2024, ACM)
2. Rotation in Event Horizon Telescope Movies

   https://doi.org/10.3847/1538-4357/acd2c8  

   Conroy, Nicholas S.; Bauböck, Michi; Dhruv, Vedant; Lee, Daeyoung; Broderick, Avery E.; Chan, Chi-kwan; Georgiev, Boris; Joshi, Abhishek V.; Prather, Ben; Gammie, Charles F. (July 2023, The Astrophysical Journal)

   Abstract The Event Horizon Telescope (EHT) has produced images of M87* and Sagittarius A*, and will soon produce time sequences of images, or movies. In anticipation of this, we describe a technique to measure the rotation rate, or pattern speed Ω_p, from movies using an autocorrelation technique. We validate the technique on Gaussian random field models with a known rotation rate and apply it to a library of synthetic images of Sgr A* based on general relativistic magnetohydrodynamics simulations. We predict that EHT movies will have Ω_p≈ 1° perGMc⁻³, which is of order 15% of the Keplerian orbital frequency in the emitting region. We can plausibly attribute the slow rotation seen in our models to the pattern speed of inward-propagating spiral shocks. We also find that Ω_pdepends strongly on inclination. Application of this technique will enable us to compare future EHT movies with the clockwise rotation of Sgr A* seen in near-infrared flares by GRAVITY. Pattern speed analysis of future EHT observations of M87* and Sgr A* may also provide novel constraints on black hole inclination and spin, as well as an independent measurement of black hole mass. 

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3. First M87 Event Horizon Telescope Results. IX. Detection of Near-horizon Circular Polarization

   https://doi.org/10.3847/2041-8213/acff70  

   Akiyama, Kazunori; Alberdi, Antxon; Alef, Walter; Algaba, Juan Carlos; Anantua, Richard; Asada, Keiichi; Azulay, Rebecca; Bach, Uwe; Baczko, Anne-Kathrin; Ball, David; et al (November 2023, The Astrophysical Journal Letters)

   Abstract Event Horizon Telescope (EHT) observations have revealed a bright ring of emission around the supermassive black hole at the center of the M87 galaxy. EHT images in linear polarization have further identified a coherent spiral pattern around the black hole, produced from ordered magnetic fields threading the emitting plasma. Here we present the first analysis of circular polarization using EHT data, acquired in 2017, which can potentially provide additional insights into the magnetic fields and plasma composition near the black hole. Interferometric closure quantities provide convincing evidence for the presence of circularly polarized emission on event-horizon scales. We produce images of the circular polarization using both traditional and newly developed methods. All methods find a moderate level of resolved circular polarization across the image (〈∣v∣〉 < 3.7%), consistent with the low image-integrated circular polarization fraction measured by the Atacama Large Millimeter/submillimeter Array (∣v_int∣ < 1%). Despite this broad agreement, the methods show substantial variation in the morphology of the circularly polarized emission, indicating that our conclusions are strongly dependent on the imaging assumptions because of the limited baseline coverage, uncertain telescope gain calibration, and weakly polarized signal. We include this upper limit in an updated comparison to general relativistic magnetohydrodynamic simulation models. This analysis reinforces the previously reported preference for magnetically arrested accretion flow models. We find that most simulations naturally produce a low level of circular polarization consistent with our upper limit and that Faraday conversion is likely the dominant production mechanism for circular polarization at 230 GHz in M87*. 

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4. Testing general relativity with the Event Horizon Telescope

   https://doi.org/10.1007/s10714-019-2611-5  

   Psaltis, Dimitrios (October 2019, General Relativity and Gravitation)
5. Variational Image Feature Extraction for the Event Horizon Telescope

   https://doi.org/10.3847/1538-4357/ac3a6b  

   Tiede, Paul; Broderick, Avery E.; Palumbo, Daniel C. (January 2022, The Astrophysical Journal)

   Abstract Imaging algorithms form powerful analysis tools for very long baseline interferometry (VLBI) data analysis. However, these tools cannot measure certain image features (e.g., ring diameter) by their nonparametric nature. This is unfortunate since these image features are often related to astrophysically relevant quantities such as black hole mass. This paper details a new general image feature-extraction technique that applies to a wide variety of VLBI image reconstructions called variational image domain analysis . Unlike previous tools, variational image domain analysis can be applied to any image reconstruction regardless of its structure. To demonstrate its flexibility, we analyze thousands of reconstructions from previous Event Horizon Telescope synthetic data sets and recover image features such as diameter, orientation, and ellipticity. By measuring these features, our technique can help extract astrophysically relevant quantities such as the mass and orientation of the central black hole in M87. 

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