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1. First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way

   https://doi.org/10.3847/2041-8213/ac6674  

   Akiyama, Kazunori ; Alberdi, Antxon ; Alef, Walter ; Algaba, Juan Carlos ; Anantua, Richard ; Asada, Keiichi ; Azulay, Rebecca ; Bach, Uwe ; Baczko, Anne-Kathrin ; Ball, David ; et al ( May 2022 , The Astrophysical Journal Letters)

   Abstract We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength of λ = 1.3 mm. The EHT data resolve a compact emission region with intrahour variability. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of 51.8 ± 2.3 μ as (68% credible interval). The ring has modest azimuthal brightness asymmetry and a comparativelymore »dim interior. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ∼4 × 10 6 M ⊙ , which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination ( i > 50°), as well as nonspinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence of a supermassive black hole at the center of the Milky Way, and for the first time we connect the predictions from dynamical measurements of stellar orbits on scales of 10 3 –10 5 gravitational radii to event-horizon-scale images and variability. Furthermore, a comparison with the EHT results for the supermassive black hole M87* shows consistency with the predictions of general relativity spanning over three orders of magnitude in central mass.« less
2. Toward Determining the Number of Observable Supermassive Black Hole Shadows

   https://doi.org/10.3847/1538-4357/ac2eb5  

   Pesce, Dominic W. ; Palumbo, Daniel C. ; Narayan, Ramesh ; Blackburn, Lindy ; Doeleman, Sheperd S. ; Johnson, Michael D. ; Ma, Chung-Pei ; Nagar, Neil M. ; Natarajan, Priyamvada ; Ricarte, Angelo ( December 2021 , The Astrophysical Journal)

   Abstract We present estimates for the number of shadow-resolved supermassive black hole (SMBH) systems that can be detected using radio interferometers, as a function of angular resolution, flux density sensitivity, and observing frequency. Accounting for the distribution of SMBHs across mass, redshift, and accretion rate, we use a new semianalytic spectral energy distribution model to derive the number of SMBHs with detectable and optically thin horizon-scale emission. We demonstrate that (sub)millimeter interferometric observations with ∼0.1 μ as resolution and ∼1 μ Jy sensitivity could access >10 6 SMBH shadows. We then further decompose the shadow source counts into the numbermore »of black holes for which we could expect to observe the first- and second-order lensed photon rings. Accessing the bulk population of first-order photon rings requires ≲2 μ as resolution and ≲0.5 mJy sensitivity, whereas doing the same for second-order photon rings requires ≲0.1 μ as resolution and ≲5 μ Jy sensitivity. Our model predicts that with modest improvements to sensitivity, as many as ∼5 additional horizon-resolved sources should become accessible to the current Event Horizon Telescope (EHT), whereas a next-generation EHT observing at 345 GHz should have access to ∼3 times as many sources. More generally, our results can help guide enhancements of current arrays and specifications for future interferometric experiments that aim to spatially resolve a large population of SMBH shadows or higher-order photon rings.« less
3. Event Horizon Telescope observations of the jet launching and collimation in Centaurus A

   https://doi.org/10.1038/s41550-021-01417-w  

   Janssen, Michael ; Falcke, Heino ; Kadler, Matthias ; Ros, Eduardo ; Wielgus, Maciek ; Akiyama, Kazunori ; Baloković, Mislav ; Blackburn, Lindy ; Bouman, Katherine L. ; Chael, Andrew ; et al ( July 2021 , Nature Astronomy)

   null (Ed.)

   Abstract Very-long-baseline interferometry (VLBI) observations of active galactic nuclei at millimetre wavelengths have the power to reveal the launching and initial collimation region of extragalactic radio jets, down to 10–100 gravitational radii ( r g  ≡  G M / c 2 ) scales in nearby sources 1 . Centaurus A is the closest radio-loud source to Earth 2 . It bridges the gap in mass and accretion rate between the supermassive black holes (SMBHs) in Messier 87 and our Galactic Centre. A large southern declination of −43° has, however, prevented VLBI imaging of Centaurus A below a wavelength of 1 cm thusmore »far. Here we show the millimetre VLBI image of the source, which we obtained with the Event Horizon Telescope at 228 GHz. Compared with previous observations 3 , we image the jet of Centaurus A at a tenfold higher frequency and sixteen times sharper resolution and thereby probe sub-lightday structures. We reveal a highly collimated, asymmetrically edge-brightened jet as well as the fainter counterjet. We find that the source structure of Centaurus A resembles the jet in Messier 87 on ~500  r g scales remarkably well. Furthermore, we identify the location of Centaurus A’s SMBH with respect to its resolved jet core at a wavelength of 1.3 mm and conclude that the source’s event horizon shadow 4 should be visible at terahertz frequencies. This location further supports the universal scale invariance of black holes over a wide range of masses 5,6 .« less
4. Polarization whorls from M87* at the event horizon telescope

   https://doi.org/10.1098/rspa.2019.0618  

   Lupsasca, Alexandru ; Kapec, Daniel ; Shi, Yichen ; Gates, Delilah E. ; Strominger, Andrew ( May 2020 , Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   The event horizon telescope (EHT) is expected to soon produce polarimetric images of the supermassive black hole at the centre of the neighbouring galaxy M87. There are indications that this black hole is rapidly spinning. General relativity predicts that such a high-spin black hole has an emergent conformal symmetry near its event horizon. In this paper, we use this symmetry to analytically predict the polarized near-horizon emissions to be seen at the EHT and find a distinctive pattern of whorls aligned with the spin.
5. SYMBA: An end-to-end VLBI synthetic data generation pipeline: Simulating Event Horizon Telescope observations of M 87

   https://doi.org/10.1051/0004-6361/201936622  

   Roelofs, F. ; Janssen, M. ; Natarajan, I. ; Deane, R. ; Davelaar, J. ; Olivares, H. ; Porth, O. ; Paine, S. N. ; Bouman, K. L. ; Tilanus, R. P. ; et al ( April 2020 , Astronomy & Astrophysics)

   Context. Realistic synthetic observations of theoretical source models are essential for our understanding of real observational data. In using synthetic data, one can verify the extent to which source parameters can be recovered and evaluate how various data corruption effects can be calibrated. These studies are the most important when proposing observations of new sources, in the characterization of the capabilities of new or upgraded instruments, and when verifying model-based theoretical predictions in a direct comparison with observational data. Aims. We present the SYnthetic Measurement creator for long Baseline Arrays ( SYMBA ), a novel synthetic data generation pipeline formore »Very Long Baseline Interferometry (VLBI) observations. SYMBA takes into account several realistic atmospheric, instrumental, and calibration effects. Methods. We used SYMBA to create synthetic observations for the Event Horizon Telescope (EHT), a millimetre VLBI array, which has recently captured the first image of a black hole shadow. After testing SYMBA with simple source and corruption models, we study the importance of including all corruption and calibration effects, compared to the addition of thermal noise only. Using synthetic data based on two example general relativistic magnetohydrodynamics (GRMHD) model images of M 87, we performed case studies to assess the image quality that can be obtained with the current and future EHT array for different weather conditions. Results. Our synthetic observations show that the effects of atmospheric and instrumental corruptions on the measured visibilities are significant. Despite these effects, we demonstrate how the overall structure of our GRMHD source models can be recovered robustly with the EHT2017 array after performing calibration steps, which include fringe fitting, a priori amplitude and network calibration, and self-calibration. With the planned addition of new stations to the EHT array in the coming years, images could be reconstructed with higher angular resolution and dynamic range. In our case study, these improvements allowed for a distinction between a thermal and a non-thermal GRMHD model based on salient features in reconstructed images.« less