Daniel C. M. Palumbo and George N. Wong. 4/12/2022. “Photon Ring Symmetries in Simulated Linear Polarization Images of Messier 87*.” The Astrophysical Journal, 929, Pp. 49. Publisher's VersionAbstract
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.
Avery E. Broderick, Paul Tiede, Dominic W. Pesce, and Roman Gold. 3/2/2022. “Measuring Spin from Relative Photon-ring Sizes.” The Astrophysical Journal, 927, 6, Pp. 1-12. Publisher's VersionAbstract

The direct detection of a bright, ring-like structure in horizon-resolving images of M87* by the Event Horizon Telescope (EHT) is a striking validation of general relativity. The angular size and shape of the ring is a degenerate measure of the location of the emission region, mass, and spin of the black hole. However, we show that the observation of multiple rings, corresponding to the low-order photon rings, can break this degeneracy and produce mass and spin measurements independent of the shape of the rings. We describe two potential experiments that would measure the spin. In the first, observations of the direct emission and n = 1 photon ring are made at multiple epochs with different emission locations. This method is conceptually similar to spacetime constraints that arise from variable structures (or hot spots) in that it breaks the near-perfect degeneracy between emission location, mass, and spin for polar observers using temporal variability. In the second, observations of the direct emission and n = 1 and n = 2 photon rings are made during a single epoch. For both schemes, additional observations comprise a test of general relativity. Thus, comparisons of EHT observations in 2017 and 2018 may be capable of producing the first horizon-scale spin estimates of M87* inferred from strong lensing alone. Additional observation campaigns from future high-frequency, Earth-sized, and space-based radio interferometers can produce high-precision tests of general relativity.

Kaushik Satapathy et al. 1/20/2022. “The Variability of the Black Hole Image in M87 at the Dynamical Timescale.” The Astrophysical Journal, 925, 1, Pp. 13. Publisher's VersionAbstract
The black hole images obtained with the Event Horizon Telescope (EHT) are expected to be variable at the dynamical timescale near their horizons. For the black hole at the center of the M87 galaxy, this timescale (5–61 days) is comparable to the 6 day extent of the 2017 EHT observations. Closure phases along baseline triangles are robust interferometric observables that are sensitive to the expected structural changes of the images but are free of station-based atmospheric and instrumental errors. We explored the day-to-day variability in closure-phase measurements on all six linearly independent nontrivial baseline triangles that can be formed from the 2017 observations. We showed that three triangles exhibit very low day-to-day variability, with a dispersion of ∼3°–5°. The only triangles that exhibit substantially higher variability (∼90°–180°) are the ones with baselines that cross the visibility amplitude minima on the u–v plane, as expected from theoretical modeling. We used two sets of general relativistic magnetohydrodynamic simulations to explore the dependence of the predicted variability on various black hole and accretion-flow parameters. We found that changing the magnetic field configuration, electron temperature model, or black hole spin has a marginal effect on the model consistency with the observed level of variability. On the other hand, the most discriminating image characteristic of models is the fractional width of the bright ring of emission. Models that best reproduce the observed small level of variability are characterized by thin ring-like images with structures dominated by gravitational lensing effects and thus least affected by turbulence in the accreting plasmas.
Lia Medeiros, Chi-Kwan Chan, Ramesh Narayan, Feryal Özel, and Dimitrios Psaltis. 1/11/2022. “Brightness Asymmetry of Black Hole Images as a Probe of Observer Inclination.” The Astrophysical Journal, 924, Pp. 46.Abstract
The Event Horizon Telescope recently captured images of the supermassive black hole in the center of the M87 galaxy, which shows a ring-like emission structure with the south side only slightly brighter than the north side. This relatively weak asymmetry in the brightness profile along the ring has been interpreted as a consequence of the low inclination of the observer (around 17° for M87), which suppresses the Doppler beaming and boosting effects that might otherwise be expected due to the nearly relativistic velocities of the orbiting plasma. In this work, we use a large suite of general relativistic magnetohydrodynamic simulations to reassess the validity of this argument. By constructing explicit counterexamples, we show that low inclination is a sufficient but not necessary condition for images to have low brightness asymmetry. Accretion flow models with high accumulated magnetic flux close to the black hole horizon (the so-called magnetically arrested disks) and low black hole spins have angular velocities that are substantially smaller than the orbital velocities of test particles at the same location. As a result, such models can produce images with low brightness asymmetry even when viewed edge on.
Razieh Emami, Richard Anantua, Andrew A. Chael, and Abraham Loeb. 12/28/2021. “Positron Effects on Polarized Images and Spectra from Jet and Accretion Flow Models of M87* and Sgr A*.” The Astrophysical Journal, 923, Pp. 272, 1-27. Publisher's VersionAbstract
We study the effects of including a nonzero positron-to-electron fraction in emitting plasma on the polarized spectral energy distributions and submillimeter images of jet and accretion flow models for near-horizon emission from M87* and Sgr A*. For M87*, we consider a semi-analytic fit to the force-free plasma regions of a general relativistic magnetohydrodynamic jet simulation, which we populate with power-law leptons with a constant electron-to-magnetic pressure ratio. For Sgr A*, we consider a standard self-similar radiatively inefficient accretion flow where the emission is predominantly from thermal leptons with a small fraction in a power-law tail. In both models, we fix the positron-to-electron ratio throughout the emission region. We generate polarized images and spectra from our models using the general relativistic ray tracing and radiative transfer from GRTRANS. We find that a substantial positron fraction reduces the circular polarization fraction at IR and higher frequencies. However, in submillimeter images, higher positron fractions increase polarization fractions due to strong effects of Faraday conversion. We find an M87* jet model that best matches the available broadband total intensity, and 230 GHz polarization data is a sub-equipartition, with positron fraction of ≃10%. We show that jet models with significant positron fractions do not satisfy the polarimetric constraints at 230 GHz from the Event Horizon Telescope (EHT). Sgr A* models show similar trends in their polarization fractions with increasing pair fraction. Both models suggest that resolved, polarized EHT images are useful to constrain the presence of pairs at 230 GHz emitting regions of M87* and Sgr A*.
Maciek Wielgus. 12/20/2021. “Photon rings of spherically symmetric black holes and robust tests of non-Kerr metrics.” Physical Review D, 104, Pp. 124058. Publisher's VersionAbstract

Under very general assumptions on the accretion flow geometry, images of a black hole illuminated by electromagnetic radiation display a sequence of photon rings (demagnified and rotated copies of the direct image) which asymptotically approach a purely theoretical critical curve—the outline of the black hole photon shell. To a distant observer, these images appear dominated by the direct emission, which forms a ring whose diameter is primarily determined by the effective radius of the emitting region. For that reason, connecting the image diameter seen by a distant observer to the properties of the underlying spacetime crucially relies on a calibration that necessarily depends on the assumed astrophysical source model. On the other hand, the diameter of the photon rings depends more on the detailed geometry of the spacetime than on the source structure. As such, a photon ring detection would allow for the spacetime metric to be probed in a less model-dependent way, enabling more robust tests of general relativity and the Kerr hypothesis. Here we present the photon ring structure of several spherically symmetric black hole spacetimes and perform comparisons with the Schwarzschild/Kerr case. We offer our perspective on future tests of the spacetime metric with photon rings, discussing the challenges and opportunities involved.

    Michael Janssen, Heino Falcke, Matthias Kadler, Eduardo Ros, and EHT the Collaboration. 7/19/2021. “Event Horizon Telescope observations of the jet launching and collimation in Centaurus A.” Nature Astronomy, 5, Pp. 1017-1028. Publisher's VersionAbstract
    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 (rg ≡ GM/c2) scales in nearby sources. Centaurus A is the closest radio-loud source to Earth. 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 thus 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, 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 rg 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 shadow4 should be visible at terahertz frequencies. This location further supports the universal scale invariance of black holes over a wide range of masses.
    Prashant Kocherlakota, Luciano Rezzolla, and Event Horizon Telescope Collaboration. 5/20/2021. “Constraints on non-Einsteinian black-hole charges with the 2019 EHT observations of M87.” Physical Review D, 103, 10, Pp. 40-47. Publisher's Version
    J. C. Algaba et al. 4/14/2021. “Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign.” ApJL , 911, Pp. L11. Publisher's VersionAbstract
    In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 109M. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
    EHT Collaboration et al. 3/24/2021. “First M87 Event Horizon Telescope Results. VII. Polarization of the Ring.” ApJL, 910, L12, Pp. 48. Publisher's Version
    EHT Collaboration et al. 3/24/2021. “First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon.” ApJL, 910, L13, Pp. 43. Publisher's Version
    C. Goddi et al. 3/24/2021. “Polarimetric Properties of Event Horizon Telescope Targets from ALMA.” ApJL, 910, L14, Pp. 54. Publisher's Version
    D. Psaltis et al. 10/2020. “Gravitational Test beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole.” Physical Review Letters, 125, 14, Pp. 141104. Publisher's Version
    T. Bronzwaer et al. 9/2020. “RAPTOR. II. Polarized radiative transfer in curved spacetime.” Astronomy & Astrophysics, 641, A126, Pp. 13. Publisher's Version
    M. Wielgus et al. 9/2020. “Monitoring the Morphology of M87* in 2009-2017 with the Event Horizon Telescope.” Astrophysical Journal, 901, 1, Pp. 28. Publisher's Version
    E. Kravchenko et al. 5/2020. “Linear polarization in the nucleus of M87 at 7 mm and 1.3 cm.” Astronomy & Astrophysics, 637, L6, Pp. 9. Publisher's Version
    J. Dexter et al. 5/2020. “A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating.” Monthly Notices of the Royal Astroonomical Society, 494, 3, Pp. 4168-4186. Publisher's Version
    F. Roelofs et al. 4/2020. “SYMBA: An end-to-end VLBI synthetic data generation pipeline. Simulating Event Horizon Telescope observations of M 87 .” Astronomy & Astrophysics, 636, A5, Pp. 19. Publisher's Version
    M. Johnson et al. 2020. “Universal interferometric signatures of a black hole's photon ring.” Science Advances, 6, 12, Pp. eaaz1310. Publisher's Version
    B. Jeter et al. 2020. “Differentiating Disk and Black Hole Driven Jets with EHT Images of Variability in M87.” MNRAS, 493, 5606. Publisher's Version