%0 Journal Article %A et D. Psaltis al. %B ApJ %G eng %T A Model for Anisotropic Interstellar Scattering and its Application to Sgr A* %0 Journal Article %A et S. Issaoun al. %B ApJ %G eng %T Persistent Non-Gaussian Structure in the Image of Sagittarius A* at 86 GHz %0 Journal Article %A Feng-Li Lin %A Avani Patel %A Hung-Yi Pu %G eng %T Black Hole Shadow with Soft Hair %U https://arxiv.org/abs/2202.13559 %X Light bending by the strong gravity around the black hole will form the so-called black hole shadow, the shape of which can shed light on the structure of the near-horizon geometry to possibly reveal novel physics of strong gravity and black hole. In this work, we adopt both analytical and ray-tracing methods to study the black hole shadow in the presence of the infrared structure of gravity theory, which manifests the asymptotic symmetries of spacetime as the supertranslation soft hairs of the black hole. Though the black hole metrics with and without the soft hair are related by large gauge transformations, the near horizon geometries relevant for the shape of the shadow are quite different. Moreover, the Hamiltonian for the geodesic seems intrinsically different, i.e., the loss of separability due to the breaking of spherical symmetry by soft hair. By applying ray-tracing computations, we find that the soft hair, although not affecting the shape of the shadow, may change the average size and position of the shadow. Images resulting from soft hair black holes with surrounding accretion flows are also discussed.
  %0 Journal Article %A Jesse Daas %A Kolja Kuijpers %A Frank Saueressig %A Michael F. Wondrak %A Heino Falcke %G eng %T Probing Quadratic Gravity with the Event Horizon Telescope %U https://arxiv.org/abs/2204.08480 %X Quadratic gravity constitutes a prototypical example of a perturbatively renormalizable quantum theory of the gravitational interactions. In this work, we construct the associated phase space of static, spherically symmetric, and asymptotically flat spacetimes. It is found that the Schwarzschild geometry is embedded in a rich solution space comprising horizonless, naked singularities and wormhole solutions. Characteristically, the deformed solutions follow the Schwarzschild solution up outside of the photon sphere while they differ substantially close to the center of gravity. We then carry out an analytic analysis of observable signatures accessible to the Event Horizon Telescope, comprising the size of the black hole shadow as well as the radiation emitted by infalling matter. On this basis, we argue that it is the brightness within the shadow region which constrains the phase space of solutions. Our work constitutes the first step towards bounding the phase space of black hole type solutions with a clear quantum gravity interpretation based on observational data. %0 Journal Article %A Angelo Ricarte %A Charles Gammie %A Ramesh Narayan %A Ben S. Prather %B Monthly Notices of the Royal Astronomical Society %G eng %T Probing Plasma Physics with Spectral Index Maps of Accreting Black Holes on Event Horizon Scales %U https://arxiv.org/abs/2202.02408 %X

The Event Horizon Telescope (EHT) collaboration has produced the first resolved images of M87*, the supermassive black hole 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 {\sc ipole} to a suite of general relativistic magnetohydrodynamic (GRMHD) simulations. We analytically explore how 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 Disk (MAD) flows due to differences in the characteristic magnetic field strength and temperature of emitting plasma.

%0 Journal Article %A Joshua Yao-Yu Lin %A Dominic W. Pesce %A George N. Wong %A Ajay Uppili Arasanipalai %A Ben S. Prather %A Charles F. Gammie %G eng %T VLBInet: Radio Interferometry Data Classification for EHT with Neural Networks %U https://arxiv.org/abs/2110.07185 %X

The Event Horizon Telescope (EHT) recently released the first horizon-scale images of the black hole in M87. Combined with other astronomical data, these images constrain the mass and spin of the hole as well as the accretion rate and magnetic flux trapped on the hole. An important question for the EHT is how well key parameters, such as trapped magnetic flux and the associated disk models, can be extracted from present and future EHT VLBI data products. The process of modeling visibilities and analyzing them is complicated by the fact that the data are sparsely sampled in the Fourier domain while most of the theory/simulation is constructed in the image domain. Here we propose a data-driven approach to analyze complex visibilities and closure quantities for radio interferometric data with neural networks. Using mock interferometric data, we show that our neural networks are able to infer the accretion state as either high magnetic flux (MAD) or low magnetic flux (SANE), suggesting that it is possible to perform parameter extraction directly in the visibility domain without image reconstruction. We have applied VLBInet to real M87 EHT data taken on four different days in 2017 (April 5, 6, 10, 11), and our neural networks give a score prediction 0.52, 0.4, 0.43, 0.76 for each day, with an average score 0.53, which shows no significant indication for the data to lean toward either the MAD or SANE state.

%0 Journal Article %A He Sun %A Katherine L. Bouman %A Paul Tiede %A Jason J. Wang %A Sarah Blunt %A Dimitri Mawet %G eng %T alpha-Deep Probabilistic Inference (alpha-DPI): efficient uncertainty quantification from exoplanet astrometry to black hole feature extraction %U https://arxiv.org/abs/2201.08506 %X

Inference is crucial in modern astronomical research, where hidden astrophysical features and patterns are often estimated from indirect and noisy measurements. Inferring the posterior of hidden features, conditioned on the observed measurements, is essential for understanding the uncertainty of results and downstream scientific interpretations. Traditional approaches for posterior estimation include sampling-based methods and variational inference. However, sampling-based methods are typically slow for high-dimensional inverse problems, while variational inference often lacks estimation accuracy. In this paper, we propose alpha-DPI, a deep learning framework that first learns an approximate posterior using alpha-divergence variational inference paired with a generative neural network, and then produces more accurate posterior samples through importance re-weighting of the network samples. It inherits strengths from both sampling and variational inference methods: it is fast, accurate, and scalable to high-dimensional problems. We apply our approach to two high-impact astronomical inference problems using real data: exoplanet astrometry and black hole feature extraction.

%0 Journal Article %A et Kaushik Satapathy al. %B The Astrophysical Journal %D 2022 %G eng %N 1 %P 13 %T The Variability of the Black Hole Image in M87 at the Dynamical Timescale %U https://doi.org/10.3847/1538-4357/ac332e %V 925 %X 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. %0 Journal Article %A George N. Wong %A Ben S. Prather %A Vedant Dhruv %A Benjamin R. Ryan %A Monika Mościbrodzka %A Chi-Kwan Chan %A Abhishek V. Joshi %A Ricardo Yarza %A Angelo Ricarte %A Hotaka Shiokawa %A Joshua C. Dolence %A Scott C. Noble %A Jonathan C. McKinney %A Charles F. Gammie %B The Astrophysical Journal Supplement Series %D 2022 %G eng %N 2, 64 %T PATOKA: Simulating Electromagnetic Observables of Black Hole Accretion %U https://doi.org/10.3847/1538-4365/ac582e %V 259 %X The Event Horizon Telescope (EHT) has released analyses of reconstructed images of horizon-scale millimeter emission near the supermassive black hole at the center of the M87 galaxy. Parts of the analyses made use of a large library of synthetic black hole images and spectra, which were produced using numerical general relativistic magnetohydrodynamics fluid simulations and polarized ray tracing. In this article, we describe the PATOKA pipeline, which was used to generate the Illinois contribution to the EHT simulation library. We begin by describing the relevant accretion systems and radiative processes. We then describe the details of the three numerical codes we use, iharm, ipole, and igrmonty, paying particular attention to differences between the current generation of the codes and the originally published versions. Finally, we provide a brief overview of simulated data as produced by PATOKA and conclude with a discussion of limitations and future directions. %0 Journal Article %A Daniel C. M. Palumbo %A George N. Wong %B The Astrophysical Journal %D 2022 %G eng %P 49 %T Photon Ring Symmetries in Simulated Linear Polarization Images of Messier 87* %U https://doi.org/10.3847/1538-4357/ac59b4 %V 929 %X 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. %0 Journal Article %A Lia Medeiros %A Chi-Kwan Chan %A Ramesh Narayan %A Feryal Özel %A Dimitrios Psaltis %B The Astrophysical Journal %D 2022 %G eng %P 46 %T Brightness Asymmetry of Black Hole Images as a Probe of Observer Inclination %V 924 %X 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. %0 Journal Article %A Joseph Farah %A Peter Galison %A Kazunori Akiyama %A Katherine L. Bouman %A Geoffrey C. Bower %A Andrew Chael %A Antonio Fuentes %A José L. Gómez %A Mareki Honma %A Michael D. Johnson %A Yutaro Kofuji %A Daniel P. Marrone %A Kotaro Moriyama %A Ramesh Narayan %A Dominic W. Pesce %A Paul Tiede %A Maciek Wielgus %A Guang-Yao Zhao %A The Event Horizon Telescope Collaboration %B The Astrophysical Journal Letters %D 2022 %G eng %N L18 %P 1-21 %T Selective Dynamical Imaging of Interferometric Data %U https://doi.org/10.3847/2041-8213/ac6615 %V 930 %X Recent developments in very long baseline interferometry (VLBI) have made it possible for the Event Horizon Telescope (EHT) to resolve the innermost accretion flows of the largest supermassive black holes on the sky. The sparse nature of the EHT's (u, v)-coverage presents a challenge when attempting to resolve highly time-variable sources. We demonstrate that the changing (u, v)-coverage of the EHT can contain regions of time over the course of a single observation that facilitate dynamical imaging. These optimal time regions typically have projected baseline distributions that are approximately angularly isotropic and radially homogeneous. We derive a metric of coverage quality based on baseline isotropy and density that is capable of ranking array configurations by their ability to produce accurate dynamical reconstructions. We compare this metric to existing metrics in the literature and investigate their utility by performing dynamical reconstructions on synthetic data from simulated EHT observations of sources with simple orbital variability. We then use these results to make recommendations for imaging the 2017 EHT Sgr A* data set. %0 Journal Article %A Maciek Wielgus %A Nicola Marchili %A Iván Martí-Vidal %A Garrett K. Keating %A Venkatessh Ramakrishnan %A Paul Tiede %A Sara Issaoun %A Joey Neilsen %A Michael A. Nowak %A Lindy Blackburn %A Ciriaco Goddi %A Daryl Haggard %A Daeyoung Lee %A Monika Moscibrodzka %A Alexandra J. Tetarenko %A Geoffrey C. Bower %A Chi-Kwan Chan %A Koushik Chatterjee %A Paul M. Chesler %A Jason Dexter %A Sheperd S. Doeleman %A Boris Georgiev %A Mark Gurwell %A Michael D. Johnson %A Daniel P. Marrone %A Alejandro Mus %A Dimitrios Psaltis %A Gunther Witzel %A The Event Horizon Telescope Collaboration %B The Astrophysical Journal Letters %D 2022 %G eng %N L19 %P 1-32 %T Millimeter Light Curves of Sagittarius A* Observed during the 2017 Event Horizon Telescope Campaign %U https://doi.org/10.3847/2041-8213/ac6428 %V 930 %X

The Event Horizon Telescope (EHT) observed the compact radio source, Sagittarius A* (Sgr A*), in the Galactic Center on 2017 April 5-11 in the 1.3 mm wavelength band. At the same time, interferometric array data from the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array were collected, providing Sgr A* light curves simultaneous with the EHT observations. These data sets, complementing the EHT very long baseline interferometry, are characterized by a cadence and signal-to-noise ratio previously unattainable for Sgr A* at millimeter wavelengths, and they allow for the investigation of source variability on timescales as short as a minute. While most of the light curves correspond to a low variability state of Sgr A*, the April 11 observations follow an X-ray flare and exhibit strongly enhanced variability. All of the light curves are consistent with a red-noise process, with a power spectral density (PSD) slope measured to be between -2 and -3 on timescales between 1 minute and several hours. Our results indicate a steepening of the PSD slope for timescales shorter than 0.3 hr. The spectral energy distribution is flat at 220 GHz, and there are no time lags between the 213 and 229 GHz frequency bands, suggesting low optical depth for the event horizon scale source. We characterize Sgr A*'s variability, highlighting the different behavior observed just after the X-ray flare, and use Gaussian process modeling to extract a decorrelation timescale and a PSD slope. We also investigate the systematic calibration uncertainties by analyzing data from independent data reduction pipelines.

%0 Journal Article %A Boris Georgiev %A Dominic W. Pesce %A Avery E. Broderick %A Vedant Dhruv %A Charles F. Gammie %A Chi-Kwan Chan %A Koushik Chatterjee %A Razieh Emami %A Yosuke Mizuno %A Roman Gold %A Christian M. Fromm %A Angelo Ricarte %A Doosoo Yoon %A Abhishek V. Joshi %A Ben Prather %A Alejandro Cruz-Osorio %A Michael D. Johnson %A Oliver Porth %A Héctor Olivares %A Ziri Younsi %A Luciano Rezzolla %A Jesse Vos %A Richard Qiu %A Antonios Nathanail %A Ramesh Narayan %A Andrew Chael %A Richard Anantua %A Monika Moscibrodzka %A The Event Horizon Telescope Collaboration %B The Astrophysical Journal Letters %D 2022 %G eng %N L20 %P 1-32 %T A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows %V 930 %X

We present a framework for characterizing the spatiotemporal power spectrum of the variability expected from the horizon-scale emission structure around supermassive black holes, and we apply this framework to a library of general relativistic magnetohydrodynamic (GRMHD) simulations and associated general relativistic ray-traced images relevant for Event Horizon Telescope (EHT) observations of Sgr A*. We find that the variability power spectrum is generically a red-noise process in both the temporal and spatial dimensions, with the peak in power occurring on the longest timescales and largest spatial scales. When both the time-averaged source structure and the spatially integrated light-curve variability are removed, the residual power spectrum exhibits a universal broken power-law behavior. On small spatial frequencies, the residual power spectrum rises as the square of the spatial frequency and is proportional to the variance in the centroid of emission. Beyond some peak in variability power, the residual power spectrum falls as that of the time-averaged source structure, which is similar across simulations; this behavior can be naturally explained if the variability arises from a multiplicative random field that has a steeper high-frequency power-law index than that of the time-averaged source structure. We briefly explore the ability of power spectral variability studies to constrain physical parameters relevant for the GRMHD simulations, which can be scaled to provide predictions for black holes in a range of systems in the optically thin regime. We present specific expectations for the behavior of the M87* and Sgr A* accretion flows as observed by the EHT.

%0 Journal Article %A Avery E. Broderick %A Roman Gold %A Boris Georgiev %A Dominic W. Pesce %A Paul Tiede %A Chunchong Ni %A Kotaro Moriyama %A The Event Horizon Telescope Collaboration %B The Astrophysical Journal Letters %D 2022 %G eng %N L21 %P 1-30 %T Characterizing and Mitigating Intraday Variability: Reconstructing Source Structure in Accreting Black Holes with mm-VLBI %V 930 %X The extraordinary physical resolution afforded by the Event Horizon Telescope has opened a window onto the astrophysical phenomena unfolding on horizon scales in two known black holes, M87* and Sgr A*. However, with this leap in resolution has come a new set of practical complications. Sgr A* exhibits intraday variability that violates the assumptions underlying Earth aperture synthesis, limiting traditional image reconstruction methods to short timescales and data sets with very sparse (u, v) coverage. We present a new set of tools to detect and mitigate this variability. We develop a data-driven, model-agnostic procedure to detect and characterize the spatial structure of intraday variability. This method is calibrated against a large set of mock data sets, producing an empirical estimator of the spatial power spectrum of the brightness fluctuations. We present a novel Bayesian noise modeling algorithm that simultaneously reconstructs an average image and statistical measure of the fluctuations about it using a parameterized form for the excess variance in the complex visibilities not otherwise explained by the statistical errors. These methods are validated using a variety of simulated data, including general relativistic magnetohydrodynamic simulations appropriate for Sgr A* and M87*. We find that the reconstructed source structure and variability are robust to changes in the underlying image model. We apply these methods to the 2017 EHT observations of M87*, finding evidence for variability across the EHT observing campaign. The variability mitigation strategies presented are widely applicable to very long baseline interferometry observations of variable sources generally, for which they provide a data-informed averaging procedure and natural characterization of inter-epoch image consistency. %0 Journal Article %A The Event Horizon Telescope Collaboration %B The Astrophysical Journal Letters %D 2022 %G eng %N L12 %T First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way %U https://doi.org/10.3847/2041-8213/ac6674 %V 930 %X

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 μas (68% credible interval). The ring has modest azimuthal brightness asymmetry and a comparatively 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 × 106 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 103-105 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.

%0 Journal Article %A The Event Horizon Telescope Collaboration %B The Astrophysical Journal Letters %D 2022 %G eng %N L13 %T First Sagittarius A* Event Horizon Telescope Results. II. EHT and Multiwavelength Observations, Data Processing, and Calibration %V 930 %X

We present Event Horizon Telescope (EHT) 1.3 mm measurements of the radio source located at the position of the supermassive black hole Sagittarius A* (Sgr A*), collected during the 2017 April 5-11 campaign. The observations were carried out with eight facilities at six locations across the globe. Novel calibration methods are employed to account for Sgr A*'s flux variability. The majority of the 1.3 mm emission arises from horizon scales, where intrinsic structural source variability is detected on timescales of minutes to hours. The effects of interstellar scattering on the image and its variability are found to be subdominant to intrinsic source structure. The calibrated visibility amplitudes, particularly the locations of the visibility minima, are broadly consistent with a blurred ring with a diameter of ~50 μas, as determined in later works in this series. Contemporaneous multiwavelength monitoring of Sgr A* was performed at 22, 43, and 86 GHz and at near-infrared and X-ray wavelengths. Several X-ray flares from Sgr A* are detected by Chandra, one at low significance jointly with Swift on 2017 April 7 and the other at higher significance jointly with NuSTAR on 2017 April 11. The brighter April 11 flare is not observed simultaneously by the EHT but is followed by a significant increase in millimeter flux variability immediately after the X-ray outburst, indicating a likely connection in the emission physics near the event horizon. We compare Sgr A*'s broadband flux during the EHT campaign to its historical spectral energy distribution and find that both the quiescent emission and flare emission are consistent with its long-term behavior.

%0 Journal Article %A The Event Horizon Telescope Collaboration %B The Astrophysical Journal Letters %D 2022 %G eng %N L14 %T First Sagittarius A* Event Horizon Telescope Results. III. Imaging of the Galactic Center Supermassive Black Hole %U https://doi.org/10.3847/2041-8213/ac6429 %V 930 %X

We present the first event-horizon-scale images and spatiotemporal analysis of Sgr A* taken with the Event Horizon Telescope in 2017 April at a wavelength of 1.3 mm. Imaging of Sgr A* has been conducted through surveys over a wide range of imaging assumptions using the classical CLEAN algorithm, regularized maximum likelihood methods, and a Bayesian posterior sampling method. Different prescriptions have been used to account for scattering effects by the interstellar medium toward the Galactic center. Mitigation of the rapid intraday variability that characterizes Sgr A* has been carried out through the addition of a "variability noise budget" in the observed visibilities, facilitating the reconstruction of static full-track images. Our static reconstructions of Sgr A* can be clustered into four representative morphologies that correspond to ring images with three different azimuthal brightness distributions and a small cluster that contains diverse nonring morphologies. Based on our extensive analysis of the effects of sparse (u, v)-coverage, source variability, and interstellar scattering, as well as studies of simulated visibility data, we conclude that the Event Horizon Telescope Sgr A* data show compelling evidence for an image that is dominated by a bright ring of emission with a ring diameter of ~50 μas, consistent with the expected "shadow" of a 4 × 106 M ⊙ black hole in the Galactic center located at a distance of 8 kpc.

%0 Journal Article %A The Event Horizon Telescope Collaboration %B The Astrophysical Journal Letters %D 2022 %G eng %N L15 %T First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass %V 930 %X

In this paper we quantify the temporal variability and image morphology of the horizon-scale emission from Sgr A*, as observed by the EHT in 2017 April at a wavelength of 1.3 mm. We find that the Sgr A* data exhibit variability that exceeds what can be explained by the uncertainties in the data or by the effects of interstellar scattering. The magnitude of this variability can be a substantial fraction of the correlated flux density, reaching ~100% on some baselines. Through an exploration of simple geometric source models, we demonstrate that ring-like morphologies provide better fits to the Sgr A* data than do other morphologies with comparable complexity. We develop two strategies for fitting static geometric ring models to the time-variable Sgr A* data; one strategy fits models to short segments of data over which the source is static and averages these independent fits, while the other fits models to the full data set using a parametric model for the structural variability power spectrum around the average source structure. Both geometric modeling and image-domain feature extraction techniques determine the ring diameter to be 51.8 ± 2.3 μas (68% credible intervals), with the ring thickness constrained to have an FWHM between ~30% and 50% of the ring diameter. To bring the diameter measurements to a common physical scale, we calibrate them using synthetic data generated from GRMHD simulations. This calibration constrains the angular size of the gravitational radius to be ${4.8}_{-0.7}^{+1.4}$ μas, which we combine with an independent distance measurement from maser parallaxes to determine the mass of Sgr A* to be ${4.0}_{-0.6}^{+1.1}\times {10}^{6}$ M ⊙.

%0 Journal Article %A The Event Horizon Telescope Collaboration %B The Astrophysical Journal Letters %D 2022 %G eng %N L16 %T First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole %V 930 %X

In this paper we provide a first physical interpretation for the Event Horizon Telescope's (EHT) 2017 observations of Sgr A*. Our main approach is to compare resolved EHT data at 230 GHz and unresolved non-EHT observations from radio to X-ray wavelengths to predictions from a library of models based on time-dependent general relativistic magnetohydrodynamics simulations, including aligned, tilted, and stellar-wind-fed simulations; radiative transfer is performed assuming both thermal and nonthermal electron distribution functions. We test the models against 11 constraints drawn from EHT 230 GHz data and observations at 86 GHz, 2.2 μm, and in the X-ray. All models fail at least one constraint. Light-curve variability provides a particularly severe constraint, failing nearly all strongly magnetized (magnetically arrested disk (MAD)) models and a large fraction of weakly magnetized models. A number of models fail only the variability constraints. We identify a promising cluster of these models, which are MAD and have inclination i ≤ 30°. They have accretion rate (5.2-9.5) × 10-9 M ⊙ yr-1, bolometric luminosity (6.8-9.2) × 1035 erg s-1, and outflow power (1.3-4.8) × 1038 erg s-1. We also find that all models with i ≥ 70° fail at least two constraints, as do all models with equal ion and electron temperature; exploratory, nonthermal model sets tend to have higher 2.2 μm flux density; and the population of cold electrons is limited by X-ray constraints due to the risk of bremsstrahlung overproduction. Finally, we discuss physical and numerical limitations of the models, highlighting the possible importance of kinetic effects and duration of the simulations.

%0 Journal Article %A The Event Horizon Telescope Collaboration %B The Astrophysical Journal Letters %D 2022 %G eng %N L17 %T First Sagittarius A* Event Horizon Telescope Results. VI. Testing the Black Hole Metric %V 930 %X

Astrophysical black holes are expected to be described by the Kerr metric. This is the only stationary, vacuum, axisymmetric metric, without electromagnetic charge, that satisfies Einstein's equations and does not have pathologies outside of the event horizon. We present new constraints on potential deviations from the Kerr prediction based on 2017 EHT observations of Sagittarius A* (Sgr A*). We calibrate the relationship between the geometrically defined black hole shadow and the observed size of the ring-like images using a library that includes both Kerr and non-Kerr simulations. We use the exquisite prior constraints on the mass-to-distance ratio for Sgr A* to show that the observed image size is within ~10% of the Kerr predictions. We use these bounds to constrain metrics that are parametrically different from Kerr, as well as the charges of several known spacetimes. To consider alternatives to the presence of an event horizon, we explore the possibility that Sgr A* is a compact object with a surface that either absorbs and thermally reemits incident radiation or partially reflects it. Using the observed image size and the broadband spectrum of Sgr A*, we conclude that a thermal surface can be ruled out and a fully reflective one is unlikely. We compare our results to the broader landscape of gravitational tests. Together with the bounds found for stellar-mass black holes and the M87 black hole, our observations provide further support that the external spacetimes of all black holes are described by the Kerr metric, independent of their mass.

%0 Journal Article %A Antonios Nathanail %A Prasun Dhang %A Christian M. Fromm %B Monthly Notices of the Royal Astronomical Society %D 2022 %G eng %N 4 %P 5204-5210 %T Magnetic field structure in the vicinity of a supermassive black hole in low-luminosity galaxies: the case of Sgr A* %U https://doi.org/10.1093/mnras/stac1276 %V 513 %X Observations of SgrA∗ have provided a lot of insight on low-luminosity accretion, with a handful of bright flares accompanied with orbital motion close to the horizon. It has been proposed that gas supply comes from stellar winds in the neighborhood of the supermassive black hole. We here argue that the flow at the vicinity of the black hole has a low magnetization and a structure of alternating polarity, totally dictated by the well-studied and long-ago proposed MRI turbulent process. This can be the case, provided that in larger distances from the black hole magnetic diffusivity is dominant, and thus, the magnetic field will never reach equipartition values. For SgrA∗⁠, we show the immediate consequences of this specific magnetic field geometry, which are: (i) an intermittent flow that passes from quiescent states to flaring activity, (ii) no quasi-steady-state jet, (iii) no possibility of a magnetically arrested configuration. Moreover, a further distinctive feature of this geometry is the intense magnetic reconnection events, occurring as layers of opposite magnetic polarity, accreted in the vicinity of the black hole. Finally, we argue that the absence of a jet structure in such case will be a smoking gun in 43 and 86 GHz observations. %0 Journal Article %A Christian M. Fromm %A Alejandro Cruz-Osorio %A Yosuke Mizuno %A Antonios Nathanail %A Ziri Younsi %A Oliver Porth %A Hector Olivares %A Jordy Davelaar %A Heino Falcke %A Michael Kramer %A Luciano Rezzolla %B Astronomy & Astrophysics %D 2022 %G eng %N A107 %P 1-22 %T Impact of non-thermal particles on the spectral and structural properties of M87 %U https://doi.org/10.1051/0004-6361/202142295 %V 660 %X Context. The recent 230 GHz observations of the Event Horizon Telescope are able to image the innermost structure of M 87 and show a ring-like structure that agrees with thermal synchrotron emission generated in a torus surrounding a supermassive black hole. However, at lower frequencies, M 87 is characterised by a large-scale and edge-brightened jet with clear signatures of non-thermal emission. In order to bridge the gap between these scales and to provide a theoretical interpretation of these observations, we perform general relativistic magnetohydrodynamic simulations of accretion onto black holes and jet launching.
Aims: M 87 has been the target for multiple observations across the entire electromagnetic spectrum. Among these, very large baseline interferometry (VLBI) observations provide unique details of the collimation profile of the jet down to several gravitational radii. We aim to model the observed broad-band spectrum of M 87 from the radio to the near-IR regime and at the same time, fit the jet structure as observed with global millimeter-VLBI at 86 GHz.
Methods: We used general relativistic magnetohydrodynamics and simulated the accretion of the magnetised plasma onto Kerr black holes in 3D. The radiative signatures of these simulations were computed taking different electron distribution functions into account, and a detailed parameter survey was performed in order to match the observations.
Results: The results of our simulations show that magnetically arrested disks around fast-spinning black holes (a⋆ ≥ 0.5) together with a mixture of thermal and non-thermal particle distributions are able to simultaneously model the broad-band spectrum and the innermost jet structure of M 87. %0 Journal Article %A Avery E. Broderick %A Paul Tiede %A Dominic W. Pesce %A Roman Gold %B The Astrophysical Journal %D 2022 %G eng %N 6 %P 1-12 %T Measuring Spin from Relative Photon-ring Sizes %U https://doi.org/10.3847/1538-4357/ac4970 %V 927 %X

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.

%0 Journal Article %A Prashant Kocherlakota %A Luciano Rezzolla %A Event Horizon Telescope Collaboration %B Physical Review D %D 2021 %G eng %N 10 %P 40-47 %T Constraints on non-Einsteinian black-hole charges with the 2019 EHT observations of M87 %U https://journals.aps.org/prd/abstract/10.1103/PhysRevD.103.104047 %V 103 %0 Journal Article %A et EHT Collaboration al. %B ApJL %D 2021 %G eng %N L12 %P 48 %T First M87 Event Horizon Telescope Results. VII. Polarization of the Ring %U https://iopscience.iop.org/article/10.3847/2041-8213/abe71d %V 910 %0 Journal Article %A et EHT Collaboration al. %B ApJL %D 2021 %G eng %N L13 %P 43 %T First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon %U https://iopscience.iop.org/article/10.3847/2041-8213/abe4de %V 910 %0 Journal Article %A et C. Goddi al. %B ApJL %D 2021 %G eng %N L14 %P 54 %T Polarimetric Properties of Event Horizon Telescope Targets from ALMA %U https://iopscience.iop.org/article/10.3847/2041-8213/abee6a %V 910 %0 Journal Article %A et J. C. Algaba al. %B ApJL %D 2021 %G eng %P L11 %T Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign %U https://iopscience.iop.org/article/10.3847/2041-8213/abef71 %V 911 %X 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. %0 Journal Article %A Michael Janssen %A Heino Falcke %A Matthias Kadler %A Eduardo Ros %A the EHT Collaboration %B Nature Astronomy %D 2021 %G eng %P 1017-1028 %T Event Horizon Telescope observations of the jet launching and collimation in Centaurus A %U https://doi.org/10.1038/s41550-021-01417-w %V 5 %X 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. %0 Journal Article %A Razieh Emami %A Richard Anantua %A Andrew A. Chael %A Abraham Loeb %B The Astrophysical Journal %D 2021 %G eng %P 272, 1-27 %T Positron Effects on Polarized Images and Spectra from Jet and Accretion Flow Models of M87* and Sgr A* %U https://doi.org/10.3847/1538-4357/ac2950 %V 923 %X 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*. %0 Journal Article %A Maciek Wielgus %B Physical Review D %D 2021 %G eng %P 124058 %T Photon rings of spherically symmetric black holes and robust tests of non-Kerr metrics %U https://doi.org/10.1103/PhysRevD.104.124058 %V 104 %X

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.

%0 Journal Article %A et B. Jeter al. %B MNRAS %D 2020 %G eng %N 5606 %T Differentiating Disk and Black Hole Driven Jets with EHT Images of Variability in M87 %U https://academic.oup.com/mnras/article-abstract/493/4/5606/5804779 %V 493 %0 Journal Article %A et J. Y. Kim al. %B Astronomy and Astrophysics %D 2020 %G eng %N A69 %P 21 %T Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution %U https://www.aanda.org/articles/aa/abs/2020/08/aa37493-20/aa37493-20.html %V 640 %X

3C 279 is an archetypal blazar with a prominent radio jet that show broadband flux density variability across the entire electromagnetic spectrum. We use an ultra-high angular resolution technique - global Very Long Baseline Interferometry (VLBI) at 1.3 mm (230 GHz) - to resolve the innermost jet of 3C 279 in order to study its fine-scale morphology close to the jet base where highly variable γ-ray emission is thought to originate, according to various models. The source was observed during four days in April 2017 with the Event Horizon Telescope at 230 GHz, including the phased Atacama Large Millimeter/submillimeter Array (ALMA), at an angular resolution of ∼20 μas (at a redshift of z = 0.536 this corresponds to ∼0.13 pc ∼ 1700 Schwarzschild radii with a black hole mass MBH = 8 × 108 M☉). Imaging and model-fitting techniques were applied to the data to parameterize the fine-scale source structure and its variation. We find a multicomponent inner jet morphology with the northernmost component elongated perpendicular to the direction of the jet, as imaged at longer wavelengths. The elongated nuclear structure is consistent on all four observing days and across different imaging methods and model-fitting techniques, and therefore appears robust. Owing to its compactness and brightness, we associate the northern nuclear structure as the VLBI "core". This morphology can be interpreted as either a broad resolved jet base or a spatially bent jet. We also find significant day-to-day variations in the closure phases, which appear most pronounced on the triangles with the longest baselines. Our analysis shows that this variation is related to a systematic change of the source structure. Two inner jet components move non-radially at apparent speeds of ∼15 c and ∼20 c (∼1.3 and ∼1.7 μas day-1, respectively), which more strongly supports the scenario of traveling shocks or instabilities in a bent, possibly rotating jet. The observed apparent speeds are also coincident with the 3C 279 large-scale jet kinematics observed at longer (cm) wavelengths, suggesting no significant jet acceleration between the 1.3 mm core and the outer jet. The intrinsic brightness temperature of the jet components are ≲1010 K, a magnitude or more lower than typical values seen at ≥7 mm wavelengths. The low brightness temperature and morphological complexity suggest that the core region of 3C 279 becomes optically thin at short (mm) wavelengths.

 

The data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/640/A69 and at http://https://eventhorizontelescope.org/for-astronomers/data %0 Journal Article %A et R. Gold al. %B The Astrophysical Journal %D 2020 %G eng %N 2 %P id. 148 %T Verification of Radiative Transfer Schemes for the EHT %U https://iopscience.iop.org/article/10.3847/1538-4357/ab96c6 %V 897 %0 Journal Article %A et A. Broderick al. %B The Astrophysical Journal %D 2020 %G eng %N 2 %P id. 139 %T THEMIS: A Parameter Estimation Framework for the Event Horizon Telescope %U https://iopscience.iop.org/article/10.3847/1538-4357/ab91a4 %V 897 %0 Journal Article %A et F. Roelofs al. %B Astronomy & Astrophysics %D 2020 %G eng %N A5 %P 19 %T SYMBA: An end-to-end VLBI synthetic data generation pipeline. 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Verhagen %B Physical Review Letters %D 2015 %G eng %N 20 %P 203904 %R 10.1103/PhysRevLett.115.203904 %T Perturbing Open Cavities: Anomalous Resonance Frequency Shifts in a Hybrid Cavity-Nanoantenna System %V 115 %0 Journal Article %A M. D. Johnson %A A. Loeb %A H. Shiokawa %A A. A. Chael %A S. S. Doeleman %B ApJ %D 2015 %G eng %P 132 %R 10.1088/0004-637X/813/2/132 %T Measuring the Direction and Angular Velocity of a Black Hole Accretion Disk via Lagged Interferometric Covariance %V 813 %8 11/2015 %0 Journal Article %A J. Wagner %A A. L. Roy %A T. P. Krichbaum %A W. Alef %A A. Bansod %A A. Bertarini %A R. Güsten %A D. Graham %A J. Hodgson %A R. Märtens %A K. Menten %A D. Muders %A H. Rottmann %A G. Tuccari %A A. Weiss %A G. Wieching %A M. Wunderlich %A J. A. Zensus %A J. P. Araneda %A O. Arriagada %A M. Cantzler %A C. Duran %A F. M. Montenegro-Montes %A R. Olivares %A P. Caro %A P. Bergman %A J. Conway %A R. Haas %A J. Johansson %A M. Lindqvist %A H. Olofsson %A M. Pantaleev %A S. 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Yun %B PASJ %D 2014 %G eng %P 103 %R 10.1093/pasj/psu104 %T VLBI observations of bright AGN jets with the KVN and VERA Array (KaVA): Evaluation of imaging capability %V 66 %0 Journal Article %A M. Honma %A K. Akiyama %A M. Uemura %A S. Ikeda %B PASJ %D 2014 %G eng %P 95 %R 10.1093/pasj/psu070 %T Super-resolution imaging with radio interferometry using sparse modeling %V 66 %0 Journal Article %A K. Hada %A M. Giroletti %A M. Kino %A G. Giovannini %A F. D'Ammando %A C. C. Cheung %A M. Beilicke %A H. Nagai %A A. Doi %A K. Akiyama %A M. Honma %A K. Niinuma %A C. Casadio %A M. Orienti %A H. Krawczynski %A J. L. Gómez %A S. Sawada-Satoh %A S. Koyama %A A. Cesarini %A S. Nakahara %A M. A. Gurwell %B ApJ %D 2014 %G eng %P 165 %R 10.1088/0004-637X/788/2/165 %T A Strong Radio Brightening at the Jet Base of M 87 during the Elevated Very High Energy Gamma-Ray State in 2012 %V 788 %0 Journal Article %A M. Kino %A F. Takahara %A K. Hada %A A. Doi %B ApJ %D 2014 %G eng %P 5 %R 10.1088/0004-637X/786/1/5 %T Relativistic Electrons and Magnetic Fields of the M87 Jet on the \~10 Schwarzschild Radii Scale %V 786 %0 Journal Article %A C. Y. Kuo %A K. Asada %A R. Rao %A M. Nakamura %A J. C. Algaba %A H. B. Liu %A M. Inoue %A P. M. Koch %A P. T. P. Ho %A S. Matsushita %A H.-Y. Pu %A K. Akiyama %A H. Nishioka %A N. Pradel %B ApJ %D 2014 %G eng %P L33 %R 10.1088/2041-8205/783/2/L33 %T Measuring Mass Accretion Rate onto the Supermassive Black Hole in M87 Using Faraday Rotation Measure with the Submillimeter Array %V 783 %0 Journal Article %A M. J. Reid %A M. Honma %B ARA&A %D 2014 %G eng %P 339-372 %R 10.1146/annurev-astro-081913-040006 %T Microarcsecond Radio Astrometry %V 52 %0 Journal Article %A A. E. Broderick %A T. Johannsen %A A. Loeb %A D. Psaltis %B ApJ %D 2014 %G eng %N 1 %P 7 %T Testing the No-hair Theorem with Event Horizon Telescope Observations of Sagittarius A* %V 784 %0 Journal Article %A M. Nakamura %A K. Asada %B ApJ %D 2013 %G eng %P 118 %R 10.1088/0004-637X/775/2/118 %T The Parabolic Jet Structure in M87 as a Magnetohydrodynamic Nozzle %V 775 %X The structure and dynamics of the M87 jet from sub-milliarcsec to arcsecond scales are continuously examined. We analyzed the Very Long Baseline Array archival data taken at 43 and 86 GHz to measure the size of very long baseline interferometry (VLBI) cores. Millimeter/sub-millimeter VLBI cores are considered as innermost jet emissions, which has been originally suggested by Blandford & Königl. Those components fairly follow an extrapolated parabolic streamline in our previous study so that the jet has a single power-law structure with nearly 5 orders of magnitude in the distance starting from the vicinity of the supermassive black hole (SMBH), less than 10 Schwarzschild radius (r  s). We further inspect the jet parabolic structure as a counterpart of the magnetohydrodynamic (MHD) nozzle in order to identify the property of a bulk acceleration. We interpret that the parabolic jet consists of Poynting-flux dominated flows, powered by large-amplitude, nonlinear torsional Alfvén waves. We examine the non-relativistic MHD nozzle equation in a parabolic shape. The nature of trans-fast magnetosonic flow is similar to the one of transonic solution of Parker's hydrodynamic solar wind; the jet becomes super-escape as well as super-fast magnetosonic at around ~103 r  s, while the upstream trans-Alfvénic flow speed increases linearly as a function of the distance at ~102-103 r  s. We here point out that this is the first evidence to identify these features in astrophysical jets. 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