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Please pay attention to the location of the JSP opportunities that interest you, as you are responsible for transportation to and from the job shadow site. The simulated data were then imaged with the same pipeline used for the observed images. The similarities between the simulated images bottom row of Figure 4 and the observed images Figure 3 are remarkable. Figure 4. Top: three example models of some of the best-fitting snapshots from the image library of GRMHD simulations for April 11 corresponding to different spin parameters and accretion flows.

Bottom: the same theoretical models, processed through a VLBI simulation pipeline with the same schedule, telescope characteristics, and weather parameters as in the April 11 run and imaged in the same way as Figure 3. Note that although the fit to the observations is equally good in the three cases, they refer to radically different physical scenarios; this highlights that a single good fit does not imply that a model is preferred over others see Paper V.

First, the observed image is consistent with the hypothesis that it is produced by a magnetized accretion flow orbiting within a few r g of the event horizon of a Kerr black hole. The asymmetric ring is produced by a combination of strong gravitational lensing and relativistic beaming, while the central flux depression is the observational signature of the black hole shadow.


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Second, the north—south asymmetry in the emission ring is controlled by the black hole spin and can be used to deduce its orientation. In corotating disk models where the angular momentum of the matter and of the black hole are aligned , the funnel wall, or jet sheath, rotates with the disk and the black hole; in counterrotating disk models, instead, the luminous funnel wall rotates with the black hole but against the disk. The north—south asymmetry is consistent with models in which the black hole spin is pointing away from Earth and inconsistent with models in which the spin points toward Earth.

This is consistent with the rotation of the ionized gas on scales of 20 pc, i. Furthermore, in those models that produce a sufficiently powerful jet, it is powered by extraction of black hole spin energy through mechanisms akin to the Blandford—Znajek process. In Paper VI , the black hole mass is derived from fitting to the visibility data of geometric and GRMHD models, as well as from measurements of the ring diameter in the image domain.

Our measurements remain consistent across methodologies, algorithms, data representations, and observed data sets. We used two distinct Bayesian-inference algorithms and demonstrate that such crescent models are statistically preferred over other comparably complex geometric models that we have explored. We find that the crescent models provide fits to the data that are statistically comparable to those of the reconstructed images presented in Section 5 , allowing us to determine the basic parameters of the crescents.


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The diameters of the geometric crescent models measure the characteristic sizes of the emitting regions that surround the shadows and not the sizes of the shadows themselves see, e. To account for this turbulent structure, we developed a formalism and multiple algorithms that estimate the statistics of the stochastic components using ensembles of images from individual GRMHD simulations.

We find that the visibility data are not inconsistent with being a realization of many of the GRMHD simulations. We conclude that the recovered model parameters are consistent across algorithms. Finally, we extract ring diameter, width, and shape directly from reconstructed images see Section 5.

The results are consistent with the parameter estimates from geometric crescent models. For the distance of Table 1 summarizes the measured parameters of the image features and the inferred black hole properties based on data from all bands and all days combined. A number of elements reinforce the robustness of our image and the conclusion that it is consistent with the shadow of a black hole as predicted by GR.

First, our analysis has used multiple independent calibration and imaging techniques, as well as four independent data sets taken on four different days in two separate frequency bands. Second, the image structure matches previous predictions well Dexter et al. Fourth, the observed emission ring reconstructed in our images is close to circular with an axial ratio ; similarly, the time average images from our GRMHD simulations also show a circular shape. This stability is consistent with the expectation that the size of the shadow is a feature tied to the mass of the black hole and not to properties of a variable plasma flow.

It is also straightforward to reject some alternative astrophysical interpretations. For instance, the image is unlikely to be produced by a jet-feature as multi-epoch VLBI observations of the plasma jet in M87 Walker et al. Similarly, were the apparent ring a random alignment of emission blobs, they should also have moved away at relativistic speeds, i.

Finally, an Einstein ring formed by gravitational lensing of a bright region in the counter-jet would require a fine-tuned alignment and a size larger than that measured in and Some of such exotic compact objects can already be shown to be incompatible with our observations given our maximum mass prior.

Also, some commonly used types of wormholes Bambi predict much smaller shadows than we have measured. However, other compact-object candidates need to be analyzed with more care. Boson stars are an example of compact objects having circular photon orbits but without a surface or an event horizon. In such a spacetime, null geodesics are redirected outwards toward distant observers Cunha et al.

More importantly, accretion flows onto boson stars behave differently as they do not produce jets but stalled accretion tori that make them distinguishable from black holes Olivares et al. Gravastars provide examples of compact objects having unstable photon orbits and a hard surface, but not an event horizon.

In this case, while a single image of the accretion flow could in principle be very similar to that coming from a black hole, differences of the flow dynamics at the stellar surface H. Olivares et al. Our constraints on deviations from the Kerr geometry rely only on the validity of the equivalence principle and are agnostic about the underlying theory of gravity, but can be used to measure, with ever improved precision, the parameters of the background metric.

On the other hand, current gravitational-wave observations of mergers probe the dynamics of the underlying theory, but cannot rely on the possibility of multiple and repeated measurements of the same source. To underline the complementarity of gravitational-wave and electromagnetic observations of black holes, we note that a basic feature of black holes in GR is that their size scales linearly with mass. Combining our constraints on the supermassive black hole in M87 with those on the stellar-mass black holes detected via gravitational waves we can infer that this property holds over eight orders of magnitude.

While this invariance is a basic prediction of GR, which is a scale-free theory, it need not be satisfied in other theories that introduce a scale to the gravitational field.

First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole

Finally, the radio core in M87 is quite typical for powerful radio jets in general. It falls on the fundamental plane of black hole activity for radio cores Falcke et al. This suggests that they are powered by a scale-invariant common object. Comparing the data with an extensive library of synthetic images obtained from GRMHD simulations covering different physical scenarios and plasma conditions reveals that the basic features of our image are relatively independent of the detailed astrophysical model.

Based on our modeling and information on the inclination angle, we derive the sense of rotation of the black hole to be in the clockwise direction, i. The brightness excess in the south part of the emission ring is explained as relativistic beaming of material rotating in the clockwise direction as seen by the observer, i. Future observations and further analysis will test the stability, shape, and depth of the shadow more accurately.

Polarimetric analysis of the images, which we will report in the future, will provide information on the accretion rate via Faraday rotation Bower et al. Higher-resolution images can be achieved by going to a shorter wavelength, i. Roelofs et al. Time dependent nonimaging analysis can be used to potentially track the motion of emitting matter near the black hole, as reported recently through interferometric observations in the near-infrared Gravity Collaboration et al.

In conclusion, we have shown that direct studies of the event horizon shadow of supermassive black hole candidates are now possible via electromagnetic waves, thus transforming this elusive boundary from a mathematical concept to a physical entity that can be studied and tested via repeated astronomical observations.

We thank the staff at the participating observatories, correlation centers, and institutions for their enthusiastic support. ALMA We gratefully acknowledge the support provided by the extended staff of the ALMA, both from the inception of the ALMA Phasing Project through the observational campaigns of and We would like to thank A.

Deller and W. More exotic spacetimes, such as dilaton black holes, boson stars, and gravastars, have also been considered Paper V. We here use Heaviside units, where a factor of is absorbed into the definition of the magnetic field. ADS Google Scholar. Crossref Google Scholar. Google Scholar. IOPscience Google Scholar.

This site uses cookies. By continuing to use this site you agree to our use of cookies. To find out more, see our Privacy and Cookies policy. Close this notification. Article data. Share this article. Article information. Author affiliations. Related links. Introduction Black holes are a fundamental prediction of the theory of general relativity GR; Einstein Zoom In Zoom Out Reset image size. Table 1. Abbott B. Abramowski A. Akiyama K. Asada K. Baade W. Baars J. Balbus S.

Balick B. Bambi C. Barausse E. Bardeen J. DeWitt and B. Berti E. Bird S. Biretta J. Blackburn L. Blakeslee J. Blandford R. Blecher T.


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Boccardi B. Bolton J. Bouman K. Bower G. Bridle A. Broderick A. Bromley B. Bronzwaer T. Cantiello M. Chael A. Chan C. Chirenti C. Clark B. Cohen M. Cunha P. Curtis H. Davelaar J. Deller A. Dexter J. Dibi S. Doeleman S. Eckart A. EHT Collaboration et al. Einstein A. Falcke H.

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Holt and W. Fish V. Ford H. Gammie C. Gebhardt K. Ghez A. Giddings S. Goddi C. Gold R. Gravity Collaboration, Abuter R. Greisen E. Grenzebach A. Greve A. Hada K. Harms R. Hawking S. Hilbert D. Honma M. Ichimaru S. Janssen M. Johannsen T. George N. Frederick K. Bradford A. Jay M. Iain M. Thomas M. Sergio A.

Wendeline B. Joseph R. Christopher H. Nils W. Erik M. Kyle D. Chi H. Michael A. Scott N. Rurik A. Alexandra S. David R. Anthony A. Sjoerd T. Jan G. Create citation alert. Journal RSS feed. Sign up for new issue notifications. When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.

This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days.

Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. Our radio-wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei.

They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible. Original content from this work may be used under the terms of the Creative Commons Attribution 3. Any further distribution of this work must maintain attribution to the author s and the title of the work, journal citation and DOI. Black holes are a fundamental prediction of the theory of general relativity GR; Einstein A defining feature of black holes is their event horizon, a one-way causal boundary in spacetime from which not even light can escape Schwarzschild The production of black holes is generic in GR Penrose , and more than a century after Schwarzschild, they remain at the heart of fundamental questions in unifying GR with quantum physics Hawking ; Giddings Black holes are common in astrophysics and are found over a wide range of masses.

Active galactic nuclei AGNs are central bright regions that can outshine the entire stellar population of their host galaxy. Some of these objects, quasars, are the most luminous steady sources in the universe Schmidt ; Sanders et al. The near-horizon emission from low-luminosity active galactic nuclei LLAGNs; Ho is produced by synchrotron radiation that peaks from the radio through the far-infrared. This emission may be produced either in the accretion flow Narayan et al.

When viewed from infinity, a nonrotating Schwarzschild black hole has a photon capture radius , where is the characteristic lengthscale of a black hole. The simulations of Luminet showed that for a black hole embedded in a geometrically thin, optically thick accretion disk, the photon capture radius would appear to a distant observer as a thin emission ring inside a lensed image of the accretion disk.

For accreting black holes embedded in a geometrically thick, optically thin emission region, as in LLAGNs, the combination of an event horizon and light bending leads to the appearance of a dark "shadow" together with a bright emission ring that should be detectable through very long baseline interferometery VLBI experiments Falcke et al. Its shape can appear as a "crescent" because of fast rotation and relativistic beaming Falcke et al. The observed projected diameter of the emission ring, which contains radiation primarily from the gravitationally lensed photon ring, is proportional to R c and hence to the mass of the black hole, but also depends nontrivially on a number of factors: the observing resolution, the spin vector of the black hole and its inclination, as well as the size and structure of the emitting region.

These factors are typically of order unity and can be calibrated using theoretical models. VLBI at an observing wavelength of 1. Early pathfinder experiments Padin et al. Over the following decade, a program to improve sensitivity of 1. The accompanying papers give a more extensive description of the instrument EHT Collaboration et al. It is also well studied in the optical Biretta et al. The upstream end of the jet is marked by a compact radio source Cohen et al.

The radio structures of the large-scale jet Owen et al. VLBI observations at 1. These observations, however, were not able to image the black hole shadow due to limited baseline coverage. Based on three recent stellar population measurements, we here adopt a distance to M87 of On the other hand, mass measurements modeling the kinematic structure of the gas disk Harms et al. These two mass estimates, from stellar and gas dynamics, predict a theoretical shadow diameter for a Schwarzschild black hole of and , respectively. As the Earth rotates, each telescope pair in the network samples many spatial frequencies.

To measure interferometric visibilities, the widely separated telescopes simultaneously sample and coherently record the radiation field from the source. Synchronization using the Global Positioning System typically achieves temporal alignment of these recordings within tens of nanoseconds. Each station is equipped with a hydrogen maser frequency standard. With the atmospheric conditions during our observations the coherent integration time was typically 10 s see Figure 2 in Paper II. Use of hydrogen maser frequency standards at all EHT sites ensures coherence across the array over this timescale.

After observations, recordings are staged at a central location, aligned in time, and signals from each telescope-pair are cross-correlated. Challenges at shorter wavelengths include increased noise in radio receiver electronics, higher atmospheric opacity, increased phase fluctuations caused by atmospheric turbulence, and decreased efficiency and size of radio telescopes in the millimeter and submillimeter observing bands. Started in Doeleman et al.

For the observations at a wavelength of 1. Figure 1. Eight stations of the EHT campaign over six geographic locations as viewed from the equatorial plane. Figure 2. Error bars correspond to thermal statistical uncertainties. Weather was uniformly good to excellent with nightly median zenith atmospheric opacities at GHz ranging from 0. At each station, the astronomical signal in both polarizations and two adjacent 2 GHz wide frequency bands centered at Correlation of the data was carried out using a software correlator Deller et al.

Differences between the two independent correlators were shown to be negligible through the exchange of a few identical scans for cross comparison. A subsequent fringe-fitting step identified detections in correlated signal power while phase calibrating the data for residual delays and atmospheric effects. Using ALMA as a highly sensitive reference station enabled critical corrections for ionospheric and tropospheric distortions at the other sites.

Fringe fitting was performed with three independent automated pipelines, each tailored to the specific characteristics of the EHT observations, such as the wide bandwidth, susceptibility to atmospheric turbulence, and array heterogeneity Blackburn et al. The pipelines made use of standard software for the processing of radio-interferometric data Greisen ; Whitney et al. Following data validation and pipeline comparisons, a single pipeline output was designated as the primary data set of the first EHT science data release and used for subsequent results, while the outputs of the other two pipelines offer supporting validation data sets.

The final calibrated complex visibilities V u , v correspond to the Fourier components of the brightness distribution on the sky at spatial frequency u , v determined by the projected baseline expressed in units of the observing wavelength van Cittert ; Thompson et al.

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The visibility amplitudes resemble those of a thin ring i. This particular ring model, shown with a dashed line in the bottom panel of Figure 2 , is only illustrative and does not fit all features in the data. Second, differences in the depth of the first minimum as a function of orientation, as well as highly nonzero measured closure phases, indicate some degree of asymmetry in the source Papers III , VI. Finally, the visibility amplitudes represent only half of the information available to us.

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We will next explore images and more complex geometrical models that can fit the measured visibility amplitudes and phases. We reconstructed images from the calibrated EHT visibilities, which provide results that are independent of models Paper IV. However, there are two major challenges in reconstructing images from EHT data. Because the u , v plane is only sparsely sampled Figure 2 , the inverse problem is under-constrained. Second, the measured visibilities lack absolute phase calibration and can have large amplitude calibration uncertainties.

To address these challenges, imaging algorithms incorporate additional assumptions and constraints that are designed to produce images that are physically plausible e. We explored two classes of algorithms for reconstructing images from EHT data. CLEAN is an inverse-modeling approach that deconvolves the interferometer point-spread function from the Fourier-transformed visibilities. When applying CLEAN , it is necessary to iteratively self-calibrate the data between rounds of imaging to solve for time-variable phase and amplitude errors in the data. The second class of algorithms is the so-called regularized maximum likelihood RML ; e.

RML is a forward-modeling approach that searches for an image that is not only consistent with the observed data but also favors specified image properties e. As with CLEAN , RML methods typically iterate between imaging and self-calibration, although they can also be used to image directly on robust closure quantities immune to station-based calibration errors. Every imaging algorithm has a variety of free parameters that can significantly affect the final image. We adopted a two-stage imaging approach to control and evaluate biases in the reconstructions from our choices of these parameters.

In the second imaging stage, we developed three imaging pipelines, each using a different software package and associated methodology. For each pipeline, we determined the single combination of fiducial imaging parameters out of the Top-Set that performed best across all the synthetic data sets and for each associated imaging methodology see Figure 11 in Paper IV.

This is shown in the bottom part of Figure 3 , which reports the images on different days see also Figure 15 in Paper IV. These results are also consistent with those obtained from visibility-domain fitting of geometric and general-relativistic magnetohydrodynamics GRMHD models Paper VI. Figure 3. The image is the average of three different imaging methods after convolving each with a circular Gaussian kernel to give matched resolutions. Bottom: similar images taken over different days showing the stability of the basic image structure and the equivalence among different days.

North is up and east is to the left. They naturally produce a powerful jet and can explain the broadband spectral energy distribution observed in LLAGNs. At a wavelength of 1. The latter does not necessarily coincide with the innermost stable circular orbit, or ISCO, and is instead related to the lensed photon ring. To explore this scenario in great detail, we have built a library of synthetic images Image Library describing magnetized accretion flows onto black holes in GR Paper V.

The images themselves are produced from a library of simulations Simulation Library collecting the results of four codes solving the equations of GRMHD Gammie et al. The elements of the Simulation Library have been coupled to three different general-relativistic ray-tracing and radiative-transfer codes GRRT, Bronzwaer et al. Younsi et al. We limit ourselves to providing here a brief description of the initial setups and the physical scenarios explored in the simulations; see Paper V for details on both the GRMHD and GRRT codes, which have been cross-validated for accuracy and consistency Gold et al.

The magnetic flux is generally nonzero because magnetic field is trapped in the black hole by the accretion flow and sustained by currents in the surrounding plasma. Once a simulation was completed, the relevant flow properties at different times are collected to be employed for the further post-processing of the GRRT codes. The generation of synthetic images requires, besides the properties of the fluid magnetic field, velocity field, and rest-mass density , also the emission and absorption coefficients, the inclination i the angle between the accretion-flow angular-momentum vector and the line of sight , the position angle PA the angle east of north, i.

Because the photons at 1. In particular, we consider the plasma to be composed of nonrelativistic ions with temperature T i and relativistic electrons with temperature T e. In SANE models, the disk jet is weakly strongly magnetized, so that low high R high models produce most of the emission in the disk jet. In MAD models, there are strongly magnetized regions everywhere and the emission is mostly from the disk midplane. While this prescription is not the only one possible, it has the advantage of being simple, sufficiently generic, and robust see Paper V for a discussion of nonthermal particles and radiative cooling.

Since each GRMHD simulation can be used to describe several different physical scenarios by changing the prescribed electron distribution function, we have used the Simulation Library to generate more than different physical scenarios. Each scenario is then used to generate hundreds of snapshots at different times in the simulation leading to more than 62, objects in the Image Library. As an example, the top row of Figure 4 shows three GRMHD model snapshots from the Image Library with different spins and flow type, which fitted closure phases and amplitudes of the April 11 data best.

For these models we produced simulated visibilities for the April 11 schedule and weather parameters and calibrated them with a synthetic data generation and calibration pipeline Blecher et al. The simulated data were then imaged with the same pipeline used for the observed images. The similarities between the simulated images bottom row of Figure 4 and the observed images Figure 3 are remarkable. Figure 4. Top: three example models of some of the best-fitting snapshots from the image library of GRMHD simulations for April 11 corresponding to different spin parameters and accretion flows.

Bottom: the same theoretical models, processed through a VLBI simulation pipeline with the same schedule, telescope characteristics, and weather parameters as in the April 11 run and imaged in the same way as Figure 3. Note that although the fit to the observations is equally good in the three cases, they refer to radically different physical scenarios; this highlights that a single good fit does not imply that a model is preferred over others see Paper V.

First, the observed image is consistent with the hypothesis that it is produced by a magnetized accretion flow orbiting within a few r g of the event horizon of a Kerr black hole. The asymmetric ring is produced by a combination of strong gravitational lensing and relativistic beaming, while the central flux depression is the observational signature of the black hole shadow.

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Second, the north—south asymmetry in the emission ring is controlled by the black hole spin and can be used to deduce its orientation. In corotating disk models where the angular momentum of the matter and of the black hole are aligned , the funnel wall, or jet sheath, rotates with the disk and the black hole; in counterrotating disk models, instead, the luminous funnel wall rotates with the black hole but against the disk.

The north—south asymmetry is consistent with models in which the black hole spin is pointing away from Earth and inconsistent with models in which the spin points toward Earth. This is consistent with the rotation of the ionized gas on scales of 20 pc, i. Furthermore, in those models that produce a sufficiently powerful jet, it is powered by extraction of black hole spin energy through mechanisms akin to the Blandford—Znajek process.

In Paper VI , the black hole mass is derived from fitting to the visibility data of geometric and GRMHD models, as well as from measurements of the ring diameter in the image domain. Our measurements remain consistent across methodologies, algorithms, data representations, and observed data sets. We used two distinct Bayesian-inference algorithms and demonstrate that such crescent models are statistically preferred over other comparably complex geometric models that we have explored. We find that the crescent models provide fits to the data that are statistically comparable to those of the reconstructed images presented in Section 5 , allowing us to determine the basic parameters of the crescents.