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The Event Horizon Telescope (EHT) is a large telescope array consisting of a global network of radio telescopes. The EHT project combines data from several very-long-baseline interferometry (VLBI) stations around Earth, which form a combined array with an angular resolution sufficient to observe objects the size of a supermassive black hole's event horizon. The project's observational targets include the two black holes with the largest angular diameter as observed from Earth: the black hole at the center of the supergiant elliptical galaxy Messier 87 (M87*, pronounced "M87-Star"), and Sagittarius A* (Sgr A*, pronounced "Sagittarius A-Star") at the center of the Milky Way.
The Event Horizon Telescope project is an international collaboration that was launched in 2009after a long period of theoretical and technical developments. On the theory side, work on the photon orbit and first simulations of what a black hole would look like progressed to predictions of VLBI imaging for the Galactic Center black hole, Sgr A*. Technical advances in radio observing moved from the first detection of Sgr A*, through VLBI at progressively shorter wavelengths, ultimately leading to detection of horizon scale structure in both Sgr A* and M87. The collaboration now comprises over 300 members, 60 institutions, working in over 20 countries and regions.
The first image of a black hole, at the center of galaxy Messier 87, was published by the EHT Collaboration on April 10, 2019, in a series of six scientific publications. mm and with a theoretical diffraction-limited resolution of 25 microarcseconds . In March 2021, the Collaboration presented, for the first time, a polarized-based image of the black hole which may help better reveal the forces giving rise to quasars. Future plans involve improving the array's resolution by adding new telescopes and by taking shorter-wavelength observations. On 12 May 2022, astronomers unveiled the first image of the supermassive black hole at the center of the Milky Way, Sagittarius A*.The array made this observation at a wavelength of 1.3
The EHT is composed of many radio observatories or radio-telescope facilities around the world, working together to produce a high-sensitivity, high-angular-resolution telescope. Through the technique of very-long-baseline interferometry (VLBI), many independent radio antennas separated by hundreds or thousands of kilometres can act as a phased array, a virtual telescope which can be pointed electronically, with an effective aperture which is the diameter of the entire planet, substantially improving its angular resolution. GHz, higher-bandwidth VLBI backends and recorders, as well as commissioning of new submillimeter VLBI sites.The effort includes development and deployment of submillimeter dual polarization receivers, highly stable frequency standards to enable very-long-baseline interferometry at 230–450
Each year since its first data capture in 2006, the EHT array has moved to add more observatories to its global network of radio telescopes. The first image of the Milky Way's supermassive black hole, Sagittarius A*, was expected to be produced from data taken in April 2017,but because there are no flights in or out of the South Pole during austral winter (April to October), the full data set could not be processed until December 2017, when the shipment of data from the South Pole Telescope arrived.
Data collected on hard drives are transported by commercial freight airplanes 40 Gbit/s network.(a so-called sneakernet) from the various telescopes to the MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy, where the data are cross-correlated and analyzed on a grid computer made from about 800 CPUs all connected through a
Because of the COVID-19 pandemic, weather patterns, and celestial mechanics, the 2020 observational campaign was postponed to March 2021.
The Event Horizon Telescope Collaboration announced its first results in six simultaneous press conferences worldwide on April 10, 2019.The announcement featured the first direct image of a black hole, which showed the supermassive black hole at the center of Messier 87, designated M87*. The scientific results were presented in a series of six papers published in The Astrophysical Journal Letters . Clockwise rotating black hole was observed in the 6σ region.
The image provided a test for Albert Einstein's general theory of relativity under extreme conditions.Studies have previously tested general relativity by looking at the motions of stars and gas clouds near the edge of a black hole. However, an image of a black hole brings observations even closer to the event horizon. Relativity predicts a dark shadow-like region, caused by gravitational bending and capture of light, which matches the observed image. The published paper states: "Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity." Paul T.P. Ho, EHT Board member, said: "Once we were sure we had imaged the shadow, we could compare our observations to extensive computer models that include the physics of warped space, superheated matter, and strong magnetic fields. Many of the features of the observed image match our theoretical understanding surprisingly well."
The image also provided new measurements for the mass and diameter of M87*. EHT measured the black hole's mass to be 6.5±0.7 billion solar masses and measured the diameter of its event horizon to be approximately 40 billion kilometres (270 AU; 0.0013 pc; 0.0042 ly), roughly 2.5 times smaller than the shadow that it casts, seen at the center of the image. Previous observations of M87 showed that the large-scale jet is inclined at an angle of 17° relative to the observer's line of sight and oriented on the plane of the sky at a position angle of −72°. From the enhanced brightness of the southern part of the ring due to relativistic beaming of approaching funnel wall jet emission, EHT concluded the black hole, which anchors the jet, spins clockwise, as seen from Earth. EHT simulations allow for both prograde and retrograde inner disk rotation with respect to the black hole, while excluding zero black hole spin using a conservative minimum jet power of 1042 erg/s via the Blandford–Znajek process.
Producing an image from data from an array of radio telescopes requires much mathematical work. Four independent teams created images to assess the reliability of the results.These methods included both an established algorithm in radio astronomy for image reconstruction known as CLEAN, invented by Jan Högbom, as well as self-calibrating image processing methods for astronomy such as the CHIRP algorithm created by Katherine Bouman and others. The algorithms that were ultimately used were a regularized maximum likelihood (RML) algorithm and the CLEAN algorithm.
In March 2020, astronomers proposed an improved way of seeing more of the rings in the first black hole image.In March 2021, a new photo was revealed, showing how the M87 black hole looks in polarised light. This is the first time astronomers have been able to measure polarisation so close to the edge of a black hole. The lines on the photo mark the orientation of polarisation, which is related to the magnetic field around the shadow of the black hole.
In April 2020, the EHT released the first 20 microarcsecond resolution images of the archetypal blazar 3C 279 it observed in April 2017.These images, generated from observations over 4 nights in April 2017, reveal bright components of a jet whose projection on the observer plane exhibit apparent superluminal motions with speeds up to 20 c. Such apparent superluminal motion from relativistic emitters such as an approaching jet is explained by emission originating closer to the observer (downstream along the jet) catching up with emission originating further from the observer (at the jet base) as the jet propagates close to the speed of light at small angles to the line of sight.
In July 2021, high resolution images of the jet produced by black hole sitting at the center of Centaurus A were released. With a mass around 5.5×107 M☉, the black hole is not big enough to observe its ring as with Messier M87*, but its jet extends even beyond its host galaxy while staying as a highly collimated beam which is a point of study. Edge-brightening of the jet was also observed which would exclude models of particle acceleration that are unable to reproduce this effect. The image was 16 times sharper than previous observations and utilized a 1.3 mm wavelength.
On May 12, 2022, the EHT Collaboration revealed an image of Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy. The black hole is 27,000 light-years away from Earth; it is thousands of times smaller than M87*. Sera Markoff, Co-Chair of the EHT Science Council, said: "We have two completely different types of galaxies and two very different black hole masses, but close to the edge of these black holes they look amazingly similar. This tells us that General Relativity governs these objects up close, and any differences we see further away must be due to differences in the material that surrounds the black holes."
The EHT Collaboration consists of 13 stakeholder institutes:
Institutions affiliated with the EHT include:
A black hole is a region of spacetime where gravity is so strong that nothing – no particles or even electromagnetic radiation such as light – can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of no escape is called the event horizon. Although it has an enormous effect on the fate and circumstances of an object crossing it, it has no locally detectable features according to general relativity. In many ways, a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is of the order of billionths of a kelvin for stellar black holes, making it essentially impossible to observe directly.
Messier 87 is a supergiant elliptical galaxy with several trillion stars in the constellation Virgo. One of the most massive galaxies in the local universe, it has a large population of globular clusters — about 15,000 compared with the 150–200 orbiting the Milky Way — and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.
Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. The distance between the radio telescopes is then calculated using the time difference between the arrivals of the radio signal at different telescopes. This allows observations of an object that are made simultaneously by many radio telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.
A supermassive black hole is the largest type of black hole, with its mass being on the order of millions to billions of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way has a supermassive black hole in its Galactic Center, corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei and quasars.
The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the Space ShuttleColumbia during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, enabled by the high angular resolution of its mirrors. Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes; therefore space-based telescopes are required to make these observations. Chandra is an Earth satellite in a 64-hour orbit, and its mission is ongoing as of 2022.
3C 279 is an optically violent variable quasar (OVV), which is known in the astronomical community for its variations in the visible, radio and x-ray bands. The quasar was observed to have undergone a period of extreme activity from 1987 until 1991. The Rosemary Hill Observatory (RHO) started observing 3C 279 in 1971, the object was further observed by the Compton Gamma Ray Observatory in 1991, when it was unexpectedly discovered to be one of the brightest gamma ray objects in the sky. It is also one of the brightest and most variable sources in the gamma ray sky monitored by the Fermi Space Telescope. It was used as a calibrator source for Event Horizon Telescope observations of M87* that resulted in the first image of a black hole.
Centaurus A is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta, in New South Wales, Australia. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type and distance. NGC 5128 is one of the closest radio galaxies to Earth, so its active galactic nucleus has been extensively studied by professional astronomers. The galaxy is also the fifth-brightest in the sky, making it an ideal amateur astronomy target. It is only visible from the southern hemisphere and low northern latitudes.
Sagittarius A*, abbreviated Sgr A* is the supermassive black hole at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, visually close to the Butterfly Cluster (M6) and Lambda Scorpii.
The Center for Astrophysics | Harvard & Smithsonian (CfA) is an astrophysics research institute jointly operated by the Harvard College Observatory and Smithsonian Astrophysical Observatory. Founded in 1973 and headquartered in Cambridge, Massachusetts, the CfA leads a broad program of research in astronomy, astrophysics, Earth and space sciences, as well as science education. The CfA either leads or participates in the development and operations of more than fifteen ground- and space-based astronomical research observatories across the electromagnetic spectrum, including the forthcoming Giant Magellan Telescope (GMT) and the Chandra X-ray Observatory, one of NASA's Great Observatories.
Fulvio Melia is an Italian-American astrophysicist, cosmologist and author. He is professor of physics, astronomy and the applied math program at the University of Arizona and was a scientific editor of The Astrophysical Journal and an associate editor of The Astrophysical Journal Letters. A former Presidential Young Investigator and Sloan Research Fellow, he is the author of six English books and 230 refereed articles on theoretical astrophysics and cosmology.
Roger David Blandford, FRS, FRAS is a British theoretical astrophysicist, best known for his work on black holes.
The CLEAN algorithm is a computational algorithm to perform a deconvolution on images created in radio astronomy. It was published by Jan Högbom in 1974 and several variations have been proposed since then.
Laura Ferrarese FRSC is a researcher in space science at the National Research Council of Canada. Her primary work has been performed using data from the Hubble Space Telescope and the Canada-France-Hawaii Telescope.
The Greenland Telescope is a radio telescope that is currently installed and operating at the Thule Air Base in north-western Greenland. It will later be deployed at the Summit Station research camp, located at the highest point of the Greenland ice sheet at an altitude of 3,210 meters.
CHIRP is a Bayesian algorithm used to perform a deconvolution on images created in radio astronomy. The acronym was coined by lead author Katherine L. Bouman in 2016.
Jan Arvid Högbom is a Swedish radio astronomer and astrophysicist.
Sheperd "Shep" S. Doeleman is an American astrophysicist. His research focuses on super massive black holes with sufficient resolution to directly observe the event horizon. He is a senior research fellow at the Harvard–Smithsonian Center for Astrophysics and the Founding Director of the Event Horizon Telescope (EHT) project. He led the international team of researchers that produced the first directly observed image of a black hole.
Johann Anton Zensus is a German radio astronomer. He is director at the Max Planck Institute for Radio Astronomy (MPIfR) and honorary professor at the University of Cologne. He is chairman of the collaboration board of the Event Horizon Telescope (EHT). The collaboration announced the first image of a black hole in April 2019.
Ramesh Narayan is an Indian-American theoretical astrophysicist, currently the Thomas Dudley Cabot Professor of the Natural Sciences in the Department of Astronomy at Harvard University. Full member of the National Academy of Sciences, Ramesh Narayan is widely known for his contributions on the theory of black hole accretion processes. Recently he is involved in the Event Horizon Telescope project, which led in 2019 to the first image of the event horizon of a black hole.
James Moran is an American radio astronomer living in Massachusetts, USA. He was a professor of Astronomy at Harvard University from 1989 through 2016, a senior radio astronomer at the Smithsonian Astrophysical Observatory from 1981 through 2020 and the director of the Submillimeter Array during its construction and early operational phases from 1995 through 2005. In 1998 he was elected to the National Academy of Sciences, in 2010 to the American Academy of Arts and Sciences, and in 2020 to the American Philosophical Society. He is currently the Donald H. Menzel Professor of Astrophysics, Emeritus, at Harvard University.
“For the case of M87*, which is the designation of this black hole, a (very nice) name has been proposed, but it has not received an official IAU approval,” says Christensen.
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