Event Horizon Telescope

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 with 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), and Sagittarius A* (Sgr A*) at the center of the Milky Way.[1][2][3]

Event Horizon Telescope
Alternative namesEHT 
Websiteeventhorizontelescope.org
TelescopesAtacama Large Millimeter Array
Atacama Pathfinder Experiment
Heinrich Hertz Submillimeter Telescope
IRAM 30m telescope
James Clerk Maxwell Telescope
Large Millimeter Telescope
South Pole Telescope
Submillimeter Array 
Related media on Wikimedia Commons

The Event Horizon Telescope project is an international collaboration launched in 2009[1] after a long period of theoretical and technical developments. On the theory side, work on the photon orbit[4] and first simulations of what a black hole would look like[5] progressed to predictions of VLBI imaging for the Galactic Center black hole, Sgr A*.[6] Technical advances in radio observing moved from the first detection of Sgr A*,[7] through VLBI at progressively shorter wavelengths, ultimately leading to detection of horizon scale structure in both Sgr A* and M87.[8] The collaboration now comprises over 300[9] members, 60 institutions, working over 20 countries and regions.[3]

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.[10] The array made this observation at a wavelength of 1.3 mm and with a theoretical diffraction-limited resolution of 25 microarcseconds. Future plans involve improving the array's resolution by adding new telescopes and by taking shorter-wavelength observations.[2][11]

Telescope array
Soft X-ray image of Sagittarius A* (center) and two light echoes from a recent explosion (circled)
A schematic diagram of the VLBI mechanism of EHT. Each antenna, spread out over vast distances, has an extremely precise atomic clock. Analogue signals collected by the antenna are converted to digital signals and stored on hard drives together with the time signals provided by the atomic clock. The hard drives are then shipped to a central location to be synchronized. An astronomical observation image is obtained by processing the data gathered from multiple locations.

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.[12] The effort includes development and deployment of submillimeter dual polarization receivers, highly stable frequency standards to enable very-long-baseline interferometry at 230–450 GHz, higher-bandwidth VLBI backends and recorders, as well as commissioning of new submillimeter VLBI sites.[13]

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 in April 2017,[14][15] but because the South Pole Telescope is closed during winter (April to October), the data shipment delayed the processing to December 2017 when the shipment arrived.[16]

Data collected on hard drives are transported by commercial freight airplanes[17] (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 40 Gbit/s network.[18]

Because of COVID-19 pandemic, weather patterns, and celestial mechanics, the 2020 observational campaign was postponed to March 2021.[19]

Messier 87*
First image of the shadow of a black hole (M87*) captured by the Event Horizon Telescope[20][21]

The Event Horizon Telescope Collaboration announced its first results in six simultaneous press conferences worldwide on April 10, 2019.[22] 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*.[2][23][24] The scientific results were presented in a series of six papers published in The Astrophysical Journal Letters.[25]

The image provided a test for Albert Einstein's general theory of relativity under extreme conditions.[12][15] 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.[26] 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."[27] 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."[25]

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.[25][26] 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°.[2][28] 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.[2][11] 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.[2][29]

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.[30] These methods included both an established algorithm in radio astronomy for image reconstruction known as CLEAN, invented by Jan Högbom,[31] as well as self-calibrating image processing methods[32] for astronomy such as the CHIRP algorithm created by Katherine Bouman and others.[30][33] The algorithms that were ultimately used were a regularized maximum likelihood (RML)[34] algorithm and the CLEAN algorithm.[30]

In March 2020, astronomers proposed a way of better seeing more of the rings in the first black hole image.[35][36]

3C 279
EHT image of the archetypal blazar 3C 279 showing a relativistic jet down to the AGN core surrounding the supermassive 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.[37] 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.[38] 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.

Collaboration

The EHT Collaboration consists of 13 stakeholder institutes:[3]

Institutions affiliated with the EHT include:[39]

gollark: ++search wernicke's area
gollark: I think it has been shown that we have language processing brain areas at least. Not that that says much about how flexible they are.
gollark: Wait, no, ABR is sentient, never mind.
gollark: It is not a property of conscious minds, then.
gollark: ++bee you

References
  1. Doeleman, Sheperd (June 21, 2009). "Imaging an Event Horizon: submm-VLBI of a Super Massive Black Hole". Astro2010: The Astronomy and Astrophysics Decadal Survey, Science White Papers. 2010: 68. arXiv:0906.3899. Bibcode:2009astro2010S..68D.
  2. The Event Horizon Telescope Collaboration (April 10, 2019). "First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole". The Astrophysical Journal Letters. 875 (1): L1. arXiv:1906.11238. Bibcode:2019ApJ...875L...1E. doi:10.3847/2041-8213/ab0ec7.
  3. "Event Horizon Telescope Official Website". eventhorizontelescope.org. Retrieved April 22, 2018.
  4. Bardeen, James (1973). "Black holes. Edited by C. DeWitt and B. S. DeWitt". Les Houches Ecole d'Ete de Physique Theorique. Bibcode:1973blho.conf.....D.
  5. Luminet, Jean-Pierre (July 31, 1979). "Image of a spherical black hole with thin accretion disk". Astronomy and Astrophysics. 75: 228. Bibcode:1979A&A....75..228L.
  7. Balick, Bruce; Brown, R.L. (December 1, 1974). "Intense sub-arcsecond structure in the galactic center". The Astrophysical Journal. 194 (1): 265–279. Bibcode:1974ApJ...194..265B. doi:10.1086/153242.
  9. "Winners Of The 2020 Breakthrough Prize In Life Sciences, Fundamental Physics And Mathematics Announced". Breakthrough Prize. Retrieved March 15, 2020.
  10. Shep Doeleman, on behalf of the EHT Collaboration (April 2019). "Focus on the First Event Horizon Telescope Results". The Astrophysical Journal Letters. Retrieved April 10, 2019.
  11. Susanna Kohler (April 10, 2019). "First Images of a Black Hole from the Event Horizon Telescope". AAS Nova. Retrieved April 10, 2019.
  12. O'Neill, Ian (July 2, 2015). "Event Horizon Telescope Will Probe Spacetime's Mysteries". Discovery News. Archived from the original on September 5, 2015. Retrieved August 21, 2015.
  13. "MIT Haystack Observatory: Astronomy Wideband VLBI Millimeter Wavelength". www.haystack.mit.edu.
  14. Webb, Jonathan (January 8, 2016). "Event horizon snapshot due in 2017". BBC News. Retrieved March 24, 2016.
  15. Davide Castelvecchi (March 23, 2017). "How to hunt for a black hole with a telescope the size of Earth". Nature. 543 (7646): 478–480. Bibcode:2017Natur.543..478C. doi:10.1038/543478a. PMID 28332538.
  16. "EHT Status Update, December 15 2017". eventhorizontelescope.org. Retrieved February 9, 2018.
  17. "The Hidden Shipping and Handling Behind That Black-Hole Picture". The Atlantic. Retrieved April 14, 2019.
  18. Mearian, Lucas (August 18, 2015). "Massive telescope array aims for black hole, gets gusher of data". Computerworld. Retrieved August 21, 2015.
  19. "EHT Observing Campaign 2020 Canceled Due to the COVID-19 Outbreak". eventhorizontelescope.org. Retrieved March 29, 2020.
  20. Overbye, Dennis (April 10, 2019). "Black Hole Picture Revealed for the First Time – Astronomers at last have captured an image of the darkest entities in the cosmos". The New York Times. Retrieved April 10, 2019.
  21. Landau, Elizabeth (April 10, 2019). "Black Hole Image Makes History". NASA. Retrieved April 10, 2019.
  22. "Media Advisory: First Results from the Event Horizon Telescope to be Presented on April 10th". Event Horizon official blog. Event Horizon Telescope. April 1, 2019. Retrieved April 10, 2019.
  23. Lu, Donna (April 12, 2019). "How do you name a black hole? It is actually pretty complicated". New Scientist. London. Retrieved April 12, 2019. “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.
  24. Gardiner, Aidan (April 12, 2018). "When a Black Hole Finally Reveals Itself, It Helps to Have Our Very Own Cosmic Reporter - Astronomers announced Wednesday that they had captured the first image of a black hole. The Times's Dennis Overbye answers readers' questions". The New York Times. Retrieved April 15, 2019.
  25. "Astronomers Capture First Image of a Black Hole". European Southern Observatory. April 10, 2019. Retrieved April 10, 2019.
  26. Lisa Grossman, Emily Conover (April 10, 2019). "The first picture of a black hole opens a new era of astrophysics". Science News. Retrieved April 10, 2019.
  27. Jake Parks (April 10, 2019). "The nature of M87: EHT's look at a supermassive black hole". Astronomy. Retrieved April 10, 2019.
  28. R. C. Walker, P. E. Hardee, F. B. Davies, C. Ly and W. Junor, "The Structure and Dynamics of the Subparsec Jet in M87 Based on 50 VLBA Observations over 17 Years at 43 GHz", ApJ 855:128 (2018).
  29. R. D. Blandford and R. L. Znajek, "Electromagnetic extraction of energy from Kerr black holes", Mon. Not. R. Astr. Soc. 179:433-456 (1977).
  30. The Event Horizon Telescope Collaboration (2019). "First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole". Astrophysical Journal Letters. 87 (1): L4. arXiv:1906.11241. Bibcode:2019ApJ...875L...4E. doi:10.3847/2041-8213/ab0e85.
  31. Högbom, Jan A. (1974). "Aperture Synthesis with a Non-Regular Distribution of Interferometer Baselines". Astronomy and Astrophysics Supplement. 15: 417–426. Bibcode:1974A&AS...15..417H.
  32. SAO/NASA Astrophysics Data System (ADS): Seitz, Schneider, and Bartelmann (1998) Entropy-regularized maximum-likelihood cluster mass reconstruction cites Narayan and Nityananda 1986.
  33. "The creation of the algorithm that made the first black hole image possible was led by MIT grad student Katie Bouman". TechCrunch. Retrieved April 15, 2019.
  34. Narayan, Ramesh and Nityananda, Rajaram (1986) "Maximum entropy image restoration in astronomy" Annual Review of Astronomy and Astrophysics Volume 24 (A87-26730 10-90). Palo Alto, CA, Annual Reviews, Inc. p. 127–170.
  35. Overbye, Dennis (March 28, 2020). "Infinite Visions Were Hiding in the First Black Hole Image's Rings - Scientists proposed a technique that would allow us to see more of the unseeable". The New York Times. Retrieved March 29, 2020.
  36. Johnson, Michael D.; et al. (March 18, 2020). "Universal interferometric signatures of a black hole's photon ring". Science Advances. 6 (12, eaaz1310). doi:10.1126/sciadv.aaz1310. Retrieved March 29, 2020.
  38. "Something is Lurking in the Heart of Quasar 3C 279". Event Horizon Telescope. Retrieved April 20, 2019.
  39. "Affiliated Institutes". eventhorizontelescope.org. Retrieved April 10, 2019.
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