Event Horizon Telescope


May 17, 2022

The Event Horizon Telescope (EHT) is a network of radio telescopes used to study distant black holes using Very Long Baseline Interferometry (VLBI). Radio telescopes around the world record signals caused by the black holes. The resulting series of measurements are stored (the amounts of data are too large to be sent via the Internet) and taken to computer centers (such as the VLBI correlator at the Max Planck Institute for Radio Astronomy in Bonn) on data carriers (racks with hard drives), where they are evaluated . The large distance between the telescopes on the earth's surface enables an angular resolution that is far above that of the individual radio telescopes. The first two targets of the cluster are the supermassive black hole Sagittarius A* at the center of the Milky Way and the black hole at the center of the giant elliptical galaxy M87. This should be used to check predictions of the general theory of relativity and to find explanations for the formation of the extremely energetic jets of supermassive black holes. First suggestions, such as the "event horizon" or more precisely the "shadow" (you don't "see" the event horizon but that through the curvature of the light image of the surroundings formed by the black hole with a shadow in the middle) could be observed with interconnected radio telescopes was made in 2000 by Heino Falcke, Fulvio Melia and Eric Agol. The plans for the EHT were finalized at a meeting of radio astronomers in Tucson in January 2012 (“Bringing Black Holes into Focus: The Event Horizon Telescope”). It is observed at a wavelength of 1.3 mm (230 GHz); Observations at even shorter wavelengths (0.87 mm, equivalent to 345 GHz) are in preparation for 2019. The key element to the success of the EHT is that the ALMA radio interferometer was made VLBI-capable in 2016 at 230 GHz (also in the 86 GHz band, corresponding to 3.5 mm wavelength, in the Global Millimeter VLBI Array, GMVA). It first participated in VLBI observations at 86 GHz (GMVA) and 230 GHz (EHT) in early April 2017, greatly improving north-south resolution by a factor of three and sensitivity. In 2022, a picture of Sagittarius A* was also achieved. Director of the EHT since August 2020 is Huib Jan van Langevelde. Founding Director is Shep Doeleman. The Chair of the EHT Board is Colin J. Lonsdale; The founding chairman of the board is Anton Zensus. The Scientific Advisory Board is headed by Daniel Marrone, successor to Heino Falcke. The EHT collaboration was awarded the Breakthrough Prize in Fundamental Physics and the Albert Einstein Medal (both for 2020) and Breakthrough of the Year for 2019.

Black hole in galaxy M87

On April 10, 2019, the first high-resolution images of the active core of the galaxy M87 were presented to the public. This was the end result of months of analysis using complex image processing algorithms and the exclusion of interference effects. M87 is 55 million light-years from Earth, and the black hole at its center has a mass estimated at 6.6 billion solar masses prior to the EHT observation now available. The accretion flows of heated matter around a black hole, which are distorted by gravitation, were shown for the first time. The ring shown is 42 ± 3 micro arc seconds in diameter and less than 20 micro arc seconds in width. The inner edge of the ring can be identified from comparison with various computer simulations with the so-called shadow of the black hole. The shadow is called the gravitationally distorted projection of the area from which no light escapes and bounded by the photon orbit on which d