30 Mar 2017

Observations in a world-wide radio telescope network in April 2017

The „Event Horizon Telescope”(EHT) is a project in international collaboration with the goal to directly image the immediate environment of a black hole. This will allow to test Einstein’s Theory of General Relativity in an extreme regime. A number of European research institutes including the Max Planck Institute for Radio Astronomy in Bonn/Germany participate in the observations within the framework of an international consortium.

The EHT consortium consists of 14 institutes with about 200 participants in Europe, Asia, Africa and America. President of the ERC council is Prof. J. Anton Zensus from the Max Planck Institute for Radio Astronomy (MPIfR), Director of the ERC is Dr. Shepherd S. Doeleman (Harvard & MIT, USA).
The research department of Prof. Michael Kramer at MPIfR is participating via the „BlackHoleCam“ (BHC) project, founded by the European Research Council (ERC), in collaboration with Prof. Heino Falcke (Radboud University Nijwegen, The Netherlands) and Prof. Luciano Rezzolla (Frankfurt University, Germany).

The technique applied for the EHT observations is called Very Long Baseline Interferometry (VLBI). VLBI enables the highest resolutions in astronomy by coupling a number of radio telecopes distributed across different countries on Earth. This method is used for the investigation of the direct environment of supermassive black holes in active galactic nuclei, in particular jets of high-energy particles emitted from the central regions. In the framework of the EHT project it will become possible to directly image the central black holes in addition to the jets. This is achieved by observations at shorter radio waves of only 1.3 mm wavelength. The resolution of the world-wide network of radio telescopes at that wavelength corresponds to a magnification factor of two million or the size of a tennis ball in the distance of the moon.
To minimize the impact of the Earth’s atmosphere at that wavelength, the observations are only possible at high-altitude dry sites like the Atacama desert in Chile, the Sierra Nevada in southern Spain, high volcanoes at Hawaii or even the South Pole.

Including the Atacama Large Millimeter Arrays (ALMA) with its 64 dishes in total provides a very high sensitivity. In total it is synthesizing a radio telescope with an equivalent diameter of 84 meters, superior to the usual Millimeter-wave radio telescopes with 15 to 30 meters in diameter. After a preparation phase of several years observations within the EHT project will now take place between April 04 and April 14 this year (see below).
VLBI data sets are analyzed in dedicated super computers, the so-called correlators. For the analysis of the EHT observations two correlators will be used, at the Max Planck Institute for Radio Astronomy in Bonn (Head of the Correlator group: Walter Alef) and at Haystack Observatory in Haystack, Massachusetts, USA.

For an overall picture of the physics of black holes the observations are complemented by numerical simulations and tests with synthetical data.
The obervations are co-financed by the European ERC project BlackHoleCam (BHC) and supported among others by Max Planck Society.
As a part of the BHC project, MPIfR scientists are searching for pulsars in the direct neighbourhood of the black hole in the centre of the Milky Way in order to establish independent measurements of its properties.

[Image]
(A) An artist’s impression of the quasar 3C 279. Astronomers connected the Atacama Pathfinder Experiment (APEX), in Chile, to the Submillimeter Array (SMA) in Hawaii, USA, and the Submillimeter Telescope (SMT) in Arizona, USA for the first time, to make the sharpest observations ever, of the centre of a distant galaxy, the bright quasar 3C 279. Quasars are the very bright centres of distant galaxies that are powered by supermassive black holes. This quasar contains a black hole with a mass about one billion times that of the Sun, and is so far from Earth that its light has taken more than 5 billion years to reach us. The team were able to probe scales of less than a light-year across the quasar — a remarkable achievement for a target that is billions of light-years away.

(B) Antennas of the Event Horizon Telescope used in April 2017 (clockwise from upper left): APEX, Pico Veleta, LMT, JCMT, ALMA, SMT (Heinrich Hertz Telescope), SMA, SPT.