The Event Horizon Telescope (EHT) project will use a global network of radio telescopes employing the sophisticated technique of Very Long Baseline Interferometry (VLBI) to resolve and image the regions near two nearby supermassive black holes. One in the center of our own Milky Way Galaxy ("SgrA*", pronounced "Sagittarius-A star") has a mass four million times that of our own Sun; the other, in the center of a massive galaxy 50 million light years away (known as "M87"), has a measured mass several billion times that of the Sun. These regions will be imaged at radio wavelengths, showing detail on the scale of the apparent black-hole diameter ("Event Horizon"). The study of massive black holes has many applications in astrophysics and is of great popular interest; such objects are thought to fuel the most powerful activity seen in far-distant galaxies. Supermassive black holes are extreme examples of Einstein's theory of General Relativity, but the actual mechanism by which they form remains uncertain. Because of the popularity of black holes, the numerous public outreach efforts of this project will enthrall thousands of members of the general public.

The Event Horizon Telescope includes using the recently completed, NSF-funded Atacama Large Millimeter/submillimeter Array (ALMA), as well as other large, high-precision radio telescopes worldwide. This project will support assembly of a wideband VLBI correlator to process the torrent (5 Petabytes per session) of EHT data, construction of a VLBI system at the Large Millimeter Telescope (LMT) in Mexico, and development of a robust data processing pipeline. Through these developments, the EHT will track time-variable structure at the Event Horizon of SgrA*, and search for periodic signatures of orbiting inhomogeneities in the innermost accretion flow. Observations of M87 will target study of the jet-launching region of a radio-loud AGN, to cleanly distinguish between models of jet genesis whose predictions differ markedly close to the black hole. New polarimetric EHT data will reveal Schwarzschild scale magnetic structures, providing unique constraints on accretion models, as well as General Relativistic magneto-hydrodynamical (MHD) simulations.

The Event Horizon Telescope will test General Relativity (GR) in the strong gravity limit, and as its ultimate and most fundamental prize, the EHT aims at detecting the signature of the photon orbit (and enclosed "shadow") predicted by GR. In addition, while only a few sources can potentially be imaged in the innermost regions, the array has the ability to provide unique and vital time domain information with high angular resolution on the structure and magnetic fields for many other active galactic nuclei (AGN), probing a spatial region of the jet outflow which is very poorly studied with current instrumentation. These topics are all at the forefront of work in AGN physics, and the anticipated data have the potential to revolutionize our understanding of black holes and their environments and to greatly enhance our understanding of the formation and evolution of jetted sources.

Broader impacts of the work include hands-on training of students and early career scientists in radio astronomy instrumentation and mm/submm interferometry, and those mentored within this project will emerge with unique observational and hardware skills that will be invaluable in the era of ALMA. All EHT data will be made available to the astronomy community, which will maximize its broad astronomical impact across many fields and disciplines, both nationally and internationally. The project includes a public outreach effort through a Scientific American blog, a possible partnership with the Smithsonian Channel, and Center for Astrophysics' traveling exhibition.

National Science Foundation (NSF)
Division of Astronomical Sciences (AST)
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Philip J. Puxley
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Smithsonian Institution Astrophysical Observatory
United States
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