Gadget in Extremis: EHT radio telescope array captures first picture of Black hole

  • Miles
  • April 12, 2019
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Gadget in Extremis: EHT radio telescope captures first picture of Black hole

We’re talking the rather-large black hole at the heart of Messier 87 in the galaxy Virgo A, which has now been captured by the Event Horizon Telescope (EHT) collaboration – a planet-scale array of eight ground-based radio telescopes.

It’s been linking radio dishes across the globe to create an “Earth-sized interferometer” in order to measure the size of the emission regions of two supermassive black holes (SgrA* at the centre of the Milky Way and M87 in the galaxy Virgo A).

In a nutshell, the image shows the ring formed as light bends in the gravity around the black hole.

Supermassive black hole

Altogether, it’s an amazing scientific experiment – showing what can be achieve when the finest minds work together with expensive technology. Officially, the EHT is described as a very long baseline interferometry (VLBI) array, which comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth.

“We have taken the first picture of a black hole,” said EHT project director Sheperd S. Doeleman of the Center for Astrophysics at Harvard & Smithsonian. “This is an extraordinary scientific feat accomplished by a team of more than 200 researchers.”

Reportedly, the level of detail involved, captured at such a distance, is equivalent to seeing the seeing details of an orange on the moon while positioned on the Earth.

What is shown in the now-famous image is the calibration and reduction of the 1.3 mm radio wavelength observations of the supermassive black hole that were taken during the 2017 April 5–11 observing campaign.

And the numbers involved are amazing – the black hole’s boundary measures 40 billion km across, which 6.5 billion times bigger than the Sun, says EHT.

VLBI

The EHT writes of the Very Long Baseline Interferometry (VLBI):

Addition of key millimeter and submillimeter wavelength facilities at high altitude sites has now opened the possibility of imaging such features and sensing the dynamic evolution of black hole accretion. The EHT project includes theoretical and simulation studies that are framing questions rooted at the black hole boundary that may soon be answered through observations.

By linking together existing telescopes using novel systems, the EHT leverages considerable global investment to create a fundamentally new instrument with angular resolving power that is the highest possible from the surface of the Earth. Over the coming years, the international EHT team will mount observing campaigns of increasing resolving power and sensitivity, aiming to bring black holes into focus.

The infographic below shows a simulation of the outflow (bright red) from a black hole and the accretion disk around it, with simulated images of the three potential shapes of the event horizon’s shadow. Credit due to: ESO/N. Bartmann/A. Broderick/C.K. Chan/D. Psaltis/F. Ozel

Eight observatories

To build the virtual telescope, eight observatories at six geographical locations were involved: the ALMA and the Atacama Pathfinder Experiment (APEX, in the Atacama Desert in Chile), the Large Millimeter Telescope Alfonso Serrano (LMT, on the Volcán Sierra Negra in Mexico), the South Pole Telescope (SPT, at the geographic south pole), the IRAM 30m telescope (PV, on Pico Veleta in Spain), the Submillimeter Telescope (SMT, on Mount Graham in Arizona), and the Submillimeter Array (SMA) and the James Clerk Maxwell Telescope (JCMT, on Maunakea in Hawaii).

“These global very long baseline interferometric observations include for the first time the highly sensitive Atacama Large Millimeter/submillimeter Array (ALMA); reaching an angular resolution of 25 µas, with characteristic sensitivity limits of ~1 mJy on baselines to ALMA and ~10 mJy on other baselines. The observations present challenges for existing data processing tools, arising from the rapid atmospheric phase fluctuations, wide recording bandwidth, and highly heterogeneous array. In response, we developed three independent pipelines for phase calibration and fringe detection, each tailored to the specific needs of the EHT. The final data products include calibrated total intensity amplitude and phase information. They are validated through a series of quality assurance tests that show consistency across pipelines and set limits on baseline systematic errors of 2% in amplitude and 1° in phase.”

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