Astronomers Have Taken The Closest Look Yet At A Supermassive Black Hole

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At the very heart of the Milky Way lies a supermassive black hole called Sagittarius A*. We know about its properties thanks to indirect methods, like observing passing stars, but astronomers have worked hard over the past several years to obtain direct observations of it. We are not there yet, but we are really close.

In The Astrophysical Journal, researchers report the closest observation of Sagittarius A* ever made. The team looked at a region with a 36-million-kilometer (22-million-mile) radius, three times the actual radius of the supermassive black hole. These observations have allowed the researchers to find the two best models to explain how the matter around the black hole is distributed.

“We started to figure out what the horizon-scale structure may look like, rather than just draw generic conclusions from the visibilities that we sampled," lead author Ru-Sen Lu, from the Max Planck Institute for Radio Astronomy, said in a statement. "It is very encouraging to see that the fitting of a ring-like structure agrees very well with the data, though we cannot exclude other models, e.g., a composition of bright spots.” 

Schematics of the observational approach and potential models for Sagittarius A*. Eduardo Ros/Thomas Krichbaum (MPIfR)

These observations were conducted in 2013 using radio telescopes at four different sites. Radio telescope observations can be improved by a technique called interferometry. Basically, if you use distant radio telescopes together, your image resolution is proportional to their separation. The further apart they are, the better. In 2013, APEX, which is located in Chile, joined three telescopes from the Northern Hemisphere, and their combined power delivered this result.

But this is just the beginning. Last year, more telescopes joined this mission and astronomers were able to use them as the single Event Horizon Telescope (EHT). This was named after the last visible surface of a black hole and, thanks to telescopes from Europe to Antarctica, was effectively the size of our planet.

“The results are an important step to ongoing development of the Event Horizon Telescope”, said Sheperd Doeleman from the Harvard-Smithsonian Center for Astrophysics and director of the EHT project. “The analysis of new observations, which since 2017 also include ALMA, will bring us another step closer to imaging the black hole in the centre of our Galaxy.”

The EHT observed Sagittarius A* in April 2017 and researchers are currently busy analyzing the data. The first image of a supermassive black hole could be with us very soon.


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