Why was the image of the Milky Way’s black hole obtained 3 years later than expected?

It’s here at last: First-ever image of the Milky Way’s central black hole Just revealed! After five years of calculations and analysis of thousands of images, researchers have finally succeeded in visualizing arc A*.

The saga of this tech feat began with a surveillance campaign in April 2017 by Interferometry Very long base, also called VLBI. This monitoring technology consists of the simultaneous use of several radio telescopes In the world, in order to create the equivalent of a giant Interferometer Earth size. By doing so, the Precision Get the angular that sets the smallest angular size Researchers can observe them in detail, they become so small that many normally invisible things become visible.

It is with M87 * and Sagittarius A*, two black holes with similar apparent diameters, and they require a resolution that VLBI can reach. Actually, M87* It is much larger and much more distant than Sgr A*, with 6.5 billion masses Solar cells and a distance of 50 millionlight yearsso its visible diameter from Earth is equivalent to that of Sgr A*.

A total of eight radio telescopes They were used at one time, located in Chile, the United States, Mexico, Spain and South Pole ! Since then, three more telescopes Join the teamevent horizon telescope with, in particular, Greenland Telescope It is located northwest of Greenland and is the second observatory of IRAM NovemberIt is located in the French Alps. This process requires a lot of preparation, because the synchronization must be perfect: radio telescopes have also been used. atomic clocks, which causes a delay of only one second every 10 million years. The only monitoring campaign in this way took place in 2017, more precisely from 4 to 14 April 2017. Then came the analysis of data, and here everything became complicated!

This stage is as difficult as the first. In total, more than 350 people took part in this technological feat. And so, in 2019, The first photo of the M87 * It was revealed, but the arc A*, which the scientific community was also expecting, was not.

The reason for the delay: the diameter of the black hole is too small!

Since Sgr A* is much smaller, the acoustic material Accumulation disk It turns much faster, at a frantic pace of rotation in just 4 minutes and 30! Thus, during the monitoring campaign, the researchers observed these differences in real time, so to speak, because the campaigns lasted several hours, several times a day, for about two weeks.

« if Gas Nearby black holes are moving at the same speed – roughly the speed of light – around Sgr A* and M87*. But while it takes days or even weeks to circle the large M87*, it only takes minutes to do so around the much smaller Sgr A*. This means that the file brightness And the gas pattern around Sgr A* changed rapidly as observed by the EHT collaboration—much like trying to take a clear picture of a puppy chasing its tail ‘, explained in CNRS Press Release Chi Kwan (“CK”) Chan, a researcher at the Steward Observatory, Department of Astronomy and Data Science Institute at the University of Arizona, USA.

So many means have been implemented to get a static image of Sgr A*, in particular a huge library of black hole simulations that have been compared with real images. All of these calculations also require what is called supercomputers which enables unimaginable computing power, with tens of thousands of Treatments.

« These rapid changes in luminosity and composition around Sgr A* generate a data set much more complex than that of M87*, so we had to develop new tools Digital simulation and analysis a movement gases to generate a sharp imagesaid Frederic Gith, a researcher at the National Center for Scientific Research and deputy director of the Institute for Millimeter Radio Astronomy (IRAM). Which explains why it took us three more years to visualize Sgr A*. »