It took five years to “develop” the first photo of a black hole, captured in 2017. So many to be revealed only now, that is to say three years after what everyone has called the “photo of the century” which portrayed another black hole: M87.
Although the two cosmic giants were observed on the same days, “see” Sagittarius A* – the supermassive black hole that occupies the center of our galaxy, the Milky Way – proved to be a challenge. First of all for a matter of “restlessness” of the portrayed subject, a little like a photo that is “moved” because whoever is in front of the camera never stands still. And for the fact that he hides behind a thick blanket of dust.
In addition to collecting as much radio signal as possible that comes from 26,000 light years awaytherefore, it was necessary to find a new way to clean it and transform it into a vision scientifically valid: “The main problem concerned the variability of Sagittarius A *, very difficult to treat – explains to Italian Tech Ciriaco Goddi, astrophysicist of the University of Cagliari, affiliated with INAF and member of the international collaboration Event Horizon Telescope (Eht) – a variability that was not there for M87 because it is at least 1,500 times larger.
Space
The first image of the black hole at the center of the Milky Way has been revealed
by Matteo Marini
12 Maggio 2022
The black hole at the center of our galaxy it is so small that the photons emitted by the gases barely take up ten minutes to walk around it. We are seeing an image produced by observations made during one night, but its appearance, during the night, changed hundreds of times ”.
A telescope the size of the planet
The problem of variability, according to Goddi, took longer to solve: “We had to develop five software to go from measurement to image. The big problem is the fact that the data is not uniformly sampled ”. The technique that Eht uses is that ofvery broad-based interferometry (Very-long-baseline interferometry, Vlbi). It is used to observe very distant sources which also emit radio waves, such as plasma around a black hole.
Several radio telescopes observe the same object at the same time, and it is as if they constitute a virtual parabola the size of the whole Earth, because they are located in different parts of the world. In 2017 there were eight, from Hawaii to Spain, from the USA to Chile to the South Pole. But in this “disk” there are missing pixels due to the distance that separates them: “In three years we have produced millions of sample images using various software and parameters. We couldn’t choose one, the final one is the average of all the images produced. Only one thing has never changed: the size of the ring, that tells us that there is a black hole in the center ”.
However, imaging is the last step in a very complicated process. For observation, first of all, all the telescopes in the network need to look in the same direction, in a limited period of time, during a night, to collect as much light as possible.
As the Earth turns, each observatory points towards the ‘target’. Goddi has coordinated since 2014 the European group of Eht under the guidance of BlackHoleCam. In the past years, he has been responsible for the observations, calibration, and data analysis of the Atacama Large Millimeter / submillimeter Array (Alma), in Chile, among the eight radio telescopes of the EHT network, the largest and most sensitive. So every night the count started. “We have to coordinate. If multiple telescopes have weather problems, for example, it is not observed. But even if Alma alone is not in good condition, no observations are made that night, because Alma is essential ”he specifies.
A ton of data
At the end of the observation campaign, the scientists had to put together a huge amount of raw data. “The data from each telescope is saved on disks, each of 10 Terabytes. Together they will weigh a ton, for a total of about 5 Petabytes of data. Impossible to share on the Net, as already for M87, we physically shipped them by courier, to the processing centers at MIT in Boston and in Germany ”says Goddi. There, independently, they were “related”, that is all electromagnetic waves from 26,000 light years away they have been synchronized using atomic clocks whose error is miniscule, on the order of one billionth of a second per year. To form a single, detailed observation.
Behind a curtain of dust
The second phase instead involved having to deal with another obstacle: the thick cloud of gas and dust that surrounds Sagittarius A *. It is not a question of playing hide and seek, the radio waves, in fact, have the advantage of “piercing” these blankets and making their way up to us. But having to cross them disturbs the signal: “The emissions are influenced by the ionized gas of the clouds, the phenomenon is called” scattering “: the electrons scatter the signal and the radiation that comes out is modified – specifies the astrophysicist – also for this reason we had to write software through which flow and calibrate all data “.
The result of one night of observing kept busy for about five years 300 researchers around the world. The result, however, pays off the effort: “For M87, the stress of so much work was dictated by the fact that we didn’t know if we would succeed. This time we were aware that the tool worked. But we didn’t know if all that interference would mask the source we were looking at. It was a great satisfaction to see her ”concludes Goddi.