Behind the photo of the black hole at the center of the Milky Way: China is planning to build a submillimeter-wave VLBI telescope

  ”The supermassive black hole in the center of the M87 galaxy was detected last time, and the supermassive black hole in the center of our galaxy was observed this time. These two targets are also two supermassive black holes that are relatively close to humans.” Chen Xuelei, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, said .
  The Shanghai Observatory of the Chinese Academy of Sciences held a press conference on “Breakthrough Results from the Event Horizon Telescope on the Galactic Center”, and released a breakthrough on the Galactic Center from the Event Horizon Telescope (EHT).
  Experts have shown the first images of the supermassive black hole at the center of the Milky Way. It is reported that the Event Horizon Telescope Cooperation Organization “taken” this photo by relying on a network of radio telescopes located in many places around the world.

The Event Horizon Telescope EHT consists of eight radio telescopes that together form a virtual telescope the size of Earth

  Not only is it the first image of the supermassive black hole Sgr A* at the center of the Milky Way, but it is also the first direct visual evidence that such a black hole actually exists. To be more precise, this photo was captured by the virtual telescope EHT, which is located on the earth. The telescope consists of 8 radio telescopes and forms a virtual telescope comparable to the size of the earth. They are Telescopes from Antarctica, Chile, Mexico, Arizona, USA, Hawaii, USA (operated by China), Spain. During the observation period, the EHT observed Sagittarius A* on multiple nights, and each acquisition lasted for several hours, which is similar to the principle of long exposure of the same camera.

  It is reported that the earth we live in is 27,000 light-years away from the black hole at the center of the Milky Way. Regarding the difficulty of detecting the black hole, experts from the Shanghai Observatory likened it to being as difficult as using a telescope on Earth to check a punctuation on a newspaper on the moon.
  The photos released this time were actually taken in 2017. In this observation, EHT not only captured the image of the black hole at the center of the Milky Way announced today, but also captured the M87 black hole released in 2019. Because M87 is farther away from humans, the researchers did data processing on it first. In 2019, since the announcement of M87, scientists have spent another three years processing data on the black hole Sgr A* at the center of the Milky Way.
  Talking about related technologies, Chen Xuelei said, “Due to the insufficient number of antennas, the results of reconstructing the interference data are not unique, and the image of the black hole at the center of the Milky Way changes rapidly with time, so it is impossible to simply combine observations at different times like M87. Together, this time they used a method that averaged different imaging results. Obviously, this is also a new way to solve these imaging difficulties.”
  As a “truth photo” of the massive object at the center of the Milky Way, the astronomers Long-awaited for it. Before that, in the center of the Milky Way, which is surrounded by a large number of stars, astronomers have found that there is a kind of celestial body orbiting there, which has the characteristics of non-visibility, compactness, and extremely high mass. The implication of this discovery is that the celestial body named Sagittatius A* (Sagittatius A*: Sgr A*) should be a black hole. Scientists who have previously discovered the “black hole at the center of the Milky Way” have also won the Nobel Prize in Physics for their work.
  It is reported that in this discovery, scientists studied a huge imaging parameter space, and finally were able to determine the photo of this black hole. Regarding the difficulty of determining the photo, Chen Xuelei said: “They use the interference imaging method, which is characterized by not direct imaging, but interference between antennas. We can decompose an image into many different directions and scales. The components of , the interference results between each pair of antennas can give a component of a direction and scale, but due to the small number of antennas, only a few components are actually measured, and most components do not, so it cannot be unique The restored image can only be ‘guessed’, so there will be a lot of parameters.”
Shooting purpose: to crack the “true face of Mount Lu” of the dense radio source

  Why take such a shooting and observation? Researchers from the Shanghai Observatory said that our current impression of the Milky Way is mostly a band of light. Since humans live inside the Milky Way, we see the Milky Way as a band of light. However, if you observe from outside the Milky Way, the Milky Way is very different.
  It can be seen from the structure map of the Milky Way based on the Bezier plan that the center of the Milky Way is located in the middle, and the position of the sun is marked in red. If human beings are fortunate enough to carry out interstellar travel, when they go straight from the earth to the Milky Way, they will When you see the rich scene, especially near the center of the Milky Way, you will find a very dense and bright radio source, and this radio source is the protagonist of the conference.
  From previous observations of the Milky Way, we already know that the center of the Milky Way is a region full of physical environments. And this dense radio source can be seen in many astronomical photos. However, we have never seen the details of the dense radio source.
  In order to decipher the true face of Mount Lu of the compact radio source, higher-resolution observations are required. For the observation of this compact radio source, Sgr A*, various efforts have been made by humans since 50 years ago. In the 1960s, scientists discovered quasars, high-brightness objects observed at extremely large distances. In 1971, British astronomer Donald Linden Bell proposed that there is usually a black hole hidden in a galaxy, and the Milky Way is no exception. Therefore, he proposed to use high-resolution very long baseline interferometry (VLBI) to search for a dense radio source similar to a black hole at the center of the Milky Way.

Panorama of the Milky Way. The Milky Way seen from Earth (left) and the Milky Way seen from outside the Milky Way (right) are very different. On the right is a diagram of the structure of the Milky Way drawn by the Bezier Project

  Sure enough, in 1974, American astronomer H.W. Babcock and his collaborators used an interferometer to explore such a dense radio source in the center of the Milky Way, and named it Sgr A*. In 1976, American astronomer Raymond Davis and others proposed that the signal sent from the center of the Milky Way observed on Earth may have been affected by interstellar scattering.

  In order to overcome the effects of interstellar scattering, improve resolution, and reveal the true face of dense radio sources, scientists from all over the world have used shorter wavelengths for observations since then. In particular, in 1998, German physicist Thomas Kribaum et al. detected Sgr A* at a λ (wavelength unit) of 1.3 mm.
  Although these previous works have not yet allowed people to observe their true colors, they have also given people a basic understanding of Sgr A* and laid the foundation for this EHT discovery.
  The protagonist of this time to photograph the black hole of the Milky Way is the Event Horizon Telescope (EHT), which is an international VLBI array composed of a group of submillimeter wave point telescopes with frequencies of at least 230 GHz.
  After the shooting, the scientists spent three years processing the data for the Sgr A* black hole. Why does it take so long, Jiang Wu, associate researcher at the Shanghai Observatory of the Chinese Academy of Sciences and head of the EHT imaging effort, explained that there is a large amount of gas and dust between us and the center of the Milky Way, which scatters the radiation of electron waves from the center of the Milky Way. effect. Simply put, scattering causes blur and magnification in the captured image. And the longer the wavelength, the more obvious the scattering effect. This is one of the reasons why shorter millimeter waves are used for black hole imaging.
  Furthermore, for the galactic center, the biggest challenge in imaging is the rapid identification of fast-moving objects. During the shooting in 2017, in just 10 hours, the brightness of the center of the Milky Way had a rapid and significant change process.
  Because black holes don’t emit light, we can’t see the black hole itself, but the glowing gas that orbits signals its presence: a faint central region (called a shadow) surrounded by bright rings. The (radio) light shown in the photo is caused by the powerful gravitational bending of the black hole at the center of the Milky Way, which is more than 4 million times the mass of the sun.

The angular diameter of the ring structure and the mass of the black hole of Sgr A*
Construction of China’s submillimeter-wave VLBI telescope is being planned

  In 2019 and 2022, the photos of these two black holes of different sizes were released successively, which made scientists very excited, which allowed them to have better conditions for comparative research. Currently, scientists are using the new data to test theories and models of the behavior of gas around supermassive black holes. Although the process of this behavior is unknown, it is widely believed to be critical to the formation and evolution of galaxies.
  In a nutshell, Sgr A* is the closest supermassive black hole to us, providing scientists with a unique “laboratory” where they can test general relativity and explore black hole astrophysics.
  After the release of the first photo of the black hole at the center of the Milky Way, scientists will use polarization observation data to study the magnetic field around the black hole, and the structural changes of X-ray flare activity will also be their research objects.

  In fact, after 2017, new telescopes have been gradually added, and the data recording bandwidth has gradually increased, which has improved the sensitivity of the EHT array, which has gradually enhanced the imaging capability of the variable source SgrA*. In the future, more submillimeter wave telescopes will be added to the imaging observation, and the all-weather relay imaging observation of Sgr A* may be realized by then. At that point, a dynamic camera of the physical environment around the black hole will “become reality.” Experts from the Shanghai Observatory who participated in this study believe that if a submillimeter-wave VLBI telescope is built in China and participates in related observations, it will play an important role.
  For a long time, EHT has been on the road of observation. In March 2021, it just completed a joint observation with other telescopes. In the future, its goal is no longer to take pictures, but to make movies. Making a “movie” for the black hole at the center of the Milky Way is also the pursuit of the next-generation event horizon telescope.
  Regarding the “movie” here, Chen Xuelei said: “It is to get dynamic images at different times. Of course, such images give us much more information, just like small videos have much more than static photos.”
  At present, relevant domestic experts are The construction of China’s submillimeter-wave VLBI telescope is planned, and if the construction is successful, it will participate in the 24-hour uninterrupted relay observation of Sgr A*.
  In this regard, Chen Xuelei said: “There are two important conditions here, one is the need to have a good site, and the other is the need to develop related technologies. In terms of site, submillimeter waves are easily affected by the atmosphere, especially water vapor. Therefore, it is necessary to find a plateau and a dry place, and the site selection can be carried out on the Qinghai-Tibet Plateau in my country. In terms of technology, our country has a certain technical foundation, and of course, further research is required.”

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