Gas bubble swirls around the heart of the Milky Way

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Researchers discover a hot spot near the Sagittarius A* black hole with the Alma radio telescope.

Still: The illustration shows where the hot spot should be, according to modelin
Still: The illustration shows where the hot spot should be, according to modeling of Alma data, and at what distance it is racing around the black hole Sagittarius A* in the center of our Milky Way. Sagittarius A* is seen here in the image obtained with the Event Horizon Telescope. © EHT Collaboration, ESO/M. Kornmesser (Acknowledgment: M. Wielgus).

There is a black hole in the center of our Milky Way. In the immediate vicinity of this mass monster called Sagittarius A*, things are turbulent. Now, an international group led by Maciek Wielgus of the Max Planck Institute for Radio Astronomy in Bonn has discovered an object that orbits the black hole on a very narrow path in only about 70 minutes. The observation of this hot spot was made with the Alma telescope facility in the Chilean Andes.

In April 2017, the Event Horizon Telescope (EHT) pointed eight radio telescopes at the center of our Milky Way. The result of this campaign, which included Alma, was the first image of the galactic black hole published last May. For the calibration, the team around Maciek Wielgus used the Alma data recorded in parallel with the EHT observations of Sagittarius A*.

Some of the measurements happened by chance shortly after a burst of X-ray energy from the center of our Galaxy. Such flares are probably associated with hot spots - hot bubbles of gas orbiting very fast and close to the black hole. This bubble orbits Sagittarius A* in 70 minutes on an orbit similar to that of the planet Mercury. In the process, the object apparently has the incredible speed of about 30 percent of the speed of light," Wielgus says.

What is new is that such flares were previously only found in X-ray and infrared observations of Sagittarius A*: Now, for the first time, we see a very strong indication that orbiting hot spots are also present in radio observations," adds Maciek Wielgus, who is also active at the Nicolaus Copernicus Astronomical Centre in Poland and in the Black Hole Initiative at Harvard University.

Jesse Vos of Radboud University also suspects that the hot spot originally discovered at infrared wavelengths and now observed in the radio range is a manifestation of the same physical phenomenon: "As such a bubble cools, it becomes visible at longer wavelengths."

For a long time, the flares were thought to be caused by magnetic interactions in the very hot gas orbiting in close proximity to Sagittarius A*. The new results support this idea. According to Monika Moscibrodzka of Radboud University, there is strong evidence for a magnetic origin of these flares; moreover, the observations provide a clue to the geometry of the process. This is because Alma makes it possible to study the polarized - that is, oscillating in one direction - radio emission from Sagittarius A*. The team used these measurements, along with theoretical models, to learn more about the formation of the hot spot and the environment in which it is embedded.

The observations confirm some of the earlier discoveries made at the European Southern Observatory’s Very Large Telescope (VLT). For example, data from the VLT’s infrared instrument and from Alma both suggest that the burst of radiation originates in a clump of gas swirling clockwise around the black hole at about 30 percent the speed of light. In the process, we are apparently looking at the orbit almost from above.

The team hopes to use the Event Horizon Telescope to observe the orbiting gas clump directly and get even closer to the black hole. But Anton Zensus, director at the Max Planck Institute for Radio Astronomy, is already satisfied: "The results impressively demonstrate that further developments on the Alma telescope for imaging with the EHT also provide insights into the variability of such sources at scales that were not previously possible."