September 26, 2022

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Astronomers discover a hot gas bubble in the Milky Way's supermassive black hole

Astronomers discover a hot gas bubble in the Milky Way’s supermassive black hole

A still image of the Sagittarius A* supermassive black hole, as seen by the Event Horizon Collaboration (EHT), with an illustration showing where ALMA data modeling predicts the hotspot and its orbit around the black hole.

Photo: EHT Collaboration, ESO / M. Kornmesser (Acknowledgments: M. Wielgus)

Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers saw signs of a hot spot orbiting Sagittarius A*, the black hole at the center of our Milky Way. This discovery gives us further insight into the baffling and dynamic environment of this supermassive black hole. “We think we’re seeing a hot gas bubble here orbiting Mercury around Sagittarius A*, making a complete orbit in just about seventy minutes.

This means that it is moving about thirty percent of the speed of light – an astonishing speed! and astrophysics. The observations were made using the ALMA Radio Telescope in the Chilean Andes, owned by the European Southern Observatory (ESO) during the collaborative Event Horizon Telescope (EHT) campaign to image black holes. In April 2017, the EHT connected eight radio telescopes located around the world, including ALMA, providing the first-ever image of Sagittarius A*. To calibrate the EHT data, Wielgus and colleagues, members of the EHT Collaboration, used ALMA data obtained simultaneously with the EHT observations of Sagittarius A*. To the team’s surprise, ALMA’s measurements contained more clues about the nature of the black hole.

By chance, some observations were made shortly after X-ray energy was emitted from the center of our galaxy, which was spotted by NASA’s Chandra Space Telescope. Such eruptions, previously observed by X-ray and infrared telescopes, are associated with so-called “hot spots”: bubbles of hot gas swirling around the black hole at high speed. What is really new and interesting is that so far, such flames have only been clearly seen in X-ray and infrared observations of the A* arc. Here we see for the first time a very strong indication that orbital hotspots can also be observed at radio wavelengths, says Wilgus, who also belongs to the Nicolaus Copernicus Astronomical Center in Poland and the Black Hole Initiative at Harvard University (USA).

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“These hotspots observed at infrared wavelengths are probably a manifestation of the same physical phenomenon: When infrared hotspots cool, they spontaneously become visible at longer wavelengths, such as those observed by ALMA and EHT,” he adds. Jesse Voss, a Ph.D. student at Radboud University was involved in this research. The eruptions have long been suspected to be the result of magnetic interactions in the superheated gas orbiting the A* arc, and new results confirm this. “We have now found strong indications of a magnetic origin for these eruptions and our observations give us an indication of the engineering of the process. The new data is very useful for building a theoretical explanation of these events,” says co-author Monika Mościbrodzka of Radboud University.

ALMA allows astronomers to examine the polarized radio emission from Sagittarius A*, which can be used to expose the black hole’s magnetic field. In combination with theoretical models, the team used these observations to learn more about the formation of the hotspot and the environment in which it is involved, including the magnetic field around the A* arc. Their research imposes stronger limits on the shape of this magnetic field than previous observations and is helping astronomers decipher the nature of our black hole and its surroundings. The observations confirm some previous discoveries made by the GRAVITY instrument on ESO’s Very Large Telescope (VLT), which monitors infrared radiation. GRAVITY and ALMA data indicate that the explosions occur in a clump of gas that orbits the black hole clockwise at about 30 percent of the speed of light, with the hotspot’s orbit being roughly perpendicular to our line of sight.

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“In the future we should be able to track hotspots through coordinated observations with both GRAVITY and ALMA at different frequencies – if this project is successful it will be a real milestone for our understanding of the physics of explosions in the Milky Way Center,” says Ivan Marti Vidal of the University of Valencia in Spain, co-author of the study. The team also hopes to directly observe the orbital gas agglomeration with the EHT, get closer and closer to the black hole and learn more about it. “I hope one day we will be able to say with confidence that we ‘know’ what’s going on in arc A*,” concludes Wilgus.

source: ESO