University of São Paulo: Astronomers detect signs of hot spots in the Milky Way’s black hole orbit

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A team of astronomers from the European Southern Observatory (ESO) has detected a bubble of hot gas around the supermassive black hole Sagittarius A*, located at the center of our galaxy, the Milky Way. Using the Atacama Large Millimeter/submillimeter Array (Alma) radio telescope in Chile, astronomers have discovered signs of the bubble, known as the hot spot, orbiting Sagittarius A* after a burst of X-ray energy emitted from it. from the center of the Milky Way. The discovery helps to better understand the enigmatic and dynamic environment of the supermassive black hole.

The research is presented in the paper Orbital motion near Sagittarius A* – Constraints from polarimetric ALMA observations , published on the Astronomy & Astrophysics website on September 22.

One of the co-authors of the work is astrophysicist Ciriaco Goddi, from the Università degli Studi di Cagliari, in Italy, who is currently at the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) at USP. “On April 11, 2017, there was a burst of X-ray energy, and our light curve revealed a signal compatible with a hot spot,” recalls Goddi, who at the time was in Chile observing with Alma. “Thus, we were able to link the two phenomena for the first time to a mechanism known as magnetic reconnection, that is, a magnetic interaction occurring in hot gas in orbit close to the black hole.”


Using the Atacama Large Millimeter/submillimeter Array (Alma), astronomers have discovered signs of a hot spot orbiting Sagittarius A*, the black hole at the center of our galaxy. Hot spots are bubbles of gas moving at high speeds in orbits very close to the black hole.
The observations were taken with Alma in the Chilean Andes — a radio telescope that is partially owned by the European Southern Observatory (ESO) — during an EHT Collaboration (Event Horizon Telescope) campaign to image black holes. In April 2017, the EHT connected eight existing radio telescopes around the world, including Alma, to obtain data that resulted in the recently released first image of Sagittarius A*.

To calibrate the EHT data, the astronomers used Alma data recorded simultaneously with the Sagittarius A* observations. Some of the observations had been made shortly after a burst of X-ray energy emitted from the center of our galaxy, which was detected by NASA’s Chandra Space Telescope . These types of explosions, previously observed by infrared and X-ray telescopes, are thought to be associated with so-called “hot spots,” bubbles of hot gas moving at high speeds in orbits very close to the black hole.

“What’s really new and interesting is the fact that these outbursts are, so far, only clearly present in infrared and X-ray observations of Sagittarius A*,” points out Maciek Wielgus of the Max Planck Institute for Radio Astronomy in Bonn, Germany. , who led the study. “We are, for the first time, seeing strong indications that hot spots orbiting the black hole are also present in radio observations,” adds the researcher, who is also affiliated with Poland’s Nicolaus Copernicus Astronomical Center and the Black Hole Initiative at the University of Harvard in the United States.


Ciriaco Goddi – Photo: Lattes Curriculum
“Perhaps these hot spots detected at infrared wavelengths are a manifestation of the same physical phenomenon: as they cool, the hot spots that emit in the infrared become visible at longer wavelengths, such as those observed by Alma and the EHT,” adds Jesse Vos, a doctoral student at Radboud University, the Netherlands, who also participated in the research. For a long time, scientists speculated that the explosions would originate from the magnetic interactions of the very hot gas that orbits very close to Sagittarius A*. “We have now found strong evidence for a magnetic origin of these explosions and our observations give clues to the geometry of the process. The new data are extremely useful in building a theoretical interpretation of these events,” says co-author Monika Mościbrodzka,

compatible signal
Alma allows astronomers to study polarized radio wave emission from Sagittarius A*, which can be used to investigate the black hole’s magnetic field. The team used these observations along with theoretical models to learn more about the hotspot’s formation and the environment it finds itself in, including the magnetic field surrounding Sagittarius A*. Thus, it was possible to establish the limits of the magnetic field more clearly, helping astronomers to discover the nature of the black hole and its surroundings. The observations confirm some of the earlier discoveries made using the Gravity instrument on ESO’s Very Large Telescope (VLT), which observes in the infrared. Both the data ofGravity like those of Alma suggest the explosion originates from a clump of gas orbiting around the black hole at about 30% the speed of light, clockwise across the sky.

“In the future we should be able to follow hotspots across multiple frequencies, using coordinated observations at multiple wavelengths, obtained with both Gravity and Alma, and the success of such an effort would be a real milestone in our understanding of physics. of explosions in the galactic center,” says Ivan Marti-Vidal of the University of Valencia, Spain, co-author of the study.

For IAG’s Goddi, the research also reaffirms the importance of observations made by instruments such as Alma. “Polarized radiation brings with it information that allows us to identify and model emission regions considerably smaller than the effective resolution of the telescope”, he concludes.

The team also hopes to be able to directly observe clumps of gas with the EHT, to probe ever closer to the black hole and learn more about it. “Maybe one day we’ll be comfortable enough to say we ‘know’ what goes on in Sagittarius A*,” concludes Wielgus.

Also part of the work were Z. Gelles, from the Center for Astrophysics at Harvard University and the Smithsonian Institute (United States), J. Farah, from the Las Cumbres Observatory and University of California, Santa Barbara (United States), N. Marchili, from the Alma Italian Regional Center, Instituto di Radioastronomia (Inaf) and MPIfR (Italy), and H. Messias, from the Joint Alma Observatory (Chile).

Created in 1962 and supported by 16 member countries, ESO designs, builds and operates world-class ground-based observatories that astronomers use in research and to promote international collaboration in astronomy.

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