The Milky Way's supermassive black hole has a hot spot

Astronomers have spotted a bright 'hot spot' swirling around the supermassive black hole at the heart of our Milky Way galaxy, Sagittarius A* (Sgr A*). 

The team behind the discovery thinks that the 'hot spot' could be a bubble of hot gas orbiting Sgr A* as fast as 30% of the speed of light. The discovery could help astronomers and astrophysicists better understand the violent environment at the center of the Milky Way, and around Sgr A* in particular.

"We think we're looking at a hot bubble of gas zipping around Sgr A* on an orbit similar in size to that of the planet Mercury, but making a full loop in just around 70 minutes," Maciek Wielgus, an astrophysicist at the Max Planck Institute for Radio Astronomy in Germany, said in a statement. "This requires a mind-blowing velocity of about 30% of the speed of light!"

Related: The 1st photo of the Milky Way's monster black hole explained in images

Sagittarius A*, as seen by the Event Horizon Collaboration (EHT) with an illustration of the hot spot seen by astronomers. (Image credit: EHT Collaboration, ESO/M. Kornmesser (Acknowledgment: M. Wielgus))

Wielgus led a team that collected observational data using the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, comprised of 66 radio antennas spread across the Atacama Desert of northern Chile, as part of the Event Horizon Telescope (EHT) collaboration's work to image black holes.

Along with other telescopes in the EHT, ALMA started observing supermassive black holes in 2017. This led to the first-ever image of a black hole, released in 2019, which depicted the supermassive black hole at the heart of the galaxy Messier 87 (M87). Earlier this year, the same collaboration unveiled the first image of Sgr A*. 

But ALMA recorded additional data at the same time as the EHT observations of Sgr A*. Wielgus and his team found within that data clues to the nature of Sgr A* and its surroundings, buried in the measurements made by only ALMA.

The discovery comes because ALMA collected some of its data after a burst, or flare, of X-rays from the heart of the Milky Way detected by NASA's Chandra X-ray Observatory. Scientists have previously linked flares like this to magnetic interactions in hot gas bubbles that orbit close to Sgr A* at rapid speeds.

"What is really new and interesting is that such flares were so far only clearly present in X-ray and infrared observations of Sagittarius A*," Wielgus said. "Here we see for the first time a very strong indication that orbiting hot spots are also present in radio observations."

The team suggests that the hot spots detected at infrared wavelengths could be the result of gas bubbles that become visible at longer wavelengths of light (like those ALMA sees) when they cool down. 

"Now we find strong evidence for a magnetic origin of these flares and our observations give us a clue about the geometry of the process," Monika Mościbrodzka, an EHT collaboration team member and an astrophysicist at Radboud University in the Netherlands, said in the same statement. "The new data are extremely helpful for building a theoretical interpretation of these events."

Using ALMA, astronomers and astrophysicists can study polarized radio wave emissions from Sgr A*, which they can use to investigate the magnetic field surrounding the supermassive black hole. The new research could help in this investigation by better constraining the shape of this magnetic field and details of the surroundings of Sgr A*, the scientists hope.

Additionally, the results help confirm previous research based on data from the GRAVITY instrument on the Very Large Telescope (VLT) in Chile, which implied that X-ray flares come from clumps of gas swirling clockwise around black holes at 30% the speed of light.

The team now hopes that both GRAVITY and ALMA can track these hot spots in multiple wavelengths of light, which could be a milestone in the understanding of the physics of flares at the center of the Milky Way and which would build on direct observations of Sgr A* and its environment by the EHT. 

"Hopefully, one day, we will be comfortable saying that we 'know' what is going on in Sgr A*," Wielgus concluded.

A paper detailing the team's findings is published in the September issue of the journal Astronomy & Astrophysics.

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.