Looks! Astronomers find a hot ball of plasma orbiting the Milky Way’s massive black hole

The supermassive black hole lurks at the center of the Milky Way is privy to powerful flares, and new evidence shows it’s orbited by a strange blob flying through space at 30 percent the speed of light.

Thanks to the groundbreaking Event Horizon Telescope (EHT), astronomers can look deeper into the 4-million-solar-mass black hole at the core of our galaxy called Sagittarius A* (Sgr A*). The latest look at this irresistible scientific goal was published in the journal on Thursday Astronomy & Astrophysics, and it has revealed a fast-flying hotspot as well as new information about the mysterious flares going off around the black hole. The phenomena are helping scientists understand more about the chaos behind the black hole, and are fascinating in their own right.

Here’s food for thought: The gas disk surrounding Sgr A* appears a size in the sky that would be comparable to an orange on the lunar surface as seen from Earth. Now imagine a grapefruit twice the size of this orange and you get the approximate orbital size of this bubble. This is only apparent size; Models suggest the bubble has an orbital radius similar to Mercury’s around the Sun. To put the blob’s speed into perspective, imagine Mercury making a trip around the Sun in a whopping 70 minutes instead of its typical 3-month trip.

What they found – “Which comes first — a bubble or a flare — could be kind of a chicken-and-egg question,” says Maciek Wielgus, an astrophysicist at the Max Planck Institute for Radio Astronomy in Germany who led the new work Vice versa. Wielgus was part of the EHT team that produced the intriguing first image of Sgr A* in May 2022.

“Our results seem to indicate that the flare comes first, creating hot spots, or bubbles, through this powerful release of magnetic field energy into the plasma,” he adds. Plasma is a state of matter that is formed when strong forces continue to heat material.

Solar flares, which now occur more frequently as the Sun awakens into the most active part of its 11-year cycle, are similar to the explosions produced by the supermassive black hole. “The mechanism by which we believe that [black hole] Flare occurs is the magnetic reconnection,” says Wielgus. “And basically it’s similar to how flares originate on the sun’s surface.”

“However, there is a big difference in geometry, plasma properties and overall performance,” he adds. That said, just because Sgr A* far outperforms the Sun doesn’t mean it’s a titan among its own kind.

“While Sgr A* is as massive as about 4 million suns,” says Wielgus, “it only emits at about 100 suns magnitude, which is actually very inconspicuous for a supermassive black hole!”

That Sgr A* is weak compared to black holes might invite a sigh of relief. It’s in our cosmic backyard, after all. It’s 27,000 light-years away, but still a stone’s throw away by universe standards.

This gentle supermassive black hole is nonetheless capable of fleeting activity. And the new study helps clarify why these flares and the bubble are forming.

Why it matters – Co-author Monika Mościbrodzka, who is based at Radboud University in the Netherlands, says in a Thursday note from the European Southern Observatory (ESO) that they now have “strong evidence” that these flares come from magnetic interactions, if very different hot gas encircled at Sgr. A*.

Their explanation for these flares and hotspots is that when a black hole’s magnetic field line reconnects (as they do on the Sun’s surface), the event releases a lot of energy very quickly. This strikes some of the gas around the black hole, heating it up and forming what Wielgus describes as a “low-density, very high-temperature plasma bubble.”

What they have done – The Bubble is an intriguing revelation from a chunk of outlier data.

The new study is based on the observations of the Atacama Large Millimeter/submillimeter Array (ALMA), named after the Chilean desert where it was built. ESO, which published an announcement of the new work, maintains this page. Wielgus says that on April 6 and 7, 2017, ALMA observed Sgr A* for EHT, and then a few days later ALMA again observed the black hole. But this April 11 data was not included in the first image of Sagittarius A*.

“We avoided using the third day with ALMA [April 11] for the EHT results precisely because we knew there was an X-ray flare that day and we saw some evidence of increased source variability, says Wielgus. “Probably rather unsuitable for the reconstruction of a static average image.”

This was revealed by EHT on May 12, 2022. The team created a main image by averaging thousands of images. It preserves common traits and suppresses those that don’t appear often.

What’s next – Think of the fabulous May 2022 image as a portrait painting that Wielgus says “showed us Sgr A* on a still, peaceful day”. The new work is exciting because it is a vivid screenshot of an action-packed scene. With this “peek inside the flickering Sgr A*” comes the opportunity to flesh out a better model of the supermassive black hole.

X-ray data from NASA’s Chandra X-ray Observatory in space and infrared data from the GRAVITY instrument with the Very Large Telescope Interferometer (VLTI) in Chile had previously spotted the flares. But now that new radio observations from the latest study are available, there’s a new layer for scientists to explore.

Hopefully all of this will contribute to a robust understanding of the heart of the Milky Way that has previously been impossible to obtain.