The Milky Way’s black hole is hiding an explosive past, evidence suggests

Our galaxy’s supermassive black hole is famous for being one of the dimmest in the universe. Evidence from a new space telescope shows that might not always have been the case.

Sagittarius A*, located at the center of the Milky Way Galaxy, appears to have flared dramatically sometime within the past few hundred to 1,000 years, according to X-ray emissions observed by the XRISM space telescope. These surprising findings unveil new details of how supermassive black holes evolve. They also teach astronomers lessons about the history of our cosmic home.

Michigan State University researcher Stephen DiKerby worked with an international team to measure the X-rays coming from a giant cloud of gas near the galaxy’s center. The team examined the cloud in incredible detail thanks to XRISM’s ability to resolve the energy of individual X-ray photons. Their findings offer strong evidence that the cloud is glowing in response to a past outburst from Sagittarius A*.

The results, accepted by The Astrophysical Journal Letters, highlight XRISM’s laboratory-like precision, effectively changing the game for X-ray astronomy. The work was done in collaboration with Kumiko Nobukawa of Kindai University, Higashi-osaka, Osaka, Japan, and Masa Nobukawa of Nara University of Education, Nara, Nara, Japan.

“Nothing in my professional training as an X-ray astronomer had prepared me for something like this,” said DiKerby, a postdoctoral researcher in Physics and Astronomy Assistant Professor Shuo Zhang’s lab. “This is an exciting new capability and a brand-new toolbox for developing these techniques.”

A supermassive black hole is exactly what it sounds like – a massive, pitch-black hole containing millions or billions of solar masses of material, so dense that not even light can escape. Every large galaxy has one, though researchers don’t yet know why.

Many supermassive black holes are bright because gas around them heats up and emits high-energy radiation. In contrast, Sagittarius A* barely shines at all. It’s one of the dimmest known black holes in the universe, only visible because it is so close to Earth.

Several large molecular clouds float around Sagittarius A* and can act like cosmic mirrors, reflecting past X-ray flashes from the black hole. Previous space telescopes could detect these glimmers, but not with enough energy resolution to examine their fine structure or determine what produced them.

XRISM changed that. The telescope was launched in 2023 through a partnership between NASA and the Japan Aerospace Exploration Agency. Its first observations are highly anticipated because they’re a vast improvement over every existing space telescope in terms of energy resolution. Most X-ray space telescopes can distinguish the energy of a photon to about one part in 10, or even 100. XRISM can resolve one part in 1,000. The new images are like shifting from a Polaroid to a high-definition technicolor image.

DiKerby used this sharp view to zoom in on two extremely narrow X-ray emission lines coming from one of the molecular clouds. By measuring their energies and shapes with groundbreaking precision, he could determine the cloud’s motion and match it to previous radio observations. He also examined subtle features in the spectrum to test two competing explanations for the cloud’s glow.

Those details ruled out the idea that cosmic rays were responsible and instead showed that the cloud is reflecting an X-ray outburst from Sagittarius A* — effectively a “light echo” from the past. By studying multiple clouds at different distances from the black hole, astronomers can reconstruct a timeline of these ancient flares, much like using delayed echoes to map the shape of a cave.

“This remarkable measurement shows just how powerful XRISM is for uncovering the hidden history of the center of our galaxy,” Zhang said. “By resolving the iron lines with such clarity, we can now read the galactic center’s past activity in unprecedented detail.”

The data shows for the first time how XRISM’s energy resolution can measure extremely fine features in the universe. The team expects the telescope to open many new avenues of discovery.

“We’re just the lucky scientists who got to solve the problems with handling this data in this brand-new way,” DiKerby said. “One of my favorite things about being an astronomer is realizing I’m the first human to ever see this part of the sky in this way.”

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