Our galaxy's supermassive black hole is among the faintest known, yet new observations indicate it was far more active in the recent past on cosmic timescales. Sagittarius A*, at the Milky Way's center, appears to have produced a powerful X ray flare within the last few hundred to 1,000 years, leaving an observable imprint in surrounding gas.
Michigan State University researcher Stephen Di Los Angeles CA (SPX) Jan 07, 2026
Our galaxy's supermassive black hole is among the faintest known, yet new observations indicate it was far more active in the recent past on cosmic timescales. Sagittarius A*, at the Milky Way's center, appears to have produced a powerful X ray flare within the last few hundred to 1,000 years, leaving an observable imprint in surrounding gas.
Michigan State University researcher Stephen DiKerby and colleagues used the XRISM space telescope to measure X rays from a giant molecular cloud close to the galactic center. XRISM's spectrometer can resolve the energy of individual X ray photons with about one part in 1,000 precision, enabling the team to analyze fine spectral details that were previously inaccessible.
The researchers focused on two very narrow X ray emission lines from the cloud and used their energies and shapes to determine the cloud's motion, comparing the result with earlier radio measurements. Additional subtle features in the spectrum allowed the team to test different scenarios for what is powering the emission.
Their analysis ruled out a model in which cosmic rays excite the cloud and instead showed that the gas is reflecting an X ray outburst from Sagittarius A*, creating a light echo of the flare. By studying several molecular clouds at different distances from the black hole, astronomers can reconstruct a sequence of past flares and build a history of activity in the galactic center.
The findings, accepted for publication in The Astrophysical Journal Letters, demonstrate XRISM's laboratory like energy resolution and its impact on X ray astronomy. The project involved collaboration with Kumiko Nobukawa of Kindai University in Higashi osaka, Osaka, and Masa Nobukawa of Nara University of Education in Nara, Japan, reflecting the mission's international scope.
"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."
Supermassive black holes contain millions or billions of times the Sun's mass in a region so dense that light cannot escape, and every large galaxy appears to host one. Many such black holes are bright because gas around them heats up and emits high energy radiation, but Sagittarius A* is unusually dim and detectable mainly because it is relatively close to Earth.
Several massive molecular clouds orbit the region around Sagittarius A* and can act as mirrors that reflect X ray flashes from past episodes of higher activity. Earlier space telescopes detected these reflections but could not measure their energy structure with enough precision to reveal how they were produced.
Launched in 2023 as a joint mission of NASA and the Japan Aerospace Exploration Agency, XRISM delivers a major improvement in energy resolution over previous X ray observatories. While earlier instruments typically distinguished photon energies to about one part in 10 or 100, XRISM achieves resolution of about one part in 1,000, comparable to moving from a coarse snapshot to a high definition image.
DiKerby used this capability to isolate the narrow emission features from the cloud and analyze them in detail, which made it possible to distinguish between competing physical explanations. The work shows how high resolution spectroscopy can connect cloud properties, black hole activity, and the surrounding environment in the Milky Way.
"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 observations provide the first clear example of XRISM's ability to measure extremely fine spectral features in distant cosmic sources. The team expects that similar analyses of other targets will reveal additional aspects of the Milky Way's core and of supermassive black holes in general.
"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."
Research Report: Resolving the Fe Ka Doublet of the Galactic Center Molecular Cloud G0.11-0.11 with XRISM
Related Links
Michigan State University
Stellar Chemistry, The Universe And All Within It