When black holes feast, they don’t just swallow, they spill. That’s the unsettling picture painted by new results from the XRISM satellite, showing that even the smallest stellar-mass black holes fling matter back into space instead of calmly consuming it. The discovery, led by astronomers at the University of Michigan and published in The Astrophysical Journal Letters, challenges long-held assumptions about how these cosmic heavyweights grow and, by extension, how galaxies evolve.

The research team caught the black hole system known as 4U 1630-472 during one of its dramatic flare-ups. Ordinarily, the object shines as faintly as our Sun. But every two years or so, it surges, blasting out X-rays 10,000 times brighter in the span of a week. XRISM, launched in 2023 by JAXA and NASA, trained its ultra-sensitive spectrometer on the system during the decline of such an outburst in February 2024.

“Being surprised is good. Seeing expectations proven naive means progress,” said Jon Miller, an astronomer at the University of Michigan and lead author of the study. “Having entirely new avenues to chase down is tremendous.”

What Miller and colleagues saw was chaos: gas still being expelled at three percent of the speed of light, even as the X-ray flare dimmed. Intuition suggests the opposite. If a black hole is gorging rapidly, spillover makes sense. But when the feeding slows, one might expect cleaner accretion. Instead, it was like pouring a cup of water into a bucket and still splashing half onto the floor.

XRISM’s Resolve instrument made the distinction possible. Its sensitivity is roughly ten times greater than earlier X-ray spectrometers, allowing astronomers to see faint but telling shifts in the iron-rich gas surrounding the black hole. In 4U 1630-472, the winds weren’t just messy—they were unstable, changing speed and direction on timescales human telescopes rarely glimpse.

“We got to see a range of gas flow rates that we never get to observe with massive black holes at the centers of galaxies,” Miller explained.

That’s where the larger story comes in. Supermassive black holes, millions or billions of times the Sun’s mass, govern how galaxies breathe and grow. They regulate star formation by spewing winds and radiation, recycling gas back into interstellar space. But those processes unfold over millions of years. By studying their smaller cousins, astronomers can fast-forward the movie.

Still, the timing is awkward. XRISM is only halfway through its prime mission, yet U.S. support for its operations is now under threat. President Trump’s budget request proposes cutting NASA’s funding for the project, a move that could leave the Japanese space agency carrying the mission alone. For hundreds of American scientists relying on XRISM, that prospect is bleak.

The cost, Miller noted, is largely sunk. “The money that’s required to do these missions is almost all up front—it’s the building, technology development and the launch,” he said. “Operating year-to-year is a small fraction of the total.”

Whether XRISM continues to run with U.S. backing or not, its message is already clear: small black holes are not tidy engines. They are volatile, spilling gas, warping light, and refusing to play by the rules. If the same holds true for the giants at galactic centers, our theories of how galaxies coalesce and evolve will need serious revision.

Explainer: Black holes don’t just pull matter in. Many also drive powerful outflows, or “winds,” from the material circling them. These winds can carry away huge amounts of mass and energy, reshaping the environment around the black hole. Studying stellar-mass black holes like 4U 1630-472 gives astronomers a faster way to observe these processes, since their behavior changes on timescales of weeks rather than millions of years. The XRISM satellite’s Resolve spectrometer measures X-ray energies with extreme precision, allowing scientists to trace these winds in detail and link them to galaxy-scale processes.

Journal: The Astrophysical Journal Letters
DOI: 10.3847/2041-8213/ade25c