| Sep 04, 2025 |
The cutting-edge XRISM satellite has revealed small black holes don’t accrete matter as expected – a finding that could have implications for entire galaxies.
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(Nanowerk News) When stellar-mass black holes—”small” black holes that are still a few times more massive than the sun—are actively gobbling up matter, they’re even more of a hot mess than scientists expected.
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That’s according to an international research team, led by Jon Miller of the University of Michigan, that uncovered this surprise that could have galactic implications. What researchers learn from stellar-mass black holes can help them better understand supermassive black holes, the behemoths that steer the evolution of galaxies like our Milky Way.
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“Being surprised is good. Seeing expectations proven naive means progress,” said Miller, a lead author of the new study published in The Astrophysical Journal Letters (“XRISM Spectroscopy of the Stellar-mass Black Hole 4U 1630-472 in Outburst”). “Having entirely new avenues to chase down is tremendous. Astronomy is full of surprises and never boring.”
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| Conceptual illustration of a black hole X-ray binary. The strong gravity of the black hole—shown as a small black dot at the center of the disk on the right—pulls gas from the companion star (left). As the gas spirals inward, it forms a high-temperature accretion disk around the black hole. New research from an international team that includes the University of Michigan shows this process is messy, no matter how fast it’s going. (Image: JAXA)
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Megumi Shidatsu of Ehime University and Misaki Mizumoto of the University of Teacher Education Fukuoka, both in Japan, co-led the project, which included researchers from about 20 institutions across five countries.
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The team made their discovery using the X-Ray Imaging and Spectroscopy Mission, or XRISM, which is pronounced “krism” (which rhymes with “prism”). The mission is a collaboration between NASA and the Japanese Aerospace Exploration Agency, or JAXA, with contributions from the European Space Agency.
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X-rays in space
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The XRISM satellite launched in September 2023 and began its scientific operations in early 2024. Since then, it has provided long-awaited and unprecedented views into cosmic systems that emit X-rays.
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“XRISM is at least 10 times more sensitive than any prior X-ray instrument,” said Miller, the Douglas Richstone Collegiate Professor of Astronomy. “For that reason, we’re suddenly able to see really dramatic spectral lines that would have just looked like noise in the data that we’ve had for the last two decades.”
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Spectroscopy—the S in XRISM—analyzes characteristics of the light it collects, rather than using that light to construct images. X-rays are high energy forms of light and XRISM’s Resolve spectrometer can quantify their energy with unmatched precision to reveal insights about the environments that produced them.
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In the case of the new study, that system was 4U 1630-472, which is what’s known as an X-ray binary. Astronomers believe this particular binary system is made up of a stellar-mass black hole and a regular companion star like our sun.
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The immense gravity of the black hole steals matter from the companion star, siphoning it into an accretion disc that surrounds the black hole. That transferred material gets so hot—up to 10 million Kelvin or about 18 million degrees Fahrenheit—that it beams out X-rays.
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Another important feature of 4U 1630-472 is that it also varies dramatically in its brightness. In its normal, quiescent state, it’s about as luminous as our sun, Miller said. But, every two years or so, it has an outburst when its brightness can increase 10,000 times over the course of a week. On Feb. 16, 2024, XRISM caught 4U 1630-472 at the tail end of an outburst, opening a new window into black hole science.
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Just throwing mass around
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With XRISM, researchers found that even as the X-rays dimmed, 4U 1630-472 still managed to eject some gas at 3% of the speed of light, or 20 million miles per hour.
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“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 said. “The time scale to see something like that from the black hole in the Milky Way would be hundreds of millions of years. So one of the reasons we study these smaller ones is to get a clue as to how gas flows onto very massive black holes could change over the evolution of a galaxy.”
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The researchers expected to see matter moving onto the black hole in a more controlled and precise manner as the flow rate slowed, which makes sense intuitively. Miller compared it to pouring water.
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“You expect to spill a lot if you try to pour a bucket of water into a cup, but not when you are pouring a cup of water into a bucket,” he said. “Black holes seem to spill in both extremes.”
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In other words, the analogy works for black holes for bucket-to-cup flow rates, but in cup-to-bucket situations, the team found.
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“There was still mass being thrown around instead of accreted directly into the black hole,” Miller said.
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This surprising finding gives rise to even more questions about black holes and galaxy evolution, many of which XRISM is equipped to help answer. But its ability to do so is now in question with President Trump’s budget request that would effectively cancel U.S. funding for the mission.
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“We’re not done with what NASA calls the mission’s prime phase, which is a two-year period where you find examples of groundbreaking science that you can do,” Miller said. “Then you spend the rest of the mission diving into those areas once you’ve unearthed them.”
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XRISM’s nominal end of prime phase is September 2026, but missions are typically renewed beyond that whenever possible to maximize the return on investment. As an example, NASA’s Chandra X-ray Observatory launched in 1999 on a nominal 5-year mission and is still running.
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“The money that’s required to do these missions is almost all up front—it’s the building, technology development and the launch,” Miller said. “Operating year-to-year is a small fraction of the total.”
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There’s a chance JAXA could continue to operate XRISM without NASA support, Miller said, but it’s not something he and his colleagues want to bet on.
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“There’s a community of hundreds, maybe as many as a couple thousand scientists in the U.S. who stand to benefit from XRISM,” Miller said. “We’re all trying very hard to use the hell out of it right now and hoping that it isn’t our last chance.”
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