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The state-of-the-art XRISM satellite has disclosed that diminutive black holes do not accumulate matter as anticipated—a discovery that could influence entire galaxies

When stellar-mass black holes—often termed “small” black holes that are several times more massive than the sun—actively consume matter, they are messier than scientists had foreseen.

This insight comes from an international research group led by Jon Miller of the University of Michigan, which uncovered unexpected findings that may have galactic consequences. Lessons learned from stellar-mass black holes can enhance understanding of supermassive black holes, the giants that guide the evolution of galaxies such as our Milky Way.

“Being caught off guard is advantageous. Realizing that expectations were oversimplified signifies advancement,” stated Miller, a principal author of the new research published in The Astrophysical Journal Letters. “Having entirely new paths to pursue is remarkable. Astronomy is filled with surprises and is never dull.”

Megumi Shidatsu from Ehime University and Misaki Mizumoto from the University of Teacher Education Fukuoka, both in Japan, co-led the initiative, which involved researchers from roughly 20 institutions spanning five nations.

The team made their discovery employing the X-Ray Imaging and Spectroscopy Mission, abbreviated as XRISM, which is pronounced “krism” (rhyming with “prism”). The mission is a partnership between NASA and the Japanese Aerospace Exploration Agency, or JAXA, with input from the European Space Agency.

X-rays in the cosmos

The XRISM satellite was launched in September 2023 and commenced its scientific activities in early 2024. Since that time, it has offered long-awaited and unparalleled insights into cosmic systems that emit X-rays.

“XRISM is at least tenfold more sensitive than any previous X-ray apparatus,” remarked Miller, the Douglas Richstone Collegiate Professor of Astronomy. “For this reason, we can now detect extraordinarily dramatic spectral lines that would have merely appeared as noise in the data we’ve collected over the last two decades.”

Spectroscopy—the S in XRISM—examines attributes of the light it gathers, instead of utilizing that light to create images. X-rays are high-energy forms of light, and XRISM’s Resolve spectrometer can measure their energy with unmatched accuracy to unveil insights regarding the environments that generated them.

In this latest study, the system in focus was 4U 1630-472, known as an X-ray binary. Astronomers believe this specific binary system consists of a stellar-mass black hole and a regular companion star similar to our sun.

The immense gravity of the black hole extracts matter from the companion star, drawing it into an accretion disc that envelops the black hole. This transferred material becomes extremely heated—up to 10 million Kelvin or approximately 18 million degrees Fahrenheit—emitting X-rays.

A notable characteristic of 4U 1630-472 is its remarkable fluctuations in brightness. In its usual, calm state, it shines with a luminosity comparable to our sun, Miller noted. However, every two years or so, it experiences an outburst during which its brightness can surge by 10,000 times over the span of a week. On February 16, 2024, XRISM detected 4U 1630-472 at the concluding phase of an outburst, providing a new glimpse into black hole science.

Scattering mass

Utilizing XRISM, researchers observed that even as the X-rays faded, 4U 1630-472 continued to expel some gas at 3% of the speed of light, or 20 million miles per hour.

“We were able to observe a variety of gas flow rates that we never witness with massive black holes in the centers of galaxies,” Miller stated. “The timescale to observe something like that from the black hole in the Milky Way would stretch into hundreds of millions of years. Thus, one of the reasons we examine these smaller black holes is to gain insight into how gas interactions with very massive black holes could evolve throughout a galaxy’s development.”

The researchers expected to witness matter migrating toward the black hole in a more regulated and precise way as the flow rate diminished, which seems logical. Miller likened it to pouring water.

“You anticipate spilling a considerable amount if you attempt to pour a bucket of water into a cup, but not when pouring a cup of water into a bucket,” he explained. “Black holes appear to spill in both scenarios.”

In other terms, the analogy applies for black holes when pouring from bucket to cup, but the team discovered the reverse holds true.

“There was still mass being flung around rather than directly accreted into the black hole,” Miller noted.

This unexpected finding raises even more inquiries about black holes and galaxy evolution, many of which XRISM is prepared to address. However, its capacity to do so is now uncertain due to President Trump’s budget proposal that could effectively terminate U.S. funding for the mission.

“We haven’t completed what NASA refers to as the mission’s prime phase, which encompasses a two-year duration to identify examples of groundbreaking science that can be achieved,” Miller mentioned. “Following that, you devote the remainder of the mission to exploring those areas once they have been unveiled.”

XRISM’s anticipated conclusion of the prime phase is September 2026, but missions are usually extended whenever feasible to maximize the return on investment. For instance, NASA’s Chandra X-ray Observatory launched in 1999 with an intended 5-year mission and remains operational today.

“The funding necessary for these missions is predominantly needed upfront—it pertains to construction, technology development, and the launch,” Miller remarked. “Annual operating costs represent a minor fraction of the overall investment.”

There is a possibility that JAXA could continue operating XRISM without NASA funding, Miller noted, but it is not something he and his colleagues wish to rely upon.

“An extensive community of hundreds, potentially even a couple thousand scientists in the U.S., stands to gain from XRISM,” Miller emphasized. “We are all striving diligently to utilize this opportunity to the fullest and hoping that it isn’t our last chance.”

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