The night sky may look silent and serene, but beyond our eyes, the universe is restless. Stars collapse, black holes devour, and galaxies breathe through powerful winds that shape their destinies. Into this vast stage comes XRISM—the X-Ray Imaging and Spectroscopy Mission, a collaboration between JAXA (Japan), NASA, and ESA. XRISM was designed not just to look at the universe, but to listen to it in exquisite detail, using X-ray light as its language.
On February 25, 2024, XRISM turned its gaze toward an extraordinary target: GX13+1, the burned-out core of a once-mighty star. It was supposed to be a routine observation of a neutron star’s accretion disk. Instead, what XRISM found stunned astronomers, upended assumptions, and gave us a glimpse into one of the deepest mysteries of cosmic evolution.
The Furnace of GX13+1
Neutron stars are the remnants of massive stars that exploded in supernovae. What remains is a city-sized sphere packed so tightly that a teaspoon of its matter would weigh billions of tons. Around many neutron stars, matter still swirls in a glowing accretion disk—gas and dust pulled from a companion star or from the remains of its own violent past. As the material spirals inward, it heats to millions of degrees and blazes in X-rays.
GX13+1 is one of these extreme objects. It shines so brightly in X-rays that it has long intrigued astronomers. But it’s not just the inflow of matter that matters. The disk also produces outflows—cosmic winds of gas that blast outward into surrounding space. These winds don’t just decorate the scene; they reshape their environments, redistributing matter and energy across the cosmos. Understanding them means understanding how stars, galaxies, and even the large-scale universe evolves.
Resolve: A Game-Changing Eye
The real star of this story is XRISM’s Resolve instrument, a revolutionary spectrometer. Unlike traditional detectors, Resolve can measure the energies of incoming X-ray photons with astonishing accuracy. It’s like switching from a blurry radio signal to crystal-clear high-definition sound.
“When we first saw the wealth of details in the data, we felt we were witnessing a game-changing result,” said Matteo Guainazzi, ESA XRISM project scientist. “For many of us, it was the realization of a dream that we had chased for decades.”
Resolve gave astronomers the ability to probe GX13+1’s winds in unprecedented detail. What it revealed was shocking.
A Universe of Winds
Cosmic winds are everywhere. They stream from stars, from supernova explosions, and—on the grandest scales—from the disks of material that swirl around supermassive black holes at the hearts of galaxies. These colossal winds are no mere side effects. They regulate star birth, feed or starve galaxies, and shape the structure of the cosmos. Astronomers call this process feedback.
Supermassive black holes, despite their name, are architects of galaxies. Their winds can crush giant clouds of gas into star-forming nurseries—or blow them apart, halting star birth for millions of years. In this way, they can govern the growth of galaxies larger than our own Milky Way.
Because the physics behind these winds might be universal, astronomers study smaller, closer systems like GX13+1 to glimpse the same processes at work—but in sharper detail. Neutron stars are cosmic laboratories, their disks bright enough to act as test cases for theories of wind formation.
The Perfect Cosmic Coincidence
Just days before XRISM’s scheduled observation, GX13+1 erupted in brightness, reaching what astronomers call the Eddington limit. This threshold represents a cosmic tug-of-war. As matter falls inward toward a compact object, it releases energy in the form of radiation. Eventually, the outward pressure from this radiation grows strong enough to counteract gravity’s inward pull. At that limit, much of the infalling matter gets blown back out as powerful winds.
Resolve happened to be watching at exactly this moment. “We could not have scheduled this if we had tried,” said Chris Done of Durham University, the study’s lead researcher. GX13+1 was burning with such intensity that its disk became a forge of powerful winds.
A Wind Like No Other
What XRISM saw was unlike anything astronomers expected. Instead of a high-speed gale, GX13+1’s wind was dense, smooth, and strangely slow—only about 1 million kilometers per hour. That might sound blistering, but compared to winds from supermassive black holes, which can roar at 20–30% the speed of light (hundreds of millions of km/h), it was almost leisurely.
“It is still a surprise to me how ‘slow’ this wind is,” said Chris Done. “As well as how thick it is. It’s like looking at the sun through a bank of fog rolling towards us. Everything goes dimmer when the fog is thick.”
This finding raised a burning question: If both systems were operating near the Eddington limit, why were their winds so dramatically different?
Black Holes vs. Neutron Stars: A Tale of Temperature
The XRISM team believes the answer lies in the temperature of the accretion disk. Paradoxically, disks around supermassive black holes are cooler than those around stellar-mass black holes or neutron stars.
Here’s why: supermassive black holes are, quite literally, enormous. Their accretion disks stretch across vast distances. While they shine brilliantly, their energy is spread across this immense area, meaning the typical radiation they emit is in the ultraviolet range.
By contrast, disks around neutron stars like GX13+1 are smaller and more compact, meaning their emission peaks in X-rays, which are far more energetic but less effective at pushing matter. Ultraviolet light interacts strongly with gas and dust, sweeping it outward in high-speed bursts. X-rays, however, tend to slip through, creating winds that are slower but thicker.
In other words, the flavor of light matters. Supermassive black holes produce “gentler” ultraviolet light that paradoxically drives faster winds. Neutron stars produce searing X-rays that instead build up dense, heavy outflows.
Winds that Shape the Universe
This discovery is more than a technical curiosity. It challenges a long-standing assumption that cosmic winds are powered in essentially the same way, regardless of scale. If the nature of the light itself changes the character of these winds, it reshapes how we understand feedback, the process that governs the growth of galaxies.
Imagine two galaxies: one with a quasar at its heart, blasting ultrafast winds into space, sculpting its spiral arms, and regulating its star birth; another with a dense neutron star wind fogging its surroundings. These subtle differences ripple outward, influencing how matter clusters, how stars form, and even how galaxies evolve over billions of years.
A New Era of High-Resolution Astronomy
For astronomers, XRISM’s result is a tantalizing glimpse of what’s to come. The mission has already shown that with high enough resolution, the universe reveals secrets we didn’t know to look for. “The unprecedented resolution of XRISM allows us to investigate these objects—and many more—in far greater detail, paving the way for the next-generation, high-resolution X-ray telescope such as NewAthena,” said Camille Diez, ESA Research Fellow.
The discovery at GX13+1 is only the beginning. Other neutron stars, stellar-mass black holes, and distant quasars await XRISM’s gaze. Each one may tell a different story, rewriting what we thought we knew about how light, matter, and gravity dance together at the universe’s extremes.
The Cosmic Wind in Us All
Perhaps the most profound aspect of this discovery is its reminder of connection. The winds that sweep out of neutron stars and black holes are not distant curiosities. They enrich the galaxies with heavy elements, seed star formation, and sculpt the cosmic web. Without them, our own solar system might not have formed in the way it did.
We are, quite literally, children of these winds. The iron in our blood, the oxygen in our lungs, the calcium in our bones—all forged in the fiery deaths of stars and spread across space by the same cosmic outflows XRISM is now revealing.
And so, as XRISM peers into the heart of GX13+1 and beyond, it is not only telling us the story of strange winds and mysterious disks. It is telling us the story of ourselves—the story of how the restless universe shapes everything we know, from galaxies to life itself.
More information: XRISM collaboration, Stratified wind from a super-Eddington X-ray binary is slower than expected, Nature (2025). DOI: 10.1038/s41586-025-09495-w. www.nature.com/articles/s41586-025-09495-w
