Astronomers using the new X-Ray Imaging and Spectroscopy Mission (XRISM) have discovered that the fierce winds blowing from matter spiraling into a neutron star behave very differently from those coming from around black holes.
The findings, published in Nature, show that while the winds from the neutron star system are unusually dense, they are also far slower than expected.
On 25 February 2024, XRISM turned its Resolve instrument—a high-resolution X-ray spectrometer—toward a system called GX13+1.
This object is a neutron star, the collapsed core of a once-massive star, surrounded by an accretion disk of hot gas and dust.
As material in the disk spirals inward, it emits intense X-rays before crashing onto the neutron star’s surface. At the same time, some of the matter is blasted outward in the form of powerful winds.
Such cosmic outflows are not just exotic features of extreme objects. They play a central role in shaping galaxies, stirring up their surroundings and regulating star formation.
Similar winds are seen around supermassive black holes at galactic centers, where they can either trigger star birth by compressing clouds of gas or prevent it by blowing those clouds apart. Understanding how these winds are generated is one of astronomy’s major challenges.
For this reason, astronomers chose GX13+1 as a target. Because it is relatively close compared to distant supermassive black holes, its details can be studied more clearly.
But the researchers got lucky.
Just before the scheduled observation, GX13+1 flared dramatically, brightening to or even beyond the so-called Eddington limit—the maximum radiation a compact object can produce before the outward pressure of light itself pushes matter away in a powerful wind.
“We could not have planned this better,” said Chris Done of Durham University, lead researcher on the project. “The system suddenly jumped to near maximum output, producing an incredibly thick wind that Resolve was able to capture in remarkable detail.”
Yet the surprise was how slowly the wind was moving. Even at the Eddington limit, the material was streaming out at about one million kilometers per hour.
While this sounds fast, it is sluggish compared to winds from supermassive black holes under similar conditions, which can reach 20 to 30 percent of the speed of light—hundreds of millions of kilometers per hour.
“It’s still astonishing to me how slow this wind is, and how thick,” Done said. “It’s like looking through a wall of fog. The light dims, but the fog moves gently compared to the hurricane we’d expect.”
The results contrasted sharply with XRISM’s earlier observations of a supermassive black hole wind, which was ultrafast and patchy.
Why the difference? The researchers suggest it comes down to the temperature and size of the accretion disks. Around supermassive black holes, the disks are enormous and emit mainly ultraviolet radiation. Around neutron stars and stellar black holes, the disks are smaller, hotter, and emit mostly X-rays.
Ultraviolet light interacts more effectively with matter than X-rays, so despite carrying less energy, it may be better at pushing gas outward quickly. This could explain why winds around black holes move much faster than those around neutron stars, even when both are at the Eddington limit.
If this interpretation is correct, it reshapes our understanding of how energy and matter behave near compact objects and offers new insights into the processes that shape galaxies.
“The unprecedented resolution of XRISM has given us a new window into these extreme environments,” said Camille Diez, ESA research fellow. “It is just the beginning of what we’ll learn, and it points the way toward future observatories like NewAthena that will take this even further.”
By revealing such unexpected differences, XRISM has shown that even in the most extreme corners of the universe, the details of light and matter still hold surprises.
