Why are cosmic winds from neutron stars so different? XRISM reveals the mystery

An artist’s impression shows powerful winds streaming from GX13+1, a bright X-ray source. The radiation comes from hot matter in an accretion disc spiralling towards the surface of a neutron star. (Image: ESA)

When scientists pointed XRISM’s sharp eyes at a distant neutron star, they expected answers. Instead, they uncovered a puzzle that could reshape how we understand cosmic winds and their role in shaping galaxies.

What did XRISM observe?
On 25 February 2024, XRISM’s Resolve instrument studied GX13+1, a neutron star left behind after a larger star died. The system glows brightly in X-rays, produced by hot matter spiraling inwards through an accretion disc before striking the star’s surface. As this inflow occurs, powerful outflows, or cosmic winds, are created. Astronomers hoped XRISM’s unmatched resolution would reveal new details about how these winds form.

Why is this system important?
GX13+1 was chosen because it is much closer than supermassive black holes found in galactic centers. That makes it appear brighter, allowing scientists to study its winds in detail. These outflows are not trivial. They can trigger star formation by compressing molecular clouds or halt it entirely by blasting them apart. Such processes, known as feedback, can even influence the growth of entire galaxies.

What surprised astronomers during the observation?
Just days before XRISM’s scheduled observations, GX13+1 suddenly brightened, reaching or even exceeding the Eddington limit. At this limit, radiation pressure from infalling matter pushes much of it back into space as wind. By chance, XRISM captured this rare moment. The system’s radiation jumped from about half its maximum output to much higher levels, producing an unusually dense wind. Yet despite this intensity, the wind travelled at only about 1 million km/h.

How does this compare with black hole winds?
The speed was far slower than winds seen around supermassive black holes, which can reach 20 to 30 percent of light speed. Instead of being clumpy and ultrafast, the wind from GX13+1 appeared smooth and sluggish. Lead researcher Chris Done compared it to seeing the Sun through a rolling bank of fog thick enough to dim the view.

What explains the difference?

Scientists propose that the solution is in accretion disc temperature. discs are cooler, larger, and around supermassive black holes, emitting predominantly ultraviolet light. This kind of light interacts very strongly with matter, driving it more efficiently and producing more rapid winds. discs are smaller, hotter, and, around neutron stars and stellar black holes, producing mainly X-rays, which interact less efficiently and give slower winds.

What does this find imply?

The discovery demonstrates how winds from extreme objects can act very differently even within similar energy constraints. These differences may help to explain how energy and matter flow through extreme cosmic environments, sculpting galaxies over billions of years.

“The unprecedented resolution of XRISM allows us to study these systems in detail,” said Camille Diez of ESA. “This paves the way for future high-resolution missions such as NewAthena.”

The study, Multi-phase winds from a super-Eddington X-ray binary are slower than expected, was published in Nature on 17 September 2025.