A neutron star’s weird wind rewrites space physics

On February 25, 2024, XRISM used its Resolve instrument to observe the neutron star GX13+1, the compact remnant of a once larger star. GX13+1 shines brightly in X-rays that come from an accretion disk of superheated material spiraling inward and striking the star's surface.

These inward flows can also launch powerful outflows that alter the space around them. How these outflows arise is still under investigation, which is why the team targeted GX13+1.

Resolve can precisely measure the energy of individual X-ray photons, so the scientists anticipated seeing fine-grained details that had never been captured before.

"When we first saw the wealth of details in the data, we felt we were witnessing a game-changing result," says Matteo Guainazzi, ESA XRISM project scientist. "For many of us, it was the realization of a dream that we had chased for decades."

Why cosmic winds matter

These winds are not just curiosities. They drive large-scale change in the universe.

Similar winds also blow from systems with supermassive black holes at galaxy centers. They can compress giant molecular clouds to trigger star birth or heat and disperse those clouds to halt star formation. Astronomers refer to this push and pull as feedback, and in extreme cases the wind from a central black hole can regulate the growth of its entire host galaxy.

Because the processes around supermassive black holes might mirror those near GX13+1, the team chose this neutron star system as a closer, brighter target that could reveal the underlying physics in sharper detail.

A timely surge to the Eddington limit

Just before the planned observations, GX13+1 unexpectedly brightened and reached or even surpassed the Eddington limit.

This limit describes what happens as matter falls onto a compact object such as a black hole or a neutron star. More infalling matter releases more energy. As the energy output rises, the radiation exerts pressure on the incoming material and pushes it outward. At the Eddington limit, the high-energy light being produced can drive almost all of the infalling matter back into space as a wind.

Resolve recorded GX13+1 during this dramatic phase.

"We could not have scheduled this if we had tried," said Chris Done, Durham University, UK, the lead researcher on the study. "The system went from about half its maximum radiation output to something much more intense, creating a wind that was thicker than we'd ever seen before."

A slow, dense wind defies expectations

Despite the intense outburst, the wind's speed remained near 1 million km/h. That is swift on Earth but slow compared with winds near the Eddington limit around supermassive black holes, where outflows can reach 20 to 30 percent of light speed, more than 200 million km/h.

"It is still a surprise to me how 'slow' this wind is," says Chris, "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."

Neutron star vs black hole winds

This was not the only contrast. Earlier XRISM observations of a supermassive black hole at the Eddington limit revealed an ultrafast, clumpy wind. By comparison, the outflow from GX13+1 appears slow and smooth.

"The winds were utterly different but they're from systems which are about the same in terms of the Eddington limit. So if these winds really are just powered by radiation pressure, why are they different?" asks Chris.

Accretion disk temperature as the key

The team suggests the answer lies in the temperature of the accretion disk around the central object. Counterintuitively, disks around supermassive black holes tend to be cooler than those in stellar-mass systems with neutron stars or black holes.

Disks around supermassive black holes are much larger. They can be extremely luminous, yet that power is spread over a vast area, so the typical radiation they emit peaks in ultraviolet light. Stellar-mass systems radiate more strongly in X-rays.

Ultraviolet light interacts with matter more readily than X-rays. Chris and colleagues propose that this difference allows ultraviolet radiation to push material more efficiently, generating the much faster winds seen near supermassive black holes.

What this means for galaxy evolution

If this explanation holds, it will refine how scientists think about the exchange of energy and matter in extreme environments. It could also clarify how these processes influence the growth of galaxies and the broader evolution of the cosmos.

"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," says Camille Diez, ESA Research fellow.

XRISM mission at a glance

XRISM (pronounced krizz-em) launched on September 7, 2023. The mission is led by the Japan Aerospace Exploration Agency (JAXA) in partnership with NASA and ESA. It flies with two instruments: Resolve, an X-ray calorimeter that measures the energy of individual X-ray photons to deliver an unprecedented level of energy resolution (the capability of an instrument to distinguish the X-ray 'colors'), and Xtend, a wide-field X-ray CCD camera that images the surrounding region.