XRISM spacecraft reveals new details and more mysteries about cosmic winds

Astronomers caught a neutron star launching an unusually dense wind. It looked nothing like the ultrafast gusts seen around supermassive black holes. The mismatch suggests our standard ideas about how these outflows start – and how they shape their surroundings – may need a rethink.

The observation comes from XRISM, an X-ray mission operated with European and Japanese partners, using its ultra-precise Resolve spectrometer.

On February 25, 2024, the team pointed Resolve at GX13+1, a neutron star bright in X-rays because a hot disk of gas is spiraling down onto its surface.

Resolve separates incoming X-ray photons with exquisite accuracy. The first spectra from GX13+1 stunned the team.

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

Star pushes past the limit

Just days before the planned look, GX13+1 brightened dramatically. It reached, and may have exceeded, the Eddington limit – the point where radiation pushes back so hard that infalling matter is driven outward as a wind. Resolve watched the system tip into this state in real time.

“We could not have scheduled this if we had tried,” said astrophysicist Chris Done from Durham University, the study’s lead researcher.

“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.”

Winds crawl instead of race

Near the Eddington limit, cosmic engines such as supermassive black holes and neutron stars can launch winds at 20 to 30 percent of light speed, well over 124 million miles per hour (200 million kilometers per hour).

GX13+1 did something else. Its wind crawled along at roughly 621,000 miles per hour (1 million kilometers per hour). That is fast for Earth. It is sluggish for a near-Eddington engine.

“It is still a surprise to me how ‘slow’ this wind is, 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,” said Done.

XRISM has also seen an Eddington-level outflow from a supermassive black hole. That one was ultrafast and clumpy. GX13+1 is 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?” Done pondered. The contrast hints that more than raw push from light is at play.

UV light drives faster winds

One leading idea points to disk temperature. Discs around supermassive black holes are vast. Their power spreads over a larger area, so their typical radiation peaks in ultraviolet.

Discs around neutron stars and stellar-mass black holes run hotter and pump out X-rays.

Ultraviolet interacts with atoms and ions much more readily than X-rays do. If so, a UV-rich disc can shove on gas more efficiently, launching faster winds.

A hotter, X-ray-bright disc may puff up a denser outflow that moves more slowly. That simple difference in photon energy could explain why two “Eddington” systems behave so differently.

Outflows shape stars and galaxies

These outflows are engines of change. Around supermassive black holes, they can compress cold clouds to spark star formation, or heat and scatter gas to quench it. Astronomers call this “feedback.”

In extreme cases, the central wind can steer how an entire galaxy grows. Neutron star systems work on smaller scales, but they still churn their neighborhoods with heat and momentum.

Seeing a dense, slow wind near the Eddington limit forces theorists to revisit how energy couples to matter – from star-sized engines to galaxy-sized ones.

Mapping stellar winds in detail

Resolve’s sharp vision makes this possible. It picks out tiny shifts and shapes in atomic fingerprints that reveal the speed, thickness, and structure of the wind.

Those diagnostics turn a spectrum into a weather report: how fast the gas flows, how much of it there is, and whether it’s smooth or clumpy.

With more targets, patterns should emerge, and the temperature-driven explanation can be tested against alternatives.

Future missions chase slow winds

The team is already thinking about what the finding means for the next wave of instruments.

“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, an ESA research fellow.

More observations of GX13+1 and other bright accretors will show whether dense, slow winds are common at high brightness, or if this was a rare turn of events.

A neutron star crossed a critical brightness and blew a wind as thick as fog but far slower than expected. That single twist challenges the idea that Eddington winds all look the same.

With Resolve, XRISM can now map these differences in fine detail. The result is a fresh framework for how discs launch winds, how light pushes on matter, and how little engines and giant ones both help sculpt the cosmos.

The study is published in the journal Nature.

Image Credit: ESA

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–