'It was the realization of a dream that we had chased for decades.' Powerful cosmic winds around neutron star reveal secrets of monster black holes

Illustration shows winds blowing away from neutron star and its swirling plate of gas and dust (Image credit: ESA)

Astronomers have used an X-ray spacecraft called XRISM to observe powerful winds blowing from a neutron star — the findings could be a "game changer" for physics.

The team discovered unexpected differences between powerful and energetic winds blowing from swirling disks of gas and dust, called accretion disks, around extreme "dead stars," or neutron stars, and winds that flow from accretion disks that feed supermassive black holes at the hearts of large galaxies.

The discovery could reveal more about the physics surrounding the inflow of matter from accretion disks to the surfaces of both neutron stars and supermassive black holes, as well as the outflow of winds from these disks. Understanding such dynamics could, in turn, reveal how these winds influence the cosmic surroundings of supermassive black holes.

The team discovered the surprising differences between supermassive black hole and neutron star accretion disks when they used the NASA/JAXA spacecraft XRISM (X-Ray Imaging and Spectroscopy Mission) to observe powerful winds flowing from accretion disk GX13+1, located between 23,000 and 26,000 light-years from Earth in the galactic bulge of the Milky Way.The observing power of XRISM's Resolve instrument allowed the team to measure the energy of X-ray light emitted from GX13+1 and gather details about its system that had never been seen before.

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

Cosmic winds of change

It may seem strange to investigate supermassive black hole winds by studying the wind blowing from a neutron star, but the team behind this research reasoned that the mechanisms behind these different outflows are similar. Also, the closest supermassive black hole to us, the Milky Way's Sagittarius A* (Sgr A*), isn't actively feeding because it isn't surrounded by enough matter to form an accretion disk.

GX13+1 is closer and brighter than the feeding supermassive black holes in other galaxies that could be used for this kind of investigation, allowing it and the physics driving its winds to be studied in greater detail.

However, before the observations of GX13+1 could even begin, this neutron star delivered a surprise to the team, brightening so much the researchers theorized it may have reached or even exceeded the Eddington limit.

This theoretical limit concerns how much matter can be accreted to a compact body like a neutron star or black hole. The more matter accreted, the more energy emitted and thus the more outward pressure exerted on infalling material. When the Eddington limit is reached, the outward pressure of this energy is so great that the supply of material to the compact celestial body is cut off, and the surrounding material is pushed away as cosmic winds.

Via Resolve, the team watched as GX13+1 hit this ceiling.

"We could not have scheduled this if we had tried," team leader Chris Done from Durham University in the UK, said. "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 large white orb surrounded by white bands of light on a starry background — An illustration of a neutron star surrounded by a powerful magnetic field. (Image credit: ESO/L. Calçada)

However, this wasn't the end of the surprises delivered by this wind. It wasn't traveling at the speed the team was expecting. Cosmic winds produced at or around the Eddington limit can flow as fast as 124 million miles per hour and even up to about 30% the speed of light.

The wind flowing from GX13+1, however, was travelling at a relatively leisurely 620,000 mph. We say relatively because that is still around 800 times as fast as the speed of sound on Earth. What the wind lacked in speed, however, it made up for with its density. However, unlike winds seen blowing from supermassive black holes near the Eddington limit, which are clumpy, the wind from GX13+1 flowed smoothly.

"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," Done added. "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?"

a black orb surrounded by a ring of orange and white light, on a black background — An illustration shows winds blowing from material surrounding a supermassive black hole. (Image credit: NASA)

Currently, Done and colleagues think these differences may be the result of temperature variations between the accretion disks around neutron stars like the one they observed and those surrounding supermassive black holes.

The accretion disks around supermassive black holes are larger and brighter than those around neutron stars, meaning their energy is dispersed over a larger area. This means the light emitted from these larger accretion disks is in the ultraviolet region of the electromagnetic spectrum, while the electromagnetic radiation from the disks around neutron stars is in the form of X-rays, which are higher in energy.

Ultraviolet light interacts with matter more easily than X-rays, so the team theorizes that radiation from supermassive black hole accretion disks may more effectively push matter, leading to faster winds.