The X-Ray Imaging and Spectroscopy Mission (XRISM) has revealed an unexpected difference between the powerful winds launching from a disc around a neutron star and those from material circling supermassive black holes. The surprisingly dense wind blowing from the stellar system challenges our understanding of how such winds form and drive change in their surroundings.
On 25 February 2024, XRISM used its Resolve instrument to look at neutron star GX13+1, the burnt-out core of a once larger star. GX13+1 is a bright X-ray source. The X-rays are coming from a disc of hot matter, known as an accretion disc, that is gradually spiralling down to strike the neutron star ‘s surface.
Such inflows also power outflows that influence and transform the cosmic environment. Yet the details of how these outflows are produced remain a matter of ongoing research. This is why XRISM was observing GX13+1.
Given the unprecedented power of Resolve to tease out the energy of incoming X-ray photons, the XRISM team expected to see those details as never 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 realisation of a dream that we had chased for decades.”
Such cosmic winds are much more than scientific curiosities – they are the winds that drive cosmic change.
They appear also from supermassive black holes systems found at the centres of galaxies, and can cause stars to form by triggering the collapse of giant molecular clouds , or they can stop star formation by heating and blowing those clouds apart. Astronomers call this ‘feedback’, and it can be so powerful that the winds from a supermassive black hole can control the growth of its entire parent galaxy.
Since the mechanisms generating the winds from supermassive black holes may be fundamentally the same as those at work around GX13+1, the team chose to look at GX13+1 because it is closer and therefore appears brighter than the supermassive black hole varieties, meaning that it can be studied in more detail.
There was a surprise. A few days before their observations were due to take place, GX13+1 unexpectedly got brighter – reaching or even exceeding a theoretical ceiling known as the Eddington limit.
The principle behind this limit is that as more matter falls onto a compact object such as a black hole or a neutron star, more energy is released. The faster energy is released, the greater the pressure it exerts on other infalling material, pushing more of it back into space. At the Eddington limit, the amount of high-energy light being produced is essentially enough to transform almost all of the infalling matter into a cosmic wind.
And Resolve happened to be watching GX13+1 as this staggering event took place.
“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.”
But mysteriously, the wind was not travelling at the speed that the XRISM scientists were expecting. It remained around 1 million km/h. While fast by any terrestrial standard, this is decidedly sluggish when compared to the cosmic winds produced near the Eddington limit around a supermassive black hole. In that situation, the winds can reach 20 to 30 percent the speed of light, 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.”
It was not the only difference the team observed. XRISM had earlier revealed a wind from a supermassive black hole at the Eddington limit. There the wind was ultrafast and clumpy, whereas the wind in GX13+1 is slow and smooth flowing.
“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.
The team has proposed that it comes down to the temperature of the accretion disc that forms around the central object. Counterintuitively, supermassive black holes tend to have accretion discs that are lower in temperature than those around stellar mass binary systems with black holes or neutron stars.
This is because the accretion discs around supermassive black holes are larger. They are also more luminous, but their power is spread across a larger area – everything is bigger around a big black hole. So, the typical kind of radiation released by a supermassive black hole accretion disc is ultraviolet, which carries less energy than the X-rays released by the stellar binary accretion discs.
Since ultraviolet light interacts with matter much more readily than X-rays do, Chris and her colleagues speculate that this may push the matter more efficiently, creating the faster winds observed in black hole systems.
If so, the discovery promises to reshape our understanding of how energy and matter interact in some of the most extreme environments in the Universe, providing a more complete window into the complex mechanisms that shape galaxies and drive cosmic evolution.
“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.