Using the joint Japan Aerospace Exploration Agency (JAXA), European Space Agency (ESA), and NASA X-Ray Imaging and Spectroscopy Mission (XRISM), scientists uncovered a dense but surprisingly slow wind coming from a neutron star. The finding sheds new light on similar winds blowing from material surrounding supermassive black holes.
Another team used NASA’s Chandra X-ray Observatory to study an ancient supermassive black hole growing at extreme rates. The giant might help answer scientific mysteries about the early universe’s most enigmatic objects.
XRISM studies neutron star GX13+1
Launched on Sept. 6, 2023, XRISM studies X-ray signals emitted throughout the universe. On Feb. 25, 2024, it aimed at a neutron star known as GX13+1, located roughly 23,000 light-years from Earth. Neutron stars are the dense remnants left after a supergiant star’s core collapses, compressing its material to the density of an atomic nucleus.
GX13+1 forms a binary with a giant companion star, which constantly siphons material into the neutron star. As the matter spirals towards the dense object, it forms a so-called accretion disk, which produces the X-ray signal studied by XRISM.
The telescope observed the object using its Resolve instrument to analyze the X-ray signal’s spectrum. Resolve’s energy resolution exceeds that of previous instruments used to binaries like GX13+1, exceeding Chandra’s High-Energy Transmission Grating Spectrometer (HETGS) by a factor of four.
Infographic detailing XRISM, its instruments, and goals. (Credit: ESA)
“When we first saw the wealth of details in the data, we felt we were witnessing a game-changing result,” said ESA XRISM project scientist Matteo Guainazzi. “For many of us, it was the realization of a dream that we had chased for decades.”
While the team was already planning to study this neutron star, the timing of the observations proved fortuitous. A few days before XRISM observations, GX13+1 suddenly got much brighter.
“We could not have scheduled this if we had tried,” said study lead Chris Done of Durham University, United Kingdom. “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.”
When the neutron star lit up, it exceeded its so-called Eddington limit. As matter falls into the object, it releases energy, and with more matter falling in, the amount of energy released increases as well. This energy pushes back on the material falling in, capping the rate at which matter can fall into the object, creating the Eddington limit.
The intense radiation at the Eddington limit blows away the material surrounding the neutron star, creating a cosmic wind. This phenomenon is also observed around supermassive black holes, some of which, like GX13+1, appear to outshine their Eddington limit.
Illustration depicting GX13+1 releasing winds as it feeds on its companion star. (Credit: ESA)
Despite the similarities, the supermassive black holes produce winds much faster than those observed at GX13+1. Using XRISM, the team found winds travelling at roughly one million km/h, whereas the winds from supermassive black holes can reach velocities over 200 times faster, up to 30% the speed of light.
“It is still a surprise to me how ‘slow’ this wind is,” said Done, “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.”
The XRISM study was published in the journal Nature on Sept. 17, 2025.
Chandra studies quasar RACS J0320−35
Meanwhile, another team used Chandra to study a supermassive black hole growing at a rate exceeding its Eddington limit. This object, known as RACS J032021.44−352104.1 (RACS J0320−35), is located 12.8 billion light-years away, so distant that it emitted the signal measured by Chandra only 920 million years after the Big Bang. The enormous black hole comes in at about a billion times the mass of the Sun and produces more X-rays than any other black hole of its age.
The black hole sits in an active galactic nucleus (AGN), forming an object known as a quasar. As matter from the galactic nucleus falls into the black hole, it releases enormous amounts of energy, resulting in a signal much brighter than the surrounding galaxy.
Illustration of a quasar powered by a supermassive black hole. Top left: Chandra’s observations of RACS J0320−35. (Credit: X-ray: NASA/CXC/INAF-Brera/L. Ighina et al.; Illustration: NASA/CXC/SAO/M. Weiss; Image Processing: NASA/CXC/SAO/N. Wolk)
The astronomers used Chandra to observe the quasar in July and December 2023. They also studied the object using radio observations from the Giant Metrewave Radio Telescope in India, the Australia Telescope Compact Array, and the Australian Long Baseline Array. The observations revealed that this object appears to be growing at 2.4 times its Eddington limit.
“It was a bit shocking to see this black hole growing by leaps and bounds,” said study lead Luca Ighina of the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts.
While bright in both radio and X-ray signals, RACS J0320−35 stands out for how “soft” its X-ray signal is, predominantly emitting low-frequency X-rays. However, the team notes that more X-ray observations of quasars are needed to determine how common or uncommon this actually is.
One of the questions astronomers hope to answer by studying black holes like RACS J0320−35 is how these supermassive black holes formed in the early universe.
“By knowing the mass of the black hole and working out how quickly it’s growing, we’re able to work backward to estimate how massive it could have been at birth,” said co-author Alberto Moretti of INAF-Osservatorio Astronomico di Brera in Italy. “With this calculation we can now test different ideas on how black holes are born.”
Little red dots observed by the James Webb Space Telescope. (Credit: NASA/ESA/CSA/STScI/D. Kocevski (Colby College))
What’s more, the team also suggests that RACS J0320−35 might hint at the nature of recently discovered “little red dots” found scattered throughout the early universe. These dots first showed up in 2022, in some of the first observations from the James Webb Space Telescope (JWST). Astronomers have since been looking for an explanation.
These dots might actually be AGNs obscured by dust, and some suspect super-Eddington accretion could explain the unique signal. Ighina’s team believes that RACS J0320−35 could confirm this suspicion and suggests the quasar should be studied in infrared.
Meanwhile, another recent study led by Anna de Graaff of Max Planck Institute for Astronomy in Heidelberg, Germany, analyzed the light signature of a specific red dot dubbed “The Cliff” after a distinct drop off in its light spectrum. They believe this tantalizing signal hints at the red dot being a new type of star-like object, called a black hole star.
The Cliff’s signal is more similar to a star’s spectrum than the spectra emitted by AGNs. Therefore, the De Graaff’s team suspects the object is surrounded by a thick shell of hydrogen, like a star. But unlike a normal star, which is powered by nuclear fusion in its core, the center of this object is an AGN heating the layer of hydrogen.
Although neither study offers a definitive explanation for Webb’s red dots, both provide an intriguing perspective on mysterious objects dotted throughout the early universe.
Ighina et al.’s study was published in The Astrophysical Journal Letters on Sept. 8, 2025.
(Lead image: Left: illustration of a quasar like RACS J0320−35. Right: illustration of neutron star GX13+1’s accretion disc expelling winds. Credit: Left: NASA/CXC/SAO/M. Weiss; Right: ESA)
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