Intriguing Discoveries from the XRISM Telescope Shed Light on Neutron Stars
In a captivating discovery, researchers employing the XRISM telescope, which was launched in late 2023, have unveiled unexpected phenomena while examining GX13+1, a binary system of a neutron star. This observation in February 2024 unveiled a notably slow cosmic wind, providing fresh perspectives on the physics of matter accumulation in binary systems.
Findings from this study, one of the pioneering observations using XRISM, were published in Nature in September 2025. The spectral data indicated that GX13+1 was experiencing a super-Eddington phase, where the star’s radiation pressure exceeds gravitational pull, causing matter to be expelled as a slow cosmic wind. According to Joey Neilsen, PhD from Villanova University, these findings might transform our understanding of such systems.
Dr. Neilsen, a key co-investigator, collaborated with experts from institutions like the University of Durham, Osaka University, and the University of Teacher Education Fukuoka. The global team included numerous researchers from various institutions.
GX13+1 comprises a neutron star and a K5 III companion star. Neutron stars, remnants of supernovae, are extremely dense, with a teaspoon of their material weighing as much as Mount Everest. Their intense gravitational fields allow them to draw material from companion stars, forming an accretion disk that shines brightly in X-rays.
“Imagine putting a giant lightbulb in a lake,” explained Dr. Neilsen. “If it’s bright enough, it will start to boil that lake and then you would get steam, which flows away like a wind. It’s the same concept; the light can heat up and exert pressure on the accretion disk, launching a wind.”
Artist’s rendition of GX13+1’s winds, originally published in the journal Nature. Photo credit: CfA/Melissa Weiss
XRISM was initially intended to observe accretion disk winds, with GX13+1 being an ideal candidate due to its consistent brightness and well-documented history. This telescope’s precision surpasses that of previous instruments like Chandra, offering a clearer view of the gas movements around neutron stars.
Dr. Neilsen noted, “It’s like comparing a blurry image to a much sharper one. The atomic physics hasn’t changed, but you can see it much more clearly.”
The high-resolution data revealed unusually deep absorption lines, indicating that the cosmic wind was moving at a leisurely pace of 1.4 million miles per hour—unexpectedly slow for such a luminous source.
The observations confirmed GX13+1 was brighter than usual, undergoing a super-Eddington phase where radiation pressure was significant enough to reduce the star’s apparent brightness to the XRISM instrument.
“There’s a theoretical maximum luminosity that you can get out of an accreting object, called the Eddington limit. At that point, the radiation pressure from the light of the infalling gas is so large that it can actually hold the matter away,” explained Dr. Neilsen, likening it to halting a waterfall with light. “What we saw was that GX13+1 had to have been near, or maybe even above, the Eddington limit.”
Comparing new and old data revealed similarities with past observations, suggesting these super-Eddington phases might be cyclical, occurring about 10% of the time. This could imply that such phases are more common in neutron star or black hole binary systems than previously understood.
Dr. Neilsen speculated, “Temporary super-Eddington phases might actually be a thing that accreting systems do, not just something unique to this system. And if neutron stars and black holes are doing it, what about supermassive black holes? Perhaps this could pave the way for a deeper understanding of all these systems.”
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