The X-Ray Imaging and Spectroscopy Mission (XRISM) has observed an unexpectedly slow and dense wind from the neutron star system GX13+1, challenging models of radiation-driven outflows. This discovery highlights differences between winds around neutron stars and supermassive black holes. The findings could refine understanding of cosmic feedback processes.
On February 25, 2024, XRISM's Resolve instrument targeted the neutron star GX13+1, a compact remnant of a larger star that emits bright X-rays from its accretion disk of superheated material. Just before the observations, GX13+1 brightened unexpectedly, reaching or surpassing the Eddington limit, where radiation pressure drives outflows by pushing infalling matter back into space.
Despite the intense outburst, the wind from GX13+1 moved at about 1 million km/h—swift by earthly standards but slow compared to the 200 million km/h outflows near supermassive black holes at similar Eddington levels. The outflow was unusually dense and smooth, unlike the ultrafast, clumpy winds seen around black holes.
"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 realization of a dream that we had chased for decades."
Lead researcher Chris Done from Durham University, UK, noted the timing's serendipity: "We could not have scheduled this if we had tried. 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."
Done described the wind as "like looking at the Sun through a bank of fog rolling towards us. Everything goes dimmer when the fog is thick." He questioned the differences: "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?"
The team proposes that cooler ultraviolet radiation from larger accretion disks around supermassive black holes interacts more efficiently with matter than the hotter X-rays from stellar-mass systems like GX13+1, explaining the velocity contrast. These winds influence galaxy evolution through feedback, compressing or dispersing molecular clouds to regulate star formation and growth.
XRISM, launched on September 7, 2023, by JAXA with NASA and ESA partners, provides unprecedented X-ray resolution. The study appears in Nature (2025; 646 (8083): 57).