Prepare to have your mind blown: the winds around a neutron star are defying everything we thought we knew about space physics. But here's where it gets controversial... Could these bizarre winds hold the key to understanding how galaxies evolve? The X-Ray Imaging and Spectroscopy Mission (XRISM) has uncovered a startling contrast between the winds emanating from a neutron star's disk and those near supermassive black holes. What’s so surprising? The neutron star system produces an unusually dense and slow outflow, challenging our current theories about how these winds form and shape their surroundings.
On February 25, 2024, XRISM’s Resolve instrument turned its gaze to GX13+1, a neutron star—the compact remnant of a once-massive star. GX13+1 glows brilliantly in X-rays, emitted by a superheated accretion disk spiraling inward and colliding with the star’s surface. These inward flows can also generate powerful outflows that dramatically alter the surrounding space. But how? That’s what the team aimed to uncover by studying GX13+1.
Resolve’s ability to precisely measure the energy of individual X-ray photons promised to reveal details never before seen. And it delivered. “When we first analyzed the data, it felt like a paradigm shift,” says Matteo Guainazzi, ESA XRISM project scientist. “For many of us, it was the culmination of decades of dreaming and chasing this moment.”
And this is the part most people miss... These cosmic winds aren’t just fascinating phenomena—they’re universe-shapers. Similar winds from supermassive black holes at galaxy centers can compress molecular clouds to ignite star formation or heat and disperse them to halt it. Astronomers call this dynamic process “feedback,” and in extreme cases, these winds can regulate the growth of entire galaxies. By studying GX13+1, a closer and brighter target, the team hoped to uncover the underlying physics in sharper detail.
Just before the planned observations, GX13+1 threw a curveball: it brightened dramatically, reaching or even surpassing the Eddington limit. This limit describes what happens when matter falls onto a compact object like a neutron star or black hole. As more matter falls in, more energy is released, creating radiation pressure that pushes material outward. At the Eddington limit, this pressure can drive nearly all infalling matter back into space as a wind.
Resolve captured GX13+1 during this intense phase. “We couldn’t have timed this if we tried,” said Chris Done, lead researcher from Durham University, UK. “The system’s radiation output surged, creating a wind denser than anything we’d ever observed.”
Here’s where it gets puzzling: despite the outburst, the wind’s speed remained around 1 million km/h—fast by Earth standards but sluggish compared to winds near supermassive black holes, which can reach 20-30% of light speed (over 200 million km/h). “The slowness and thickness of this wind still astonish me,” Chris notes. “It’s like viewing the Sun through a thick fog—everything dims as the fog rolls in.”
Boldly highlighting the controversy... Earlier XRISM observations of a supermassive black hole at the Eddington limit revealed an ultrafast, clumpy wind. In contrast, GX13+1’s outflow was slow and smooth. “These winds are completely different, yet both systems are near the Eddington limit,” Chris points out. “If radiation pressure drives these winds, why are they so distinct?”
The team suggests the answer lies in the accretion disk’s temperature. Paradoxically, disks around supermassive black holes are cooler than those in stellar-mass systems like neutron stars. Why? Supermassive black hole disks are larger, spreading their energy over a vast area, so they emit mostly ultraviolet light. Stellar-mass systems, like GX13+1, radiate more strongly in X-rays.
Ultraviolet light interacts with matter more efficiently than X-rays. Chris and colleagues propose that this difference allows ultraviolet radiation to push material more effectively, generating the faster winds seen near supermassive black holes.
What does this mean for galaxy evolution? If this theory holds, it could revolutionize our understanding of energy and matter exchange in extreme environments. It might also clarify how these processes shape galaxies and the cosmos. “XRISM’s unprecedented resolution lets us study these objects—and many more—in incredible detail, paving the way for next-generation X-ray telescopes like NewAthena,” says Camille Diez, ESA Research Fellow.
Now, the thought-provoking question... If neutron star winds operate differently from black hole winds, could this challenge our current models of galaxy evolution? Share your thoughts in the comments—let’s spark a discussion!
