Prepare to have your understanding of space physics challenged! A recent discovery involving a neutron star's unusual winds is causing a major stir, rewriting some of the most fundamental theories about how these cosmic phenomena work. This revelation comes from the X-Ray Imaging and Spectroscopy Mission (XRISM), which has uncovered a striking difference between the winds emanating from a neutron star and those observed around supermassive black holes. But here's where it gets controversial: the neutron star's wind is unexpectedly dense, defying existing models of wind formation and its impact on the surrounding environment.
On February 25, 2024, XRISM's Resolve instrument turned its gaze toward GX13+1, a neutron star. GX13+1 is the incredibly dense remnant of a once-massive star. It shines brightly in X-rays, a result of superheated material swirling inward, forming an accretion disk that crashes onto the star's surface.
These infalling flows can also generate powerful outflows, which reshape the space around them. Scientists are still trying to understand how these outflows originate, which is why GX13+1 became a key target. The Resolve instrument is capable of precisely measuring the energy of individual X-ray photons, allowing scientists to capture details never before seen.
"When we first saw the wealth of details in the data, we felt we were witnessing a game-changing result," explains Matteo Guainazzi, the ESA XRISM project scientist. "For many of us, it was the realization of a dream that we had chased for decades."
Why should we care about cosmic winds? These winds aren't just cosmic curiosities; they're major players in shaping the universe. Similar winds are also observed around supermassive black holes at the centers of galaxies. They can compress vast clouds of gas and dust, triggering the birth of new stars, or they can heat and disperse these clouds, halting star formation. This push-and-pull dynamic, known as feedback, can even regulate the growth of an entire galaxy in extreme cases.
Because the processes around supermassive black holes might mirror those near GX13+1, the team chose this neutron star system as a closer, brighter target that could reveal the underlying physics in sharper detail.
A timely surge to the Eddington limit. Just before the scheduled observations, GX13+1 unexpectedly brightened, reaching or even exceeding the Eddington limit. This limit describes what happens as matter falls onto a compact object such as a black hole or a neutron star. More infalling matter releases more energy. As the energy output rises, the radiation exerts pressure on the incoming material and pushes it outward. At the Eddington limit, the high-energy light being produced can drive almost all of the infalling matter back into space as a wind.
Resolve recorded GX13+1 during this dramatic phase. "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."
A slow, dense wind defies expectations. Despite the intense outburst, the wind's speed remained around 1 million km/h. While this is incredibly fast by Earth standards, it's slow compared to winds near the Eddington limit around supermassive black holes, which can reach 20% to 30% of the speed of light, exceeding 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."
Neutron star vs. black hole winds. This wasn't the only surprise. Earlier observations by XRISM of a supermassive black hole at the Eddington limit revealed an ultra-fast, clumpy wind. In contrast, the outflow from GX13+1 appears slow and smooth. "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 key: accretion disk temperature. The team believes the answer lies in the temperature of the accretion disk surrounding the central object. Here's a counterintuitive point: disks around supermassive black holes tend to be cooler than those in stellar-mass systems like neutron stars or black holes. Disks around supermassive black holes are much larger. They can be incredibly luminous, but that power is spread over a vast area, so the radiation they emit peaks in ultraviolet light. Stellar-mass systems, on the other hand, radiate more strongly in X-rays.
Ultraviolet light interacts with matter more readily than X-rays. Chris and his colleagues suggest that this difference allows ultraviolet radiation to push material more efficiently, generating the much faster winds observed near supermassive black holes.
What does this mean for galaxy evolution? If this explanation holds, it will refine how scientists understand the exchange of energy and matter in extreme environments. It could also shed light on how these processes influence the growth of galaxies and the broader evolution of the cosmos. "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.
XRISM mission at a glance: XRISM (pronounced krizz-em) launched on September 7, 2023. The mission is a collaborative effort led by the Japan Aerospace Exploration Agency (JAXA) in partnership with NASA and ESA. It carries two primary instruments: Resolve, an X-ray calorimeter that measures the energy of individual X-ray photons with unprecedented precision, and Xtend, a wide-field X-ray CCD camera.
What do you think? Does this new information change your understanding of space physics? Are you surprised by the differences between neutron star and black hole winds? Share your thoughts in the comments below!
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