4 min
Villanova Astrophysicist Joey Neilsen, PhD, Plays Prominent Role in Groundbreaking XRISM Collaboration Study
A global team of researchers using the new X-ray Imaging and Spectroscopy Mission (XRISM) telescope, launched in fall 2023, discovered something unexpected while observing a well-studied neutron star system called GX13+1. Instead of simply capturing a clearer view of its usual, predictable activity, their February 2024 observation revealed a surprisingly slow cosmic wind, the cause of which could offer new insights into the fundamental physics of how matter accumulates, or “accretes,” in certain types of binary systems. The study was one of the first from XRISM looking at wind from an X-ray binary system, and its results were published in Nature—the world's leading multidisciplinary science journal—in September 2025. Spectral analysis indicated GX13+1 was at that very moment undergoing a luminous super-Eddington phase, meaning the neutron star was shining so brightly that the radiation pressure from its surface overcame gravity, leading to a powerful ejection of any infalling material (hence the slow cosmic wind). Further comparison to previous data implied that such phases may be part of a cycle, and could “change the way we think about the behavior of these systems,” according to Joey Neilsen, PhD, associate professor of Physics at Villanova University. Dr. Neilsen played a prominent role as a co-investigator and one of the corresponding authors of the project, along with colleagues at the University of Durham (United Kingdom), Osaka University (Japan), and the University of Teacher Education Fukuoka (Japan). Overall, the collaboration featured researchers from dozens of institutions across the world. GX13+1 is a binary system consisting of a neutron star orbiting a K5 III companion star—a cooler giant star nearing the end of its life. Neutron stars are small, incredibly dense cores of supergiant stars that have undergone supernovae explosions. They are so dense, Dr. Neilsen says, that one teaspoon of its material would weigh about the same as Mount Everest. Because of this, they yield an incredibly strong gravitational field. When these highly compact neutron stars orbit companion stars, they can pull in, or accrete, material from that companion. That inflowing material forms a visible rotating disk of gas and dust called an accretion disk, which is extremely hot and shines brightly in X-rays. It’s so bright that sometimes it can actually drive matter away from the neutron star. “Imagine putting a giant lightbulb in a lake,” Dr. Neilsen said. “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.” The original purpose of the study was to use XRISM to observe an accretion disk wind, with GX13+1 targeted specifically because its disk is persistently bright, it reliably produces winds, and it has been well studied using Chandra— NASA’s flagship X-ray observatory—and other telescopes for comparison. XRISM can measure the X-ray energies from these systems a factor of 10 more precisely than Chandra, allowing researchers to both demonstrate the capabilities of the new instrument and study the motion of outflowing gas around the neutron star. This can provide new insights into accretion processes. “It's like comparing a blurry image to a much sharper one,” Dr. Neilsen said. “The atomic physics hasn't changed, but you can see it much more clearly.” The researchers uncovered an exciting surprise when the higher-resolution spectrum showed much deeper absorption lines than expected. They determined that the wind was nearly opaque to X-rays and slow at “only” 1.4 million miles per hour—surprisingly leisurely for such a bright source. Based on the data, the team was able to infer that GX13+1 must have been even brighter than usual and undergoing a super-Eddington phase. So much material was ejected that it made GX13+1 appear fainter to the 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,” Dr. Neilsen said, equating it to standing at the bottom of a waterfall and shining light so brightly that the waterfall stops. “What we saw was that GX13+1 had to have been near, or maybe even above, the Eddington limit.” The team compared their XRISM data from this super-Eddington phase to a set of previous observations without the resolution to measure the absorption lines directly. They found several older observations with faint, unusually shaped X-ray spectra similar to the one seen by XRISM. “XRISM explained these periods with funny-shaped spectra as not just anomalies, but the result of this phenomenally strong accretion disk wind in all its glory,” Dr. Neilsen said. “If we hadn’t caught this exact period with XRISM, we would never have understood those earlier data.” The connection suggests that this system spends roughly 10 percent of its time in a super-Eddington phase, which means super-Eddington accretion may be more common than previously understood—perhaps even following cycles—in neutron star or black hole binary systems. “Temporary super-Eddington phases might actually be a thing that accreting systems do, not just something unique to this system,” Dr. Neilsen said. “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.”
