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Villanova Astrophysicist Joey Neilsen, PhD, Plays Prominent Role in Groundbreaking XRISM Collaboration Study

Oct 9, 2025

4 min

Joey Neilsen, PhD

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.”


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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.”


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Joey Neilsen, PhD

Joey Neilsen, PhD

Associate Professor of Physics

Professor Neilsen, PhD, uses X-ray telescopes to study black hole accretion disks, winds, and relativistic jets.

Black HolesBlack Hole AccretionSupermassive Black HolesAccretion DisksAccretion Disk Winds
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Ahead of America250, Villanova Historian Reveals How Independence Hall Almost Didn't Survive featured image

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Ahead of America250, Villanova Historian Reveals How Independence Hall Almost Didn't Survive

Philadelphia’s Independence Hall has long occupied an outsized place in the American imagination. The space where the Continental Army was established, the Declaration of Independence adopted and the United States Constitution ratified, the site was once described by President Abraham Lincoln as the source “where were collected together the wisdom, the patriotism, the devotion to principle, from which sprang the institutions under which we live.” In July, these hallowed grounds will yet again take center stage, as the country observes its semiquincentennial, or America250, celebration. In due course, House lawmakers will gather at the landmark for a special commemorative event, mayors from across the U.S. will march to the gates in a show of civic pride and solidarity, and thousands of visitors will flock to the site daily in appreciation for its significance to the cause of “Life, Liberty and the pursuit of Happiness.” However, while Independence Hall’s role in the national saga will go widely remarked and recognized, the building itself has a story that remains largely unknown. According to Whitney Martinko, PhD, associate professor of History and director of the Albert Lepage Center for History in the Public Interest at Villanova University, the “cradle of American democracy” almost never survived the country’s infancy. “Early on, the challenge was about two things,” says Dr. Martinko, who specializes in public history, historic preservation and the early U.S. “One was about ownership of what was called the ‘Old State House,’ because it was the former statehouse in the colony of Pennsylvania. And the second was about the development of the city around it.” As Dr. Martinko explains, in the early 19th century, Independence Hall—then the Old State House—was under the control of the Commonwealth of Pennsylvania, which had shifted its governmental seat from Philadelphia to Harrisburg by 1812. To fund the construction of a new capitol building in the wake of the move, Pennsylvania legislators seriously contemplated selling the site to private enterprise, with the surrounding area undergoing a development boom. “Today’s Independence Mall was built up entirely,” says Dr. Martinko. “In the 18th century, it was full of buildings, shops and houses, and by the 19th century, it had become a huge furniture district and a heart of commerce in many ways.” As plans were drawn up to deliver the hall to the highest bidder, local resistance quickly emerged. Opposed to the landmark’s loss, citizens of Philadelphia and municipal leaders rushed to the defense of the building and its lawn, arguing that their preservation entailed a necessary public good. “Everyone looked to this site as the heart of the new nation. It’s a historic site. It’s an important building. People thought of it as one of the great pieces of Georgian architecture at the time,” says Dr. Martinko. “It was also seen as a civic space, as people gathered there on Election Day. And its lawn was highly valued, with green, open space considered important even then, for air circulation. So, it was really seen as a political space, a civic space and a green space that was important for the well-being of Philadelphians and the health of Philadelphia.” Deliberations over the fate of Independence Hall would continue for a period of five years, up until 1818. After a spirited public campaign, a settlement was finally reached when the City of Philadelphia purchased the plot from the Commonwealth of Pennsylvania for $70,000 (roughly $1.85 million in today’s currency). In essence, the deal would forevermore secure Independence Hall’s place within the pantheon of great American shrines, parks and monuments. However, in a terrific irony, it would also eventually lead to the loss of a different piece of history: Between 1950 and 1967, the 19th-century development projects that once threatened Independence Hall became a casualty of the city’s efforts to make the “birthplace of America” an urban focal point, with the creation of Independence Mall. “Those buildings were all torn down in the mid-20th century, when Ed Bacon and the City Planning Commission decided to make Independence Hall a major attraction,” says Dr. Martinko. “There were debates surrounding this issue as well. The Jayne Building was one of the 19th-century buildings that was demolished and that is most well-known. So, there’s this sense of preserving 18th-century history through the demolition of 19th-century architecture.” As the nation approaches its 250th anniversary, the near loss of Independence Hall and the removal of its 19th-century neighbors stand as striking examples of the ways in which what we value, and how much we value it, evolves over time. What’s more, the historic threats to Philadelphia’s most famous site serve as a poignant reminder of the delicate nature of public memory and preservation—and the fact that the places we treasure today may not always be with us tomorrow. “Even though it seems absurd to us now, we’re still seeing debates over the line between redevelopment and connection with the past,” concludes Dr. Martinko. “It’s not that no one saw the value of Independence Hall, or that they didn’t see it as historic. It was just this debate that a lot of very reasonable people continue to have today: Is this what really needs to be preserved? And how should it be preserved?”

100 Years After the "Launch" of Aerospace Industry, Villanova Faculty Continue to Innovate the Sector featured image

4 min

100 Years After the "Launch" of Aerospace Industry, Villanova Faculty Continue to Innovate the Sector

In 1926, Robert H. Goddard launched the world’s first liquid-fueled rocket in Auburn, Mass. Goddard’s 10-foot-tall rocket was airborne for just 2.5 seconds, reaching speeds of 60 miles per hour before landing 184 feet away from the launch site. A century later, the aerospace industry is booming, with new technology and missions making headlines every day—some with incredible success, and others encountering challenges that send scientists back to the drawing board. In February 2026, NASA delayed the launch of Artemis II—its next mission to the moon—citing issues with helium flow in the rocket’s systems. By April, the mission was on track again, and Artemis II completed the first crewed flight to the moon in more than half a century. Crew members for the upcoming Artemis III mission were also recently announced, as well as a timeline and overview for Artemis IV, the first planned crewed mission to the lunar South Pole in 2028. Amid these successes and setbacks, researchers continue to innovate the field and develop new technologies designed to help expand our knowledge of the vast universe. That innovation comes from diverse and unique places, including Villanova University. Research in Flight Student interest in aerospace led to the creation of a Master of Science in Aerospace Engineering (MSAE) program at Villanova University, which began in the Fall 2025 semester. Sergey Nersesov, PhD, associate professor of Mechanical Engineering, was instrumental in the inception of the MSAE program and helped develop courses in space flight mechanics, applied aerodynamics, aerospace structures analysis and advanced flight dynamics and control, among others. The idea for the graduate program was inspired by the growing popularity of the College of Engineering’s Minor in Aerospace Engineering. The minor attracts students from across the university, drawing from other majors and colleges at Villanova. For example, Dr. Nersesov recently collaborated on a research project focused on spacecraft and satellite control systems with Aedan Disanto ‘26 CLAS, an astrophysics and planetary sciences major and aerospace engineering minor. “If you look up at the sky, sometimes you see satellites chasing each other,” said Dr. Nersesov. “Dynamics and control researchers develop algorithms to ensure proper spacing between the satellites so they can function correctly.” The spacing between satellites is crucial to avoid collision, which is also a potential issue when a spacecraft approaches a space station to dock. In this situation the velocities, rotation and orientation of both vehicles are carefully controlled so that docking mechanisms align correctly, which requires up to 12 variables to be coordinated simultaneously. Dr. Nersesov and Disanto analyzed the algorithms needed to guarantee perfect satellite function and built upon them, discovering more efficient ways to operate vehicles in space. This summer, Dr. Nersesov and his students will also begin designing a prototype for a new kind of drone. Typical drones use ample amounts of energy to become airborne and capture photos or video content because they rely entirely on thrust to hold themselves up. To improve effectiveness, Dr. Nersesov and his students aim to create a drone in the style of an airplane, with vertical takeoff and landing (VTOL) capability. The drone will take off vertically, like a helicopter, but then transition to flying horizontally like an airplane, allowing lift from the wings to reduce the energy needed to stay airborne. As a result, it could stay in the air up to ten times longer than a hovering drone. While the project focuses on a single aircraft design, it represents the type of forward-thinking research driving the aerospace field today. Aerospace Engineering with Biology Elsewhere, Qianhong Wu, PhD, chair of Mechanical Engineering in the College of Engineering, is exploring a concept called super-lubrication, inspired by the way red blood cells move through the human body. Blood cells travel through capillaries narrower than their own diameter without damaging themselves or the vessel walls. A soft, porous layer called the endothelial glycocalyx within the vessels allows cells to glide through, reducing friction. In studying this biological process, an idea emerged that could potentially be translated to the aerospace field. Dr. Wu’s team is currently applying their deep understanding of biomechanical processes to applications that might reduce aerodynamic friction on aircraft surfaces by more than 90 percent. This lower friction may also improve fuel efficiency and extend flight endurance for drones or other aircraft. “Our work is an example of how thinking outside your traditional field can lead to innovation,” said Dr. Wu. “Sometimes the solution comes from a completely different subject, like biology.” A Century of Momentum One hundred years after Goddard’s brief but groundbreaking flight, aerospace innovation has expanded far beyond its earliest experiments. Today, progress in the field depends not only on major missions and milestones, but also on the steady work of researchers refining systems and exploring new ideas. At Villanova, that work is taking shape across disciplines—from spacecraft control systems to biologically inspired materials. Together, these efforts reflect how the field continues to evolve through collaboration and creativity.

Villanova Researcher Authors Comprehensive Look at Large Outdoor Gatherings and Severe Weather Risk featured image

5 min

Villanova Researcher Authors Comprehensive Look at Large Outdoor Gatherings and Severe Weather Risk

If Benjamin Franklin had modern climatological data to consult, he may have scheduled his famous kite and key experiment for July of 1753 in Philadelphia, instead of a month sooner. July, after all, is Philadelphia’s most active month for lightning strikes, noted Stephen Strader, PhD, associate professor of Geography and the Environment and severe weather geographer at Villanova University. Lightning has been on Dr. Strader’s mind recently. In Spring 2025, he and a colleague published a study in Weather, Climate, and Society where they developed a metric to comprehensively rank more than 475 “large outdoor gatherings” in the U.S. in terms of greatest lightning risk, weak (EF0+) tornadoes and strong or significant (EF2+) tornadoes. The rankings factored in time of year, frequency of events, attendance and capacity, which was then compared to tornado/lightning climatology data for the venue location. With major outdoor events scheduled throughout the summer in Philadelphia, Dr. Strader recently reanalyzed his data to include gatherings like the FIFA World Cup, MLB All-Star Game and America 250 celebrations in the city. What he found was that many of these gatherings—when compared to other large outdoor gatherings across the country—are among the most likely to experience certain severe weather events. “While it is too soon to predict what the weather will be like for each of these events, the climatology does suggest that if severe inclement weather threatens them, many people could be exposed to deadly hazards such as lightning and tornadoes,” he said. “The key is for venue organizers, first responders and event attendees to be informed, not alarmed.” Preparedness Lessens Impact Every year, thousands of sports games, concerts, fairs, parades and days at amusement parks occur without weather-related safety incidents. But every now and again one does, sometimes with tragic results. In 2011, a spectator at Pocono Raceway was struck and killed by lightning in the parking lot after the race was halted due to weather. One year later, a sudden strong wind from an approaching storm collapsed a grandstand at the Indiana State Fair, causing seven fatalities and dozens of injuries. The objective of Dr. Strader’s research was to find what venues are most likely, comparatively, to experience these extremely rare worst-case scenario exposures. By doing so, he hopes that it will help venues and organizers to understand how they can plan, execute or update their preparedness and protocol, so that their impacts are less deadly when they do occur. “The idea isn’t to scare people,” he said. “Most of these venues and entities know that they're at risk for inclement weather. My hope is that those who do have a plan in place re-evaluate them, and think, based on the climatology, how they can adapt. For those who don’t, the goal is to make them aware of their risk so they can begin to plan. “There's a mental side to this too. Every year that goes by where there's not an issue or incident, people get increasingly relaxed. It’s a reminder to stay vigilant.” Many of the large outdoor gatherings that appear high in the research rankings do have robust protocols and infrastructure. For example, amusement parks score very high for lightning risk because they are open almost every day of the year to huge crowds, and many are located in states that experience frequent severe weather, like Florida and Texas. “But places like Disney World have comprehensive preparedness plans—they have people hired to understand and be aware of these threats,” Dr. Strader said. “Attendees have ample places to seek shelter.” The same is true, to varying extents with sporting venues, especially major professional sports stadiums. Even though gatherings held there may rank higher for these weather events, the chances of a catastrophe are lessened because of preparedness, protocol and infrastructure. Where Dr. Strader is more concerned is with gatherings like parades and fairs, especially when they last for multiple days and/or draw large crowds. “A state fair tends to be more ‘mom and pop’ at times, and they lack permanent infrastructure,” he said. “Vendors set up in temporary structures and a lot of times there are no places for attendees to seek shelter when thunderstorms threaten. You're also dealing with a transient population—people who don't know the area, and don’t know where to go when severe weather hits. “Parades are kind of a free for all in that you can come and go as you please. This is one of the reasons venues take tickets—so they understand their capacity to deal with an emergency. But if you have a place that doesn't do that, it can be tricky.” How Philadelphia’s 2026 Events Rank So, what exactly did Dr. Strader find when factoring in this summer’s events in and near the City of Brotherly Love, and what does it mean? In terms of lightning risk, several events and venues rank in the 80th percentile and greater for potential exposure compared to all other documented U.S. large outdoor gatherings. Citizens Bank Park, the host of the MLB All-Star Game in July, ranks in the 95th percentile. America 250 celebrations on July 4 rank in the 94th percentile, while FIFA World Cup events at Lincoln Financial Field rank 87th in June and 94th in July. For tornado threats to all nationwide events, Citizens Bank Park tops all local functions in July at the 96th percentile. The America 250 celebration ranks in the 95th, FIFA World Cup the 94th and MLB All-Star Game the 78th percentile for tornado exposure in that same month. According to Dr. Strader, lightning is often the bigger concern because it is far less predictable than tornadoes. While events are frequently canceled ahead of a credible tornado threat, they are less likely to be canceled due to the possibility of lightning. “Although lightning [strikes] and tornadoes are not very common in southeastern Pennsylvania, the large number of people gathered for these events sets up a ‘what if’ situation where tens of thousands of individuals could be threatened by severe weather,” he said. “Based on these findings, venue operators and governing bodies should continue to develop risk reduction strategies aimed at improving lightning and tornado safety, especially as lightning and tornado hazard threats evolve over time and across geographic space.” It is not only operators and organizers who should be prepared. Patrons, he says, can also devise and enact their own personal preparedness plan in the event of sudden severe weather. Teaching children simple sayings such as “when thunder roars go indoors” and scoping out potential places for shelter can go a long way in the event of severe weather—especially at come-and-go, transient events like America 250 festivities. “And if you see approaching clouds on July 4, maybe find another way to pay homage to Ben Franklin, and pull down your kite.”

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