Joey Neilsen, PhD

Associate Professor of Physics Villanova University

  • Villanova PA

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

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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.”

Joey Neilsen, PhD

5 min

Rubin Observatory Releases First Images, as The Villanova One Sky Center for Astrophysics Begins Celestial Partnership

If the first few frames are any indicator of a blockbuster movie, hold the 2035 Best Picture Oscar for the Vera C. Rubin Observatory and its ambitious new 10-year project. On June 23, 2025, scientists at the state-of-the-art facility in the mountains of north-central Chile gave the public its first glimpses into the capabilities of its 8.4-meter Simonyi Survey Telescope, equipped with the world’s largest digital camera—a 3.2 megapixel, 6,600-pound behemoth that can photograph the whole southern sky every few nights. Its task is a decade-long lapse record-called the Legacy Survey of Space and Time (LSST). The first shots on that journey have left both the general public and astronomical community in awe, revealing in rich detail a mind-boggling number of galaxies, stars, asteroids and other celestial bodies. “The amount of sky it covers, even in just one image, is unprecedented,” said David Chuss, PhD, chair of the Department of Physics, who viewed the first images with colleagues at an organized watch party. “It’s such high-precision, beautiful detail,” added Kelly Hambleton Prša, PhD, associate professor of Astrophysics and Planetary Sciences. “It’s just mind-blowing.” What Makes Rubin and LSST So Unique? Simply, this revolutionary instrument, embarking on an equally revolutionary initiative, will observe half the sky to a greater depth and clarity than any instrument ever has before. Consider this: "The Cosmic Treasure Chest” image released by Rubin contains 1,185 individual exposures, taken over seven nights. Each one of those individual exposures covers 10 square degrees of night sky, which is about the same as looking up at 45 full moons positioned around one another. It may seem like a small size, but click the image yourself, and zoom in and out. The amount of sky captured in that range—enough to show roughly 10 million galaxies—is astounding. Per the Observatory, “it is the only astronomical tool in existence that can assemble an image this wide and deep so quickly.” “At the end of 10 years, Rubin will have observed 20 billion galaxies, and each night in that time frame it will generate 20 terabytes of data,” Dr. Hambleton Prša said. “And, because Rubin has so many different filters, we get to see the same objects in so many different ways.” According to Dr. Hambleton Prša and Dr. Chuss, the power and precision of the Rubin LSST, combined with the shear area of the sky that will be observed, will allow for an incredibly in-depth study of myriad objects, processes and events in ways nobody has ever studied them before. “For example, in our galaxy, we expect to observe only two supernovae per century,” Dr. Hambleton Prša said. “But we're observing 20 billion galaxies. For someone studying this phenomenon, the number of supernovae that they’re going to observe will be off the charts. It is an exquisite survey.” It will also provide insight into the universe’s oldest and most puzzling enigmas. “Rubin is able to look back into our universe at times when it was much smaller during its expansion and really address some of these incredible mysteries out there, like dark energy,” Dr. Chuss said. “We know the universe is expanding and that this expansion is accelerating. Rubin will trace the history of that acceleration and, from that, provide insight into the physics of the mysterious dark energy that appears to be driving it.” To enhance the technological capabilities of its instrument, scientists were invited to contribute towards the selection of the observing strategy of the telescope. The Rubin team took into consideration continual input from the astrophysics community, separated into what they call “science collaborations.” To achieve this, the Rubin team generated proposed simulations for collecting observations, which the science collaborations then assessed for their specific science goals. “The Rubin team then iterated with the science collaborations, taking into account feedback, to ultimately obtain the best strategy for the largest number of science cases,” Dr. Hambleton Prša said. Dr. Hambleton Prša is the primary contact for the Pulsating Star Subgroup, which is part of the Transients and Variable Stars Science Collaboration, the science collaboration that focuses on objects in the sky that change with time. She was the lead author among 70 co-authors on the roadmap for this science collaboration, underscoring the significant scale of community participation for each of these areas. Joined Under One Sky Dr. Hambleton Prša, Dr. Chuss and other members of the Astrophysics and Planetary Sciences Department and Department of Physics at Villanova have a vested interest in Rubin and the LSST project. In April, the two departments joined forces to launch The Villanova One Sky Center for Astrophysics, co-directed by the two faculty members. With goals to elevate the University's longstanding record of research eminence in astronomy and astrophysics and create opportunities for more students to access the disciplines, the Center partnered with the Rubin Observatory to help realize the mission. Both Villanova and Rubin share a similar vision on expanding access to this broad field of study. Fortuitously, the launch of The Villanova One Sky Center coincided with the initial data released from Rubin. What will result, Dr. Chuss says, will be a “truly awesome impact on both our Center and institution.” Dr. Hambleton Prša will advance her own research of pulsating stars, and Andrej Prša, PhD, professor of Astrophysics and Planetary Science and the primary contact for the Binary Star Subgroup, will broaden his study of short-period binary stars. Joey Neilsen, PhD, associate professor of Physics, will expand his research in black hole astrophysics. Becka Phillipson, PhD, an assistant professor of Physics, who recently led a proposal for Villanova to join the Rubin LSST Discovery Alliance, aims to increase the scope of her study of chaotic variability of compact objects. Dr. Chuss, who generally works on infrared and microwave polarimetry, which is “outside the wavelength ranges of Rubin” is interested in its complementarity with other observations, such as those of the cosmic microwave background—the oldest light in the universe—and the evolution of the large-scale structure of the universe. Subjects, he says, which are “exactly in the wheelhouse for Rubin.” Other faculty members are interested in topics such as how Rubin’s observations may change the knowledge of both the history and structure of our solar system and the population of Milky Way satellite galaxies. That is not to mention, Dr. Hambleton Prša points out, the daily 20 terabytes of data that will become available for students and postdoctoral researchers under their tutelage, who will be heavily involved in its analysis for their own projects and ideas. “This partnership is going to greatly increase our opportunities and elevate our profile,” Dr. Chuss said. “It will make our program even more attractive for faculty, postdocs and students to come and to share their knowledge and expertise. “Together, we will all have access to an incredible movie of this epoch of our universe, and the knowledge and surprises that come with it along the way.”

Joey Neilsen, PhDDavid Chuss, PhDBecka Phillipson, PhD

3 min

Casting Light on the Dark Universe, Euclid's Mission Shows Promise

On December 7, 1968, the National Aeronautics and Space Administration (NASA) successfully launched the first functional space telescope into orbit. In the 55 years since, dozens of these crafts have embarked on missions of discovery, advancing and transforming our understanding of the cosmos. Among the latest is Euclid, a wide-angle space telescope developed by the European Space Agency (ESA). Equipped to chart portions of the universe that are not directly observable and currently shrouded in mystery, Euclid is working to generate a three-dimensional map unlike any other, surveying billions of galaxies out to 10 billion light-years. This past month, the first images from its journey were released. Joey Neilsen, PhD, is a world-renowned astrophysicist, a frequent collaborator with NASA and an assistant professor in Villanova University’s College of Liberal Arts and Sciences. From his perspective, Euclid’s early returns evidence its voyage’s incredible potential. “In Euclid’s first image of the Perseus cluster, the sheer number of galaxies is really astonishing,” said Dr. Neilsen. “We talk a lot about how the universe is mostly empty space—and it is!—but it’s also enormous, and it’s really stunning that there’s room for so many galaxies in just a small patch of sky. There are 1,000 galaxies here huddled together in this cluster and over 100,000 in the background. “I also note some pale purple patches in the image of NGC 6822. These are planetary nebulae, the layers of gas and dust blasted off by stars at the ends of their lives. It’s amazing to be able to see these so clearly in images that show the entire galaxy and its environment at the same time.” According to Dr. Neilsen, Euclid’s remarkable visuals are the product of a calculated tradeoff. The ESA craft sacrifices the fine resolution of images taken by other observatories, like NASA’s James Webb Space Telescope, to capture cosmic phenomena in greater breadth. By collecting these visuals, Euclid aims to spark breakthroughs on subjects as of yet understudied—breakthroughs that could benefit Dr. Neilsen’s field of research. “Euclid’s mission is to understand the evolution of the dark components of the universe: the invisible dark matter whose gravity holds large structures like galaxies and galaxy clusters together and the dark energy responsible for the accelerating expansion of the universe,” he explained. “Much of my research focuses on a different aspect of the dark universe (black holes), but there is a puzzle that might connect: observations of very distant galaxies show there were very massive black holes very early on. How did these behemoths grow so big so fast? If would be neat if Euclid helped us to better understand the early universe in a way that informed our understanding of the growth of black holes.” In tracking and investigating the dark entities that compose and mold the cosmos, Euclid could very well offer insights into the history and development of over 95% of all energy and matter—and perhaps into the very fabric of existence itself. It is reasonable to wonder whether, when its mission is complete in six years’ time, the telescope could provide us with answers to questions that have gone unaddressed for six billion years. “For me, the best-case scenario would be that Euclid would show clear evidence of something that’s hard to explain with our current models,” said Dr. Neilsen. “For example, right now, we have ‘Hubble tension,’ a discrepancy between measurements of the expansion of the universe from when it was young and from the current era… The moments when things don’t add up are the ones where we learn the most about how the universe works. So, I’ll keep my fingers crossed for a surprise and for more to learn over the next six billion years.”

Joey Neilsen, PhD

Media

Areas of Expertise

Black Holes
Black Hole Accretion
Supermassive Black Holes
Accretion Disks
Accretion Disk Winds
Relativistic Jets
X-ray Spectroscopy
Event Horizon Telescope
Black Holes and Neutron Stars

Biography

A pioneer in black hole astrophysics, Joey Neilsen, PhD, has established himself as a foremost expert in X-ray observations of these cosmic bodies. His research has been extensively cited in the literature and featured in esteemed scientific journals, including Science, Nature, The Astrophysical Journal and Advances in Space Research.

Scientists are just beginning to understand the power and force of these astronomical objects—and Dr. Neilsen’s work has been pivotal in expanding that knowledge base. Although black holes are invisible, astronomers can still observe them indirectly by examining the way their gravity affects stars and pulls matter into orbit.

“I use space telescopes, primarily from NASA, to study what’s happening around black holes in the universe,” says Dr. Neilsen, assistant professor of Physics. “They’re among the most prolific sources of energy in the universe, and as an observational astronomer, most of my work deals with collecting and analyzing data to try to explain their behavior.”

His work with NASA officially began nearly a decade ago, with some of the most prestigious fellowships in astrophysics: an Einstein Postdoctoral Fellowship at Boston University and a Hubble Postdoctoral Fellowship at the MIT Kavli Institute for Astrophysics and Space Research. His collaboration with the space agency has continued, and he’s had the opportunity to explore the far reaches of the galaxy using X-ray telescopes, including one NASA mounted on the International Space Station.

As a widely published researcher and invited speaker at conferences around the globe—from Buenos Aires to Istanbul to Madrid—Dr. Neilsen continues to expand the world’s view and understanding of the final frontier. “My research on the X-ray spectroscopy of black holes is part of understanding more about the world that we live in,” he says. “That’s what physics is all about, growing curiosity and enthusiasm for science.”

Education

Harvard University

PhD

Astronomy

Harvard University

MA

Astronomy

Select Media Appearances

Villanova University scientist helps create cosmic postcard of black hole

6 ABC  online

2022-05-12

Action News had the chance to chat with scientist Joey Neilsen, Ph.D., an assistant professor of physics at Villanova University, about his decade-long search for Sagittarius A* (A Star), the supermassive black hole in the Milky Way.

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An asteroid bigger than Philly's City Hall will be 'near' Earth on Saturday, but not to worry

The Philadelphia Inquirer  online

2021-11-12

In the meantime, the huge-but-harmless asteroid passing by on Saturday is a reminder that the cosmos is chock-full of debris. We spoke to Joey Neilsen, an astronomer and assistant professor of physics at Villanova University, to get the big picture.

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Villanova professor contributes to discovery of black hole's behavior

KYW NewsRadio  radio

2021-04-03

Dr. Joey Neilsen, an assistant professor of physics, said the discovery about the M87 Black Hole's nature was made through coordinated observations by more than 300 worldwide users of the Event Horizon Telescope.

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Select Academic Articles

Multistructured Accretion Flow of Sgr A*. I. Examination of a Radiatively Inefficient Accretion Flow Model

The Astrophysical Journal

2024

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First Very Long Baseline Interferometry Detections at 870 μm

The Astronomical Journal

2024

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Ordered magnetic fields around the 3C 84 central black hole

Astronomy & Astrophysics

2024

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