Researchers say the remoted white dwarfs Gandalf and Moon-Sized outline a brand new class of stellar remnant as a result of they share 5 traits, together with X-ray emission.
Across the immense scale of the Universe, a single uncommon object can immediate astronomers to search for others prefer it, typically resulting in the popularity of a wholly new class of stars.
In a paper revealed in Astronomy & Astrophysics and an arXiv preprint, researchers on the Institute of Science and Technology Austria (ISTA) describe two stellar remnants that share 5 traits, together with X-ray emission, though each seem like remoted. The group argues that these two objects are sufficient to ascertain a brand new class of stellar remnants.
Our Sun is predicted to turn out to be a white dwarf in about five to eight billion years. A white dwarf is an extremely dense stellar remnant, roughly the size of Earth, left behind after a star has used up its fuel and shed its outer layers.
Although the Sun is alone, research over the last 15 years has shown that binary and multiple-star systems are much more common than astronomers previously realized. When a compact, dense remnant such as a white dwarf is part of a binary system, it can often pull material away from its companion star. This process, called accretion, typically produces X-rays that are treated as a signature signal.
Scientists in the group of Ilaria Caiazzo, assistant professor at the Institute of Science and Technology Austria (ISTA), have now confirmed X-ray signals from two isolated objects known as Gandalf and Moon-Sized. Both are highly magnetic, spin rapidly, and are classified as “merger remnants” because each formed after a violent cosmic collision. Since they emit X-rays without a companion star, the two objects now appear to define a class of their own.
Gandalf—the Lord of the Half Rings?
Gandalf is not a completely new object. Caiazzo first observed it during her postdoctoral research and flagged it as unusual because some of its signals suggested material might be present around it.
“We initially thought it was a binary system,” says Andrei Cristea, a PhD student in the Caiazzo group and first author of the paper published in Astronomy & Astrophysics, about Gandalf. “At the remnant’s extremely high level of magnetism, its spin should be synchronized with its companion’s orbit, similarly to Earth’s rotation with the Moon’s orbit,” he adds. However, the fastest orbit period observed to date is 80 minutes. Gandalf, on the other hand, rotates on its axis every six minutes. According to Cristea, this is but one of its puzzling features.
“If Gandalf were involved in a binary system, it would have been highly unsynchronized, which might have made it even more puzzling than it already is. But we never found a companion. So, where does the circumstellar material come from?”
To investigate that puzzle, the team turned to optical emission spectra, a commonly used observational method in astronomy.
For the encompassing materials to be trapped in a half-ring configuration, the item should have a robust and uneven magnetic subject, the ISTA scientists argue. Credit: Russell C. J. Kightley
“We saw hydrogen emission spectra that exhibited a double-peaked signature, similar to cat ears,” says Cristea. “Usually, this signature indicates the presence of a disk of material surrounding a merger remnant. However, by examining the signal more closely, we realized that it was alternating between the two peaks over the remnant’s six-minute spin period.” This curious commentary matched the existence of a half-ring of fabric circling the star. “We have never seen anything like that before in any white dwarf,” he provides.
The researchers concluded that if materials across the merger remnant is being held in an uneven half ring, the item should have a robust magnetic subject that can also be uneven.
“To note, white dwarfs of similar age and evolutionary stage are typically nonmagnetic,” says Cristea. “While highly magnetic white dwarf remnants are already an exception, Gandalf is now one of only two known merger remnants to feature asymmetric magnetization.” These uncommon traits led Cristea to call the item after the riddle-loving character from J.R.R. Tolkien’s novels.
Moon-Sized—Gandalf’s extra advanced twin?
Although the researchers didn’t discover a companion for Gandalf, they could have discovered one thing like a twin elsewhere within the Universe.
Caiazzo published the discovery of a white dwarf called “Moon-Sized” in 2021, and the item already stood out for a number of uncommon causes. It is extraordinarily magnetic and spins rapidly, and it compresses concerning the mass of the Sun into an object comparable in dimension to the Moon, or barely bigger, in line with new proof in an arXiv preprint led by Aayush Desai, one other PhD scholar within the Caiazzo group.
The ISTA group discovered that Moon-Sized and Gandalf have 5 main traits in frequent. Both are ultra-massive, extremely magnetic, quickly rotating, companionless, and sources of X-rays. Because of these shared properties, the scientists suggest that Gandalf and Moon-Sized are two examples of a brand new class of remnants.
Still, the objects aren’t an identical. Gandalf reveals proof of surrounding materials, whereas Moon-Sized doesn’t. Gandalf fashioned in a collision round 60 to 70 million years in the past, whereas Moon-Sized is far older, with its merger occasion relationship to about 500 million years in the past. Gandalf additionally emits X-rays about 100 occasions extra brightly, which can imply that Moon-Sized is an older and extra advanced model that’s progressively dropping no matter powers its X-ray emission.
What are the standards for outlining a brand new class of stars or remnants?
Astronomers typically agree that close by objects usually tend to symbolize a broader inhabitants than a uncommon one off object. Even so, any object with unfamiliar options can inspire a wider search.
Caiazzo explains: “If we find one new object in the vastness of the Universe, what are the chances of it being the only one? Usually, one stellar object with new characteristics is more than enough for us to start looking for similar ones. But here, we actually found two objects with five overlapping features. This is plenty for a new class of star remnants!”
X-rays and the mysteries of stellar evolution
The group has prompt a number of doable explanations for the observations, particularly the origin of the X-rays.
In one chance, a strongly magnetized star spins quick sufficient to create a drive that pulls materials out of the star remnant itself. “This is my favorite scenario because it only accounts for the white dwarf itself rather than material originating from outside the star remnant,” says Desai. The group notes that this outflow course of is understood from extremely magnetized neutron stars referred to as pulsars, however it has not but been modeled in a white dwarf remnant.
A second chance includes materials flowing inward. In this state of affairs, a leftover stream from the unique merger might not have totally fallen onto the remnant after the collision. If that materials travels on a extremely eccentric orbit, transferring removed from the star earlier than returning near it, it may fall again onto the remnant over a whole bunch of thousands and thousands of years.
In a 3rd state of affairs, the researchers think about one other doable supply of exterior inflowing materials.
“We know that a third of white dwarfs are ‘polluted,’” says Desai. “They are so dense that we would expect external material, such as asteroids or even disrupted planetary bodies, to collapse onto them.” While Gandalf reveals some indicators of air pollution, presumably by means of carbon- or silicon-rich supplies, the group didn’t detect such alerts from the significantly older Moon-Sized. “This scenario seems less likely, as it does not fully explain why we see the X-rays in both objects right now,” Desai explains.
The group has realized necessary particulars about Moon-Sized and Gandalf, however extra analysis might be wanted to find out how objects like these might have an effect on their planetary programs.
“The two objects we identified so far have lots of similarities, but also differences,” explains Desai. “Finding more such remnants will help us exclude scenarios and perhaps find other explanations altogether.”
For now, researchers nonetheless want to find out whether or not any one of many 5 shared traits is crucial for membership on this proposed new class.
References:
“A half ring of ionized circumstellar material trapped in the magnetosphere of a white dwarf merger remnant – A new class of white dwarf merger remnants with X-ray emission” by Andrei A. Cristea, Ilaria Caiazzo, Tim Cunningham, John C. Raymond, Stephane Vennes, Adela Kawka, Aayush Desai, David R. Miller, J. J. Hermes, Jim Fuller, Jeremy Heyl, Jan van Roestel, Kevin B. Burdge, Antonio C. Rodriguez, Ingrid Pelisoli, Boris T. Gänsicke, Paula Szkody, Scott J. Kenyon, Zach Vanderbosch, Andrew Drake, Lilia Ferrario, Dayal Wickramasinghe, Viraj R. Karambelkar, Stephen Justham, Ruediger Pakmor, Kareem El-Badry, Thomas Prince, S. R. Kulkarni, Matthew J. Graham, Frank J. Masci, Steven L. Groom, Josiah Purdum, Richard Dekany and Eric C. Bellm, 10 February 2026, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202556432
“Magnetic Atmospheres and Circumstellar Interaction in J1901+1458: Revisiting the Most Compact White Dwarf Merger Remnant in the light of new UV and X-ray data” by Aayush Desai, Ilaria Caiazzo, Stephane Vennes, Adela Kawka, Tim Cunningham, Gauri Kotiwale, Andrei A. Cristea, John C. Raymond, Maria Camisassa, Leandro G. Althaus, J. J. Hermes, Iris Traulsen, James Fuller, Jeremy Heyl, Jan van Roestel, Kevin B. Burdge, Antonio C. Rodriguez, Ingrid Pelisoli, Boris T. Gänsicke, Paula Szkody, Sumit Ok. Maheshwari, Zachary P. Vanderbosch, Andrew Drake, Lilia Ferrario, Dayal Wickramasinghe, Stephen Justham, Ruediger Pakmor, Kareem El-Badry, Thomas Prince, S. R. Kulkarni, Matthew J. Graham, Ben Rusholme, Russ R. Laher, and Josiah Purdum, 3 September 2025, arXiv.
DOI: 10.48550/arXiv.2509.03216
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