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A rendering of three white stars, two of which are very close together.

What if there was one process capable of creating every type of detectable stellar-mass black hole system? Recent research suggests there might be, and that it involves a triple-star system.

Three Separate Contexts

Stellar-mass black holes, or black holes that are at most a few hundred times the mass of the Sun, pop up in a number of different environments in the Milky Way. Astronomers have known since the 1960s that these black holes are the engines behind accreting low-mass X-ray binaries; more recently, researchers at gravitational wave observatories such as LIGO have found pairs of black holes orbiting each other just prior to merging; and, in just the past few years, scientists using the Gaia spacecraft have found black holes on wide, prowling orbits around still-burning stars.

A photograph of a patch of the night sky with two nested purple ovals overlaid.

An illustration of the first black hole discovered with a star on a wide orbit. The black hole moves along the smaller inner ellipse, while its companion star orbits along the wider outer one. [ESA/Gaia/DPAC]

Although each of these scenarios involves a black hole, it’s unclear how exactly these black holes are related to one another, or if they’re related at all. For instance, do low-mass X-ray binaries form the same way as the binary black holes observed with LIGO? Are the wide star–black hole binaries discovered by Gaia destined to eventually merge as two black holes, or are they a separate population altogether?

Recent research led by Smadar Naoz (University of California, Los Angeles) offers a potential answer to this question of relatedness — that each of these situations forms through the same underlying process.

Triples Systems

An illustration of multiple stars evolving along different pathways indicated by arrows.

A schematic illustration of how triple-star systems can produce all three types of observable stellar-mass black hole systems. Click to enlarge. [Naoz et al. 2025]

The mechanism Naoz and collaborators describe would work as follows. First, three stars begin their lives all bound together via gravity. Two of these stars orbit each other fairly closely, but the third hangs back much farther away. After the two inner stars burn out and collapse into black holes, they undergo the kind of collision commonly observed by LIGO and merge together. This process gives the resulting larger black hole a “kick,” meaning it goes flying off away from the site of the impact with some new velocity.

What happens next depends on the geometry of the system and the direction of the kick. If the remnant black hole gets shot away from the third star, it might just drift off on its own and leave the star behind. If the kick isn’t too strong, the remnant will remain gravitationally bound to that third star, and the system will eventually look like the star–black hole pairs observed by Gaia. Finally, if the kick sends the remnant toward the third star, some dramatic outcomes become possible: either the black hole starts nibbling on the star and the system becomes a low-mass X-ray binary, or the black hole simply smashes into the star, destroying it completely in a large, flashy explosion paired with a gravitational wave signal.

The authors stress that this mechanism is almost certainly not the only way that these three stellar-mass black hole systems form. However, it is exciting to consider a common thread underlying such seemingly different scenarios, and with upgrades coming to gravitational wave observatories, we can hope for tests of its feasibility in the near future.

Citation

“Triples as Links Between Binary Black Hole Mergers, Their Electromagnetic Counterparts, and Galactic Black Holes,” Smadar Naoz et al 2025 ApJL 992 L12. doi:10.3847/2041-8213/ae0a20

A photograph of a dense knot of stars against a collection of more spread-out stars.

By simulating how the orbits of distant solar system objects were altered by close encounters with other stars early in the Sun’s life, astronomers have placed tight constraints on how long our home star stuck around its siblings after birth.

Born in Batches

Though our Sun currently travels on a solitary trajectory through the galaxy, its earliest childhood was not spent so lonely. Instead, the Sun was likely born as part of a litter of many other stars all collapsing out of the same cloud of precursor gas and dust. As a consequence, its early adolescence was spent in the company of dozens of other young stars, all zipping along on their own paths, destined to drift apart but initially packed close together.

A photograph of three bright stars within a circular cavity of gas.

The Hubble Space Telescope’s view of a collection of young stars still embedded within their natal nebula. [NASA, ESA, G. Duchene (Universite de Grenoble I); Image Processing: Gladys Kober (NASA/Catholic University of America)]

Despite their kinship, these young stars were not kind to one another when they passed nearby. When two stars grow close, the intense gravity of the encounter can severely disrupt their proto-planetary systems, scattering the objects orbiting farthest from their stars and potentially even ejecting some objects altogether. These early years likely left scars on the edges of our solar system that persist even today, billions of years after the early tussles.

Recent research led by Amir Siraj, Princeton University, leverages these scars or their apparent absence to ask the question: given the structure we observe in the outer solar system today, what limits can we place on the number of stars born near the Sun and the amount of time the Sun spent in its birth cluster?

Distance is Power

Several authors have asked this question over the past several decades, but Siraj and collaborators added a new twist: instead of studying either the giant planets or the cold classical Kuiper Belt, they instead focused exclusively on the “distant sednoids.” This rarefied collection of only nine known objects includes only the most distant minor planets in our solar system: the sednoids never come within 40 au of the Sun, and they spend much of their orbits beyond 400 au. Interestingly, however, all of them orbit on planes that are fairly aligned with that of the planets, and none ever strays farther than 20° from the ecliptic.

A black background with the sun at center surrounded by several large ovals, each labeled with the name of a minor planet.

An illustration of the orbits for some of the distant sednoids considered in this study. Click to enlarge. [NAOJ]

Through a suite of numerical simulations, Siraj and collaborators demonstrate that this relatively tight distribution of inclinations implies that the Sun couldn’t have been too roughed up on its way out of the cluster. By simulating many different close flybys and their influence on the distant sednoids, the researchers constrained the product of the number of stars in the Sun’s birth cluster and the time the Sun spent there to be less than or equal to 5 billion years per cubic parsec. Assuming a typical cluster density of 100 stars per cubic parsec, this suggests that the Sun cleared out of the densest and most dangerous part of the cluster within just 50 million years.

The authors stress that this conclusion leans on the assumption that the distant sednoids arrived on their extreme orbits essentially immediately, though in fact astronomers aren’t sure exactly how and when these objects ended up on the outskirts of the solar system. If the sednoids were in fact implanted onto their orbits early on, this limit on how long it took the Sun to leave its siblings is by far the strongest to date. With the Vera C. Rubin Observatory poised to discover thousands of new distant solar system objects, it’s likely that the bound will grow even more stringent in the next few years.

Citation

“Limits on Stellar Flybys in the Solar Birth Cluster,” Amir Siraj et al 2025 ApJL 993 L4. doi:10.3847/2041-8213/ae1025

Supernova remnant N132D

Beautiful bubbles of hot ionized gas, supernova remnants trace the dying days of massive stars. A recent study uses the Chandra X-ray Observatory to detail the motion and understand the origins of one supernova remnant.

Retracing Supernova Remnants

Massive stars, about 8 solar masses or larger, will end their short lives in violent core-collapse supernovae. These explosions barrel into any surrounding interstellar medium, carving out low-density cavities and blowing material outward. Appearing as a bubble or shell of hot gas, a supernova remnant carries the signatures of the type of star that produced it and the ways that star shaped its environment throughout its life. 

Nearby, in the Large Magellanic Cloud, is the 2,500-year-old supernova remnant N132D — the most X-ray luminous supernova remnant within the Local Group. Though N132D’s size, likely progenitor mass, and chemical composition are well constrained, astronomers have yet to nail down the velocity of the X-ray shock front — the outer edge of the supernova remnant that rams into the interstellar medium. This measurement is critical to understanding the local conditions the supernova first encountered and how its expansion will continue to impact the interstellar medium over time.

Chandra Observations of N132D

Determining the shock front velocity requires astronomers to focus on the thin outer edge of the soaring supernova remnant — how do we measure the motion of such a narrow strip of gas? X-ray spectroscopy of N132D yields a measurement from a single epoch of observations, but it cannot isolate the narrow shock front, making a reliable velocity measurement difficult to attain.

supernova remnant velocity

The expansion of supernova remnant N132D around the rim for each of the regions analyzed in this study. The arrow lengths are proportional to their estimated expansion velocities. Click to enlarge. [Long et al 2025]

Circumventing the challenges of spectral analyses, Xi Long (University of Hong Kong) and collaborators used two sets of X-ray observations of N132D from the Chandra X-ray Observatory taken about 14.5 years apart to measure the motion of the shock front across the sky over time. This measurement, known as proper motion, compares the location of the foreground supernova remnant to stationary background stars to estimate its angular speed. 

The authors focus on small regions on the very edge of the supernova shock front, six northern and eight southern, to carefully measure the motion of the shock between the two sets of observations. In separating the shock front into smaller regions, the authors can determine any variations in speed along the shock front. The southern edge of N132D moves with the same expansion rate of 1,620 km/s. The northern edge has an average expansion rate of 3,820 km/s, but its speed is more varied, which isn’t unexpected given its blown-out appearance.

Modeling results showing the supernova age versus ejecta mass for the northern (red) and southern (blue) regions compared to the age estimate from an optical study of N132D. For the explosion energies and estimated ejecta masses between 2 and 6 solar masses, the models are consistent for all three. Click to enlarge. [Long et al 2025]

Comparing to Prior Studies

In addition to determining the velocity of the forward shock, the authors model the evolution of the supernova remnant to estimate the initial conditions of N132D including progenitor star mass, explosion energy, and ejecta mass. Their results agree with other studies that suggested N132D originated from a roughly 15-solar-mass star that exploded 2,500 years ago into a low-density cavity. 

This study showcases the unique ability of Chandra’s high-resolution instruments to carefully measure the evolution of supernova remnants, and further studies will continue to determine the detailed characteristics of supernova remnants across the Local Group.

Citation

“Chandra Large Project Observations of the Supernova Remnant N132D: Measuring the Expansion of the Forward Shock,” Xi Long et al 2025 ApJ 993 136. doi:10.3847/1538-4357/ae07c7

A rendering of a blue planet in the foreground and a small bright star in the background.

Astronomers often assume that sub-Neptune exoplanets have magma oceans hiding beneath their atmospheres. New research suggests that this might not always be the case.

Puzzling Type of Planet

In the 30 years since the discovery of the first exoplanet, astronomers have come to appreciate that many of the most common planets in our galaxy look nothing like the worlds in our solar system. One type of planet in particular appears to be ubiquitous despite not appearing in our own cosmic neighborhood: sub-Neptunes. As their name suggests these planets are slightly smaller than Neptune, but still much larger than Earth or what we’d expect from a planet composed mostly of rock.

Four planets side by side ordered by increasing size. From left to right, they are Earth, TOI-421 b, GJ 1214 b, and Neptune.

An illustration of how two sub-Neptunes mentioned in this study (TOI-421b and GJ 1214b) compare with planets in our solar system. Click to enlarge. [NASA, ESA, CSA, Dani Player (STScI)]

What sub-Neptunes are made of and how they formed are still mysteries under active investigation by the research community. It is possible that these worlds are mostly rocky and sport puffy, light atmospheres over their surfaces; it’s also possible that these worlds are watery, and that most of their mass comes from heavy, steamy atmospheres. In most cases, though, it’s assumed that the high pressures and temperatures at the surfaces of these worlds foster a permanent, fiery magma ocean.

This key assumption that there’s no solid surface to stand on influences our models about how these planets cool and evolve over time. However, new research led by Bodie Breza (University of Maryland) suggests that this premise might not always be appropriate.

Solid Ground

To test the assumption that every sub-Neptune has a magma ocean, Breza and collaborators simulated hundreds of thousands of feasible exoplanet interiors and atmospheres. Each simulation slightly tweaked various characteristics, like the surface temperature or the planet’s total mass, then checked whether the temperature and pressure conditions at the planet’s surface would leave any rocks in solid or liquid form.

phase diagram for a common rocky material

A phase diagram for a common rocky material MgSiO3. The various lines show different model pressure–temperature profiles for the sub-Neptune GJ 1214b, while the circles indicate the location of the atmosphere/surface boundary. All circles fall into the “solid” part of the phase diagram, indicating that GJ 1214b likely has a solid surface. [Breza et al. 2025]

Somewhat surprisingly, the team found that about a third of the models they simulated had solid, not magma, surfaces. They found there were two distinct ways to solidify the magma and create a firm shell. First and most intuitively, if the surface temperature falls low enough, the magma will simply freeze. Second and more intriguingly, they found that a planet can also suppress a magma ocean if its atmosphere is heavy enough to drive up the surface pressure beyond the solid–liquid threshold.

This latter result is particularly insightful in light of recent JWST observations that suggest many sub-Neptunes have heavy atmospheres. These new models suggest that these worlds might have solid surfaces, and that models attempting to explain their compositions should be tweaked to account for this finding. In these early days where astronomers are still puzzling out the most basic properties of these strange worlds, research like this demonstrates the power of using advanced theoretical modeling to help interpret our cutting-edge observations.

Citation

“Not All Sub-Neptune Exoplanets Have Magma Oceans,” Bodie Breza et al 2025 ApJL 993 L46. doi:10.3847/2041-8213/ae0c07

A rendering of two dark spheres surrounded by bright, misaligned disks drawing near to one another.

Black holes can have messy family histories, and a recent massive merger challenged astronomers to sort out just how many generations of black holes were involved in the event. Recent research, however, may have untangled the lineages of the two progenitors.

A Massive Collision

Earlier this year, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the ripples in spacetime caused by the merger of two black holes somewhere in the distant universe. Though these detections have become increasingly common, this one in particular stood out among the more than 200 reported over the last decade. Named GW231123, this event must have caused more of a splash than a ripple since the two black holes that slammed together were unusually massive; both likely weighed more than 100 times the mass of the Sun. Not only that, each was also spinning unusually rapidly prior to the collision.

A photograph of two perpendicular straight lines that cut through a forest and meet at a complex of buildings in a clearing.

LIGO Livingston, one of the two LIGO detectors. [Caltech/MIT/LIGO Lab]

According to theory, black holes this large are not supposed to form on their own. When massive stars die, they can collapse into a black hole; but, when really massive stars die, the collapse is so violent that no material is left over to form the black hole. So, when researchers first measured the masses of the black holes involved in GW231123, they knew that the black holes involved couldn’t have formed directly from collapsed stars. Instead, they were likely second-generation black holes, meaning that they themselves formed from previous black hole–black hole mergers.

Merger Mystery

This result was celebrated as evidence of so-called “hierarchical formation,” in which black holes formed in dense environments swarm throughout their star clusters, merging with other black holes they encounter and growing ever larger.

However, there were a couple of complications with this picture. For one, theory predicts that it is challenging to form such rapidly spinning black holes from randomly oriented progenitors, since usually the spins wash out and leave the remnant rotating slowly. For another, the merging process tends to give the final black hole a “kick” that sends it flying rapidly away from the site of the impact, occasionally fast enough to leave the star cluster altogether, meaning second-generation black holes might not be able to merge into subsequent generations.

A multi-panel cartoon showing black holes merging through various combinations of 1st, 2nd, and 3rd generation black holes.

A schematic of various potential black hole family trees. The binary-star origin scenario is shown in the green box. Click to enlarge. [Stegmann et al. 2025]

Born as a Pair

New research led by Jakob Stegmann, Max Planck Institute for Astrophysics, suggests a way around each of these problems. In the team’s model, the two black holes involved in the GW231123 merger didn’t form from isolated black holes in a cluster, but rather from binary stars embedded within that cluster. In this picture, two massive stars were born bound together, orbiting one another within a larger cluster of stars. Crucially for this scenario, the massive primordial stars have aligned spins — an expected outcome of massive binary star evolution.

After both stars in the binary collapsed into black holes, these black holes merged to form a rapidly spinning second-generation black hole. This second-generation black hole eventually encountered another black hole within the cluster and merged to produce the ripples in spacetime we observed as GW231123.

Since black holes born within binary systems are expected to have smaller kicks and faster spins, this scenario neatly explains the properties observed in GW231123. As LIGO keeps listening for more black hole mergers, hopefully similar events will provide more chances to untangle their complex family trees.

Citation

“Resolving Black Hole Family Issues Among the Massive Ancestors of Very High-Spin Gravitational-Wave Events like GW231123,” Jakob Stegmann et al 2025 ApJL 992 L226. doi:10.3847/2041-8213/ae0e5f

gravitational wave illustration

Meet GW241011 and GW241101, two events from the fourth observing run of the LIGO, Virgo, and KAGRA (LVK) gravitational wave detectors. With rapid spins and mismatched black hole masses, both events provide strong evidence for black hole growth through hierarchical mergers.

"masses in the stellar graveyard"

Masses of black holes and neutron stars discovered via gravitational waves (blue and orange circles). The red and yellow circles show the black holes and neutron stars detected through electromagnetic means. Click to enlarge. [LVK/A. Geller/Northwestern]

A Decade of Discovery

Humanity is 10 years into its study of the gravitational wave universe, and detectors across the globe are about to wrap up their fourth observing run. The recently released fourth catalog of gravitational wave transients, GWTC-4.0, more than doubles the number of recorded events.

Today, we’re taking a look at two of the fourth observing run’s gravitational wave events, both of which point to black holes born in dense environments.

Two Detections in the Spotlight

GW241011 and GW241101 are gravitational wave signals that reached Earth in late 2024. Using Bayesian parameter estimation, members of the LVK collaboration determined that each of these two signals arose from the merger of a pair of black holes.

plot of posterior spin distributions

Posterior distributions for the primary black hole’s spin magnitude for GW241011 (left) and GW241101 (right). [LIGO, Virgo, and KAGRA Collaborations 2025]

Though the LVK detectors have by now spotted dozens of coalescing black hole pairs, these two detections stand out. GW241011 is the third-loudest gravitational wave signal ever detected, and its primary (i.e., more massive) black hole has one of the largest and most precisely measured spins among black holes detected via gravitational waves.

GW241101 flips the script: its primary black hole also spins rapidly, but it appears to do so in a direction opposite from the direction in which the black holes orbit. Though its spin isn’t measured as precisely as that of the primary black hole in GW241011, this event still provides the best evidence so far that the spins of some merging black holes are misaligned with their orbital motion.

In addition to their standout spin parameters, these events are also remarkable for the masses involved. In both cases, the larger black hole has a mass of 15–20 solar masses, and the smaller black hole is significantly smaller — about 6 solar masses for GW241011 and 8 solar masses for GW241101.

A Black Hole Genealogy Project

Inferred masses and spins of the ancestors of the primary black holes of GW241011 and GW241101

Inferred masses and spins of the ancestors of the primary black holes of GW241011 and GW241101. Click to enlarge. [LIGO, Virgo, and KAGRA Collaborations 2025]

The rapid spins and comparatively large masses of the primary black holes of GW241011 and GW241101 don’t line up with what’s expected for black holes arising in isolated binary systems, unsubjected to outside influences. But they do match expectations for hierarchical mergers, in which one or more of the black holes involved is itself the product of a merger.

If these signals arose from hierarchical mergers, that tells us something about the environment in which the black holes lived. When two black holes merge, the product is expected to spin rapidly and receive a “kick.” For a black hole pair in a loose grouping of stars, this kick would likely boot the merger product out of the group, preventing it from encountering other black holes and merging again.

But in dense environments like compact star clusters, even a good strong kick isn’t enough to overcome the cluster’s escape velocity and launch the resulting black hole on a lonely journey through space. Instead, these black holes are retained in the dense cluster environments of their birth, where they may dance and merge with another black hole partner. While it’s not possible to entirely rule out other origins, such as an isolated pair of massive binary stars, the data strongly support the hierarchical merger scenario.

This discovery yet again demonstrates the ability of our gravitational wave detectors to reveal the lives and histories of black holes throughout our universe. Stay tuned for even more black hole news from the current observing run!

Citation

GW241011 and GW241110: Exploring Binary Formation and Fundamental Physics with Asymmetric, High-Spin Black Hole Coalescences,” A. G. Abac et al 2025 ApJL 993 L21. doi:10.3847/2041-8213/ae0d54

A rendering of two rocky worlds mid-collision, with debris exploding outwards.

What would happen to a gaseous exoplanet if it was struck by a protoplanet? Recent research suggests that it would ring like a bell, and that just maybe, we’d be able to observe this ringing back on Earth.

Heavy Metal Mystery

Previous studies have hinted that giant planets have more heavy elements in their cores than we’d expect if they had formed from the exact same stew of gas and dust as their host stars. How exactly they end up with these enriched compositions is something of a mystery, though. Astronomers have proposed two potential theories: one somewhat dull, and one that involves dramatic collisions between the massive protoplanets.

In the first case, perhaps growing giant planets clear out the gas along their orbits, then steadily and quietly accrete the dust that spills over into the gaps. In the latter case, theorists have conjured a model in which growing gas giant planets repeatedly collide and merge with large and somewhat rocky protoplanets. Both of these mechanisms would produce the enhanced heavy element contents observed today, so how can we tell these two paths apart? Recent work by a team led by J.J. Zanazzi (University of California, Berkeley) proposes one such test.

Four spheres each showing a different oscillation mode, colored red and blue according to the change in temperature.

An illustration of the different oscillation modes within a gas giant after it’s struck by a protoplanet. [Zanazzi et al. 2025]

Ringing Planets

Through mathematical derivations, the researchers demonstrate that if a gas giant were to collide with a protoplanet, the impact would cause the planet’s surface to physically oscillate inwards and outwards, and for its surface temperature to vary. In other words, the planet would “ring” like a bell struck with a hammer, and as it physically convulses in the aftermath of the merger it would grow brighter and fainter in a repeating, periodic pattern. In principle, if the ringing lasts for long enough and the oscillations in brightness were large enough, observers back on Earth could record the amplitude and period of the changes and back out some constraints on what had happened and when.

An image of a bright disk, edge on, a coronograph mask blocking out the star, and a bright dot near the center of the mask.

An image of Beta Pictoris, including the young gas giant Beta Pictoris b and the surrounding debris disk. Click to enlarge. [ESO/A.-M. Lagrange et al.; CC BY 4.0]

Excitingly, the researchers demonstrate that not only would the oscillations last for millions of years, they would also be bright enough for JWST to detect for one specific planet named Beta Pictoris b. Should this world have undergone a merger in the last 18 million years, just a few hours of observations with JWST’s Near-Infrared Camera would reveal 45-minute pulsations in which the planet’s brightness changes by about 1%. Since the planet is too far away from its host star for these oscillations to be caused by any gravitational interactions, detecting any periodic signal would be a slam-dunk for the major merger theory.

Beta Pictoris b is a young planet in a system that’s still forming, and it orbits its host star on an eccentric path that takes it through dense regions of the disk. Hopefully, future observations will test whether this treacherous path produced any massive collisions in the relatively recent past. Until then, though, we can all enjoy charming visions of massive planets colliding with one another and ringing through the night.

Citation

“Seismic Oscillations Excited by Giant Impacts in Directly Imaged Giant Planets,” J. J. Zanazzi et al 2025 ApJ 993 3. doi:10.3847/1538-4357/ae04ec

cataclysmic variable system

Astronomers have discovered the most compact cataclysmic variable system containing a strongly magnetized white dwarf. The extreme closeness of the system suggests that the companion may be a metal-poor star — the first time such a star has been paired with a strongly magnetized white dwarf.

Meet the Cataclysmic Variable

Cataclysmic variables are binary star systems that contain a white dwarf and a companion star in uncomfortably close quarters. In these systems, the companion star transfers gas to the white dwarf, resulting in sudden, irregular, and often repeated outbursts as the stolen gas ignites on the scorching surface of the white dwarf.

These bound-together stars typically orbit one another on orbits ranging from about 80 minutes to 10 hours. Though theorists place the minimum orbital period around 76 minutes, a handful of cataclysmic variable systems have cropped up with periods below this limit. The systems that have limboed under the limit are thought to have stellar companions that are more compact than typical main-sequence stars, allowing the white dwarf to nestle in closer.

Exploring Below the Limit

Discovered in 2019 by the Gaia spacecraft, Gaia19bxc is a cataclysmic variable that fluctuates with a period of 64.42 minutes. If this variability is linked to the orbital period of the system, that would place it well below the theoretical minimum period. Adding to the intrigue, early observations also hinted that Gaia19bxc’s white dwarf is strongly magnetic, making the system what’s called a polar. In polar systems, the white dwarf’s magnetic field diverts the accreted matter toward the white dwarf’s poles as it is collected, rather than into a disk around the white dwarf’s equator.

Light curves of Gaia19bxc

Phase-folded light curves for Gaia19bxc from the Caltech HIgh-speed Multi-colour camERA (CHIMERA) on the Hale Telescope. The double-peaked light curve is evidence for cyclotron beaming, which occurs in strongly magnetized white dwarfs. [Adapted from Galiullin et al. 2025]

Now, Ilkham Galiullin (Kazan Federal University) and collaborators have analyzed photometry and spectra of Gaia19bxc from the Zwicky Transient Facility, the Hale Telescope, and the Keck I telescope to investigate the nature of this unusual system.

The team’s analysis confirmed that the stars of Gaia19bxc orbit one another every 64.42 minutes, cementing the system’s place below the period minimum for typical cataclysmic variables. The system’s double-peaked light curves and evidence for an accretion stream — rather than an accretion disk — confirm the system’s polar nature, implying a magnetic field strength greater than 10 million Gauss. This makes Gaia19bxc the most closely orbiting system to contain a strongly magnetized white dwarf.

A Sign of Discoveries to Come

histogram of cataclysmic variable orbital periods

Gaia19bxc’s orbital period compared to known polar cataclysmic variables (gray) as well as the theorized minimum periods for systems containing a metal-poor (Pop II) companion (cyan) and an evolved companion (blue). [Galiullin et al. 2025]

These observations illuminate the nature of the white dwarf — but what about the companion star?

Galiullin and coauthors saw no evidence for metal lines in Gaia19bxc’s spectrum, nor did they see spectral features arising from a hot companion star. These findings suggest that the companion is an old, cool, compact, metal-poor star, which would make Gaia19bxc the first known polar to contain a metal-poor star. It’s also one of only a handful of cataclysmic variables to contain a metal-poor star and be below the theoretical period minimum.

Though Gaia19bxc is currently in a class of its own, it may not be for long; with the start of the Vera C. Rubin Observatory’s Legacy Survey of Space and Time rapidly approaching, many more cataclysmic variable systems as faint as or fainter than Gaia19bxc may soon be discovered.

Citation

“Optical Spectroscopy of the Most Compact Accreting Binary Harboring a Magnetic White Dwarf and a Hydrogen-Rich Donor,” Ilkham Galiullin et al 2025 ApJL 990 L57. doi:10.3847/2041-8213/adff82

A glowing orange sphere set against a background image of the Milky Way's disk of stars.

Isolated brown dwarfs are some of the rarest and hardest to study objects in our stellar neighborhood. Recently, astronomers revealed that one of the most famous isolated brown dwarfs, W1935, is not actually one brown dwarf but two.

Challenging but Fascinating

Astronomers know of only 50 free-floating brown dwarfs within the nearest 200 light-years or so, and even the ones we know about are poorly understood. Since brown dwarfs are frigid compared to their stellar cousins, they consequently shine much less brightly and at longer wavelengths of light. Even taking a picture of these ~500K objects requires long stares with specialized telescopes, and these facilities already have their schedules full supporting a wide range of other areas of research from distant galaxies to exploding supernovae.

A color photograph of Jupiter showing its red bands, great red spot, and a bright spiraling blue aurora near the north pole.

An image of Jupiter and its prominent aurora taken with the Hubble Space Telescope. [NASA, ESA, and J. Nichols (University of Leicester); Acknowledgment: A. Simon (NASA/GSFC) and the OPAL team]

So, when an in-demand telescope like JWST spends time on brown dwarf science, each byte of data it sends back is both precious and likely to contain a new discovery. This has already happened several times: back in 2023, research published in the AAS journals (and summarized on AAS Nova) described JWST’s discovery of the first-ever Y+Y binary system, and in 2024, astronomers at the 243rd meeting of the AAS presented the first detection of methane emission on a brown dwarf — a sign of auroral activity.

Recently, JWST continued its pattern of discovery: the telescope took another look at the brown dwarf W1935, where methane emission was detected for the first time, and again found something surprising.

A New Binary

When JWST first took a look at this brown dwarf, it used its Near Infrared Spectrograph (NIRSpec) to record a spectrum of the object. These data revealed a large spike near 3 microns that perfectly lined up with methane emission from an aurora, similar to what we observe on Jupiter in the solar system. This was a thrilling find, since it hinted at an exciting possibility: Jupiter’s aurora is powered by its nearby moon Io, so the researchers cautiously wondered if W1935’s aurora could be evidence of a companion satellite as well.

A multi-panel figure with noisy (real) images in the left column, smooth models in the middle column, and pixelated residuals in the rightmost column. The models are in good agreement with the data.

Images of W1935 taken with MIRI (left column), the author’s model for those images (center column), and the residuals (right column). The model includes two nearby point sources that are so close together they appear as one slightly elongated source. Click to enlarge. [De Furio et al. 2025]

Soon after these observations, JWST once again turned to W1935, but this time used its Mid-Infrared Instrument (MIRI) to take images of the object at long wavelengths. A team of researchers led by Matthew de Furio, University of Texas at Austin, recently analyzed these images and noticed something interesting: W1935 isn’t just one brown dwarf, but rather two roughly equal-mass brown dwarfs packed close together on 16–28 year orbits.

Doubly Interesting

This makes W1935 just the second-ever known Y+Y dwarf binary and cements its status as one of the most interesting brown dwarf systems discovered to date. So far, it’s not clear which object is responsible for the auroral signal since the objects are so close together that their images blur together without careful modeling and image processing.

De Furio’s team pointed out that JWST has yet another instrument that could prove useful here: its Integral Field Unit spectrometer within NIRSpec. Should JWST revisit this fascinating system, it could study the aurora’s variability over time, pin it to one specific object within the binary, and hopefully shed some light on whether any moons play a role in the methane emission. Given all we have learned each time JWST has turned to these strange worlds, we can only hope that it will do so again soon.

Citation

“Discovery of the Second Y+Y Dwarf Binary System: CWISEP J193518.59-154620.3,” Matthew De Furio et al 2025 ApJL 990 L63. doi:10.3847/2041-8213/adfee1

Illustration of a tidal disruption event

Catastrophic encounters between stars and massive black holes usually take place in the nuclei of galaxies, but not always. Researchers recently reported on the brightest-ever radio emission from an off-nuclear tidal disruption event caused by a wandering or recoiling black hole.

Signature of a Roaming Black Hole

Tidal disruption events occur when a star ventures too close to a massive black hole. The tidal forces of the black hole stretch the star until it’s partially or entirely disrupted, sometimes causing jets or outflows to spray from the shredded star. One thing that often distinguishes a tidal disruption event from the sea of other possible transients is the location, close to the nucleus of a galaxy.

But not all tidal disruption events happen in the center of a galaxy. In rare cases, a massive black hole roaming elsewhere in a galaxy may encounter a star, sending out a tell-tale signal in an unexpected location.

radio images of AT 2025tvd

Radio observations of AT 2024tvd on two dates after its optical discovery. The tidal disruption event was not detected at 88 days post-discovery (left) and outshone the center of its host galaxy on 160 days post-discovery (right). Click to enlarge. [Sfaradi et al. 2025]

One such event is AT 2024tvd, which was discovered at optical wavelengths by the Zwicky Transient Facility. Though the initial identification placed it at the center of its host galaxy, follow-up observations suggested that it was in fact 2,600 light years from the center. What can radio observations tell us about this rare off-center event?

Radio Reconnaissance

Less than three months after AT 2024tvd was discovered, Itai Sfaradi (University of California, Berkeley) and collaborators launched a months-long radio-wavelength observing campaign using the Very Large Array, the Atacama Large Millimeter/submillimeter Array, the Arcminute Microkelvin Imager Large Array, the Allen Telescope Array, and the Submillimeter Array. Radio observations are critical for investigating jets and outflows from tidal disruption events.

The observations, which spanned centimeter and millimeter wavelengths, revealed two emission peaks from the tidal disruption event. The first peak occurred roughly 131 days after the event was discovered, and the second followed at day 194.

Radio emission from AT 2024tvd compared to other radio-bright tidal disruption events

Demonstration of the fast evolution of AT 2024tvd’s radio emission (red and orange stars) compared to other radio-bright tidal disruption events (other symbols). [Sfaradi et al. 2025]

About 40% of the tidal disruption events that have been identified at optical wavelengths show this kind of delayed brightening at radio wavelengths, but AT 2024tvd stands out as having the fastest radio evolution ever seen. Even among fairly fast-evolving flares, AT 2024tvd is unusual, having a brighter second peak than peer events.

Prompt or Delayed, Outflow or Jet?

To understand the origin of the fast-evolving, extremely bright radio emission from AT 2024tvd, Sfaradi’s team modeled the emission that would arise from outflows and jets. For both wide-angle outflows and narrow jets, the team considered both prompt — arising simultaneously with the event’s optical detection — and delayed sources.

The team’s modeling highlighted several possible scenarios. In the first, both bright radio peaks arose from a single outflow that was launched about 84 days after the star met its doom. The double-peaked behavior is due to the outflow interacting with a complex distribution of material surrounding the black hole. It’s also possible for the two peaks to arise from separate outflows or jets, one launched around 84 days and the other around either 170 or 190 days, depending on whether the second source is a mildly relativistic outflow or a relativistic jet.

Sfaradi and collaborators posited that AT 2024tvd’s unusual radio behavior could be due to its off-nuclear location, but they acknowledged that this event might simply occupy a region of tidal disruption event parameter space that had yet to be explored. Sensitive interferometric or polarimetric observations may reveal more about how AT 2024tvd interacts with its environment, helping to illuminate the nature of this rare event.

Citation

“The First Radio-Bright Off-Nuclear TDE 2024tvd Reveals the Fastest-Evolving Double-Peaked Radio Emission,” Itai Sfaradi et al 2025 ApJL 992 L18. doi:10.3847/2041-8213/ae0a26

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