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simulation of a supermassive black hole binary system

Researchers have undertaken a new search for a supermassive black hole binary, placing limits on its properties and exploring a promising technique for simultaneously analyzing electromagnetic and gravitational wave data.

Narrowing the Search for Supermassive Black Hole Binaries

In 2023, astronomers announced the discovery of compelling evidence for the gravitational wave background: the collective murmurs and rumblings of distant supermassive black hole binaries. The time may now be ripe for the natural next phase of discovery: the detection of gravitational waves from an individual supermassive black hole binary.

predicted frequency evolution of gravitational waves from 3C 66B

The predicted frequency evolution of gravitational waves from the black hole binary candidate in 3C 66B. Click to enlarge. [Cardinal Tremblay et al. 2026]

One of the most promising candidates for this search is 3C 66B, a poetically named elliptical galaxy roughly 300 million light-years away. More than 20 years ago, researchers discovered that the radio source at the heart of this galaxy wobbles to and fro in a way that’s consistent with the elliptical paths of supermassive black holes in a close binary. Further work predicted that the gravitational waves produced by this binary would have a frequency of about 60 nanohertz and potentially be within the observational reach of current methods.

Pulsar Timing Array

Jacob Cardinal Tremblay (Max Planck Institute for Gravitational Physics and Leibniz University Hannover) and collaborators conducted a search for gravitational waves from the candidate supermassive black hole binary in 3C 66B using the Parkes Pulsar Timing Array (PPTA).

artist's impression of the gravitational wave background from a supermassive black hole binary sweeping across an array of pulsars

Artist’s impression of a supermassive black hole binary generating gravitational waves that sweep across an array of pulsars. [Aurore Simonnet / NANOGrav; CC BY 4.0]

A pulsar timing array is a collection of pulsars monitored for signs of passing gravitational waves. Pulsars are the condensed, rapidly spinning cores of high-mass stars that exploded as supernovae. Named for their characteristics radio pulses, these extreme stars spin with exceptional regularity, and the passage of a gravitational wave can shrink or expand spacetime enough to speed up or delay the arrival of a pulsar’s pulses. By searching for coordinated changes in pulse arrival times from a collection of pulsars, researchers hope to detect low-frequency gravitational waves that are inaccessible to observatories like LIGO.

Researchers have previously searched for gravitational waves from 3C 66B in data from other pulsar timing arrays, such as the North American Nanohertz Observatory for Gravitational Waves, and no black hole binary has been detected. This is the first search of the third PPTA data release, which contains measurements of 32 pulsars over 18 years, providing a long baseline to search for the slow undulations of low-frequency gravitational waves.

Placing Limits

constraints on chirp mass of 3C 66B

Constraints placed on the chirp mass by this work (blue and green histograms) compared to constraints from electromagnetic observations (gray and peach shaded areas). Click to enlarge. [Cardinal Tremblay et al. 2026]

Using Bayesian statistical methods to analyze their pulsar timing data, the team was unable to confirm or rule out the presence of a supermassive black hole binary in 3C 66B. However, they were able to place limits on its properties, such as the chirp mass and the amplitude of the signal, and certain limits were more stringent than those placed by existing electromagnetic data.

While this analysis didn’t result in the first-ever detection of gravitational waves from a single supermassive black hole binary, it did allow the team to test a new method that could someday play a role in precision cosmology. This method simultaneously analyzes electromagnetic and gravitational wave data from known supermassive black hole binaries, establishing these sources as “standard sirens” that can complement standard candles like Type Ia supernovae for measurements of the expansion rate of the universe.

Citation

“A Multimessenger Search for the Supermassive Black Hole Binary in 3C 66B with the Parkes Pulsar Timing Array,” Jacob Cardinal Tremblay et al 2026 ApJL 998 L42. doi:10.3847/2041-8213/ae3c98

Small Magellanic Cloud

Researchers show that the highly disrupted state of the Small Magellanic Cloud can be attributed to a long history of interactions with its neighbor, the Large Magellanic Cloud.

A Disrupted Dwarf Galaxy?

The Small Magellanic Cloud (SMC) is one of the largest and nearest satellites of the Milky Way. At a distance of just 200,000 light-years, the SMC has long been used as an accessible analog for low-metallicity star-forming galaxies in the early universe.

Small Magellanic Cloud photometric and HI kinematic centers

Locations of the SMC’s photometric center and hydrogen gas kinematic center. The two locations are separated by several thousand light-years. [Adapted from Rathore et al. 2026]

However, observations increasingly suggest that the SMC may not be an ideal analog for high-redshift galaxies. The SMC appears to have been disrupted: its gas seems to rotate while its stars do not, it has an unusually large line-of-sight depth compared to its extent on the sky, and its gas distribution has two peaks.

The source of this disequilibrium might be the Milky Way’s largest satellite, the Large Magellanic Cloud (LMC). As the SMC inches along its orbit around the Milky Way, it also engages in a gravitational dance with the LMC. These galaxies may be clumsy dance partners, with data suggesting a potential direct collision between the SMC and LMC within the past 200 million years.

Modeling the Magellanic Clouds

To understand how a potential collision between the Magellanic Clouds might have impacted the present-day properties of the SMC, Himansh Rathore (University of Arizona) and collaborators used hydrodynamical simulations to explore the interconnected histories of the SMC, the LMC, and the Milky Way.

SMC–LMC separation in the authors' models

SMC–LMC separation over time in the control model (orange dot-dashed line) and the collision model (purple solid line). Click to enlarge. [Rathore et al. 2026]

At the beginning of the simulations, the SMC is in equilibrium, with its stellar and gaseous components neatly rotating in unison. From there, its fate diverged: in a control simulation, the SMC and LMC maintained a respectful distance, never getting less than about 100,000 light-years from one another. In the collision simulation, the galaxies collided 100 million years ago.

Both of these model scenarios reproduced certain features of the SMC–LMC system, such as the bridge of gas that spans the distance between them. Only the collision scenario, though, can explain the apparent discrepancy between the SMC’s stars and gas.

An Impactful Collision

SMC stellar density and viewing schematic

Left: Simulated stellar density 200 million years post-collision. Right: A schematic of our likely perspective on the post-collision SMC–LMC system. Click to enlarge. [Rathore et al. 2026]

Post-collision, the SMC’s stellar population is significantly elongated and features a tidal tail. If this tidal tail is oriented along our line of sight, it would explain why the galaxy’s depth in that direction is unexpectedly large. The presence of a gas tidal tail may also help to explain the two peaks in the galaxy’s gas density.

In terms of the galaxy’s rotation, only the stars nearest the SMC’s center rotate, while the rest move radially outward, swayed by the tidal pull of the LMC. This radial motion is also present in the galaxy’s gas. While previous work has interpreted the observed velocity dispersion of the SMC’s gas as a sign of rotation, Rathore’s team showed that radial expansion, viewed from an inclined angle, can mimic a rotational signature. 

The team also showed that the gas is far more disturbed than the stars, reflecting the pressure exerted by the LMC’s gas when the two galaxies collided. This type of interaction can transform an irregular dwarf galaxy into an ellipsoidal or spheroidal dwarf galaxy — a transition that Rathore’s team argues is underway for the SMC.

Ultimately, these simulations demonstrate that the SMC is a profoundly disrupted galaxy, likely still reeling from a recent collision with its larger neighbor. Viewing the SMC through this lens is critical to understanding its role as an analog to early universe galaxies.

Citation

“A Galactic Transformation — Understanding the SMC’s Structural and Kinematic Disequilibrium,” Himansh Rathore et al 2026 ApJ 1000 50. doi:10.3847/1538-4357/ae4507 

A close-up photograph of Jupiter. Dramatic cloud bands encircle the planet, and a large dark shadow of a nearby moon appears at left.

Jupiter’s upper atmosphere is hundreds of degrees hotter than it has any right to be, and for decades astronomers could only study it in brief, seemingly chaotic glimpses. New global maps built from years of observations have finally revealed the big picture — and it is surprisingly calm.

Order from Chaos

Although we’re well acquainted with Jupiter, having sent eight spacecraft on close passages and lived with it dominating the night sky for millennia, some of its simplest characteristics continue to puzzle us. For example, high above Jupiter’s cloud bands, temperatures exceed 800 K — far hotter than we’d expect given the meager sunlight that reaches that distance. Astronomers have long suspected that this excess is deposited by energy from the planet’s brilliant polar auroras, but previous observations provided only localized, sporadic snapshots that collectively painted a confusing picture of an ever-changing landscape.

A photograph of Jupiter seen in red/orange false color with a dark stripe down the middle.

Jupiter as seen by the slit camera on the Keck telescope during one of the team’s observations. The spectrograph slit is the dark vertical band stretching from the equator to the north pole, and the Great Red Spot appears in black at the lower right. [Adapted from Roberts et al. 2026]

To truly understand how Jupiter’s upper atmosphere got so hot and whether it was actually changing over time, astronomers needed to construct a global, comprehensive map — and then do so repeatedly over several years and look for any changes. A team led by Kate Roberts (Boston University) has now assembled just this sequence using the NIRSPEC instrument at the Keck Observatory. The researchers collected over 175,000 high-resolution infrared spectra across 14 nights from 2022 to 2025; they then processed the massive dataset to extract the emission from a charged form of hydrogen, H3+. This species is a standard natural thermometer that glows at infrared wavelengths, and by mapping the spectra to their corresponding specific locations on Jupiter, the team could construct the first true global maps of Jupiter’s upper atmospheric temperature, density, and radiance.

Consistency is Key

A look at how the maps evolved over time revealed striking order: the spatial patterns remained nearly constant, and the temperatures barely changed even over years. In general, the team noted that temperature decreases smoothly from the auroral regions near the poles (~1,150 K) to the equator (~750 K) at every longitude, and that although there is some night-to-night temperature variation, things stay remarkably steady over the years-long baseline.

A 2D heatmap with longitude on the bottom axis and latitude on the vertical axis. There is a general trend towards higher temperatures at higher latitudes.

A map of the average temperature of Jupiter’s upper atmosphere, as measured by the new Keck NIRSPEC data. Click to enlarge. [Adapted from Roberts et al. 2026]

What once “appeared stochastic,” the authors write, “exhibits predominantly spatial rather than temporal variability.” Earlier observations were simply sampling different longitudes at different times, mistaking geography for weather.

The conclusion that the upper atmosphere is complex but steady has implications beyond just Jupiter, as the team notes that JWST has already observed spatial patterns in the temperatures of the upper atmospheres of both Uranus and Neptune. As Roberts and colleagues note, “similar physical mechanisms could give rise to comparably stable structures” on those distant ice giants, meaning this analysis could ground our understanding of the upper atmospheres of giant worlds across the solar system.

Citation

“A Global View of Jupiter’s Upper Atmosphere Through H3+,” Kate Roberts et al 2026 ApJL 998 L13. doi:10.3847/2041-8213/ae3c9b

sunspot

Revealed in high resolution, researchers have spotted complex twisting and braiding motions in fine threads of plasma suspended above a sunspot. The flow of material that followed this braiding provides evidence for magnetic reconnection.

Heating Up the Corona

illustration of magnetic reconnection on the Sun

Magnetic reconnection is common throughout the universe, from Earth’s magnetic field to the jets of supermassive black holes. This illustration shows magnetic reconnection happening on the Sun. [NASA’s Conceptual Image Laboratory]

One of the leading challenges for the field of solar physics is explaining how the temperature of the Sun skyrockets from roughly 6000K at its surface, or photosphere, to more than 106K in its wispy corona. The top candidates for the resolution to this mystery are magnetohydrodynamic waves and magnetic reconnection, in which nearby field lines reorganize into a lower-energy configuration and release magnetic energy.

One way for magnetic reconnection to heat the corona is along magnetic fields that loop up into the corona and are anchored to the solar photosphere. The footprints of these field lines — the spots where they land in the photosphere — wiggle around in random ways. As this happens, the arcing magnetic field lines become braided together, and the twisted magnetic field can then undergo magnetic reconnection, heating the corona.

Braiding and Unbraiding

This process has been explored theoretically, but it’s been challenging to observe directly. As increasingly capable Sun-studying instruments have come online, though, researchers have been able to attain the high resolution needed to see this process at work.

Recently, Hechao Chen (Yunnan University; Peking University; Yunnan Key Laboratory of Solar Physics and Space Science) used data from the New Vacuum Solar Telescope, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory to study the braiding and unbraiding of fine magnetic structures above a sunspot.

plasma threads around a sunspot

Location of the sunspot featured in this study, plus a closeup of the strands of plasma around the sunspot. Click to enlarge. [Adapted from Chen et al. 2025]

The images show a bundle of narrow strands of plasma stretching about 17,000–22,000 miles (28,000–36,000 kilometers). These plasma threads trace magnetic field lines as they emerge from the sunspot before diving down again in a network region, where the magnetic field is slightly enhanced at the border of a giant convective cell.

Driven by Reconnection

The plasma threads appear to lie nearly parallel to one another at the beginning of the observation. Images from all three facilities showed the plasma threads intertwining, providing evidence for the twisting of magnetic field lines predicted to happen when their footprints move around. (As further evidence for this footprint motion, the team also observed complicated flow patterns near one of the footprints.) This intertwining was followed by a sudden brightening where the threads were most tightly knotted, creating two bright “blobs” that traveled outward toward the footprints. Afterward, the threads resumed their nearly parallel state.

braiding magnetic field structures on the Sun

Images from Solar Dynamics Observatory. The first four images show the Sun at a wavelength of 17.1 nanometers, and the final image shows the radial magnetic field. The arrows indicate the location of the magnetic field braiding and the plasma “blobs.” Click to enlarge. [Adapted from Chen et al. 2025]

The team witnessed these bright points form and travel outward repeatedly, moving with projected velocities of 20–230 km/s. They posited that the motion and heating of these blobs of plasma was driven by the release of magnetic tension, and that the threads returned to a parallel configuration once reconnection was complete. They estimated that each of these events produced 1017 Joules, roughly the energy expected for small-scale reconnection events.

This work beautifully illustrates the process of magnetic reconnection on the Sun, showing how random motions can set the stage for the buildup of magnetic tension. The heating and acceleration of plasma that follows demonstrates how small-scale reconnection events like the ones shown here can provide the energy needed to heat the solar corona.

Citation

“Witnessing Magnetic Reconnection in Tangled Superpenumbral Fibrils Around a Sunspot,” Hechao Chen et al 2025 ApJ 995 94. doi:10.3847/1538-4357/ae12e9

In September 2022, the Double Asteroid Redirection Test (DART) mission served as the first attempt at redirecting a near-Earth asteroid — an experiment testing humanity’s ability to fend off any threatening Earth-bound asteroids. A recent study presents the first 3D reconstruction of the impact ejecta, suggesting that the blown-out material is more complex than previous models considered.

DART-ing into Dimorphos 

The DART spacecraft intentionally smashed into Dimorphos, the small and unassuming moon of the larger asteroid Didymos. This space-based crash test aimed to determine if kinetic impactors like DART could successfully change an asteroid’s course through the solar system. Dimorphos’s orbit was indeed shortened by roughly 30 minutes, confirming that ramming an asteroid with a spacecraft could be a viable method of planetary defense. However, determining exactly how much an asteroid’s path will be altered is not necessarily straightforward. When a spacecraft collides with an asteroid, the impactor imparts momentum to the target, and any material excavated from the impact site can also alter the asteroid’s course.

DART’s impact launched a significant spray of rubble, or what researchers call an ejecta curtain. The ejecta curtain is typically modeled as a cone, using a simple geometry to determine its contribution to momentum transfer. However, when looking closely at imaging taken soon after the collision, the real physical structure of the ejecta appears to be more complex. Untangling the true 3D distribution of the ejecta is necessary to understand how momentum was actually distributed among escaping ejecta, better estimate the total mass ejected from Dimorphos, and improve modeling of the Didymos system’s orbit after impact.

DART ejecta

LICIACube images of the DART ejecta curtain with the 14 distinct features identified in this study outlined in red. Click to enlarge. [Deshapriya et al 2026]

Determining Debris Distribution

Seeking to better characterize the ejecta curtain, a team led by J. D. P. Deshapriya from Italy’s National Institute for Astrophysics used data from the Light Italian CubeSat for Imaging of Asteroids (LICIACube) that flew behind the DART spacecraft to capture images of the system shortly after the impact. After careful image handling to bring out diffuse and faint material, the researchers identified 14 distinct ejecta features across the flyby imaging. LICIACube viewed the aftermath both side on and face on, providing valuable 3D information about the distribution and motion of the ejecta. 

DART ejecta animation

Animation of the DART ejecta as the LICIACube spacecraft approached the impact. The left shows the images taken by the spacecraft, and the right shows the 3D model produced for the ejecta. Click to play animation. [Deshapriya et al 2026]

Using 3D modeling software, the researchers traced each of the 14 distinct features across the images and reconstructed a 3D representation of the ejecta morphology. While earlier models assumed symmetric cone-like geometries, this 3D modeling revealed clear asymmetric and non-uniform structures within the ejecta curtain. These variations point to underlying heterogeneities in the impact site on Dimorphos — surface and subsurface composition naturally link to the resulting ejecta distribution according to previous laboratory experiments. The spatial and directional features uncovered in this study highlight the limitations of models that attempt to characterize ejecta curtains with symmetric and uniform distributions. Refining these model parameters will better match observations and better predict momentum transfer in future missions. 

Planetary Defense

What does this mean for planetary defense? While simplified symmetric cone models provide helpful approximations for kinetic impactor missions, they do not fully capture the true complexity of the impact. This study shows that local variations in an asteroid’s surface and interior properties can directly influence the resulting ejecta distribution and motion. The true momentum imparted to the target asteroid depends heavily on the specific ejecta material, and these details inform how effective planetary defense missions can be in redirecting asteroids. Incorporating realistic, observation-driven ejecta distributions into modeling frameworks is essential to achieve reliable predictions for future planetary defense missions.

Citation

“3D Reconstruction of DART Ejecta at Dimorphos Reveals an Anisotropic, Filamentary Structure,” J. D. P Deshapriya et al 2026 Planet. Sci. J. 7 4. doi:10.3847/PSJ/ae2c64

illustration of M31-2014-DS1

One of the most luminous supergiant stars in the Andromeda Galaxy, M31-2014-DS1, has disappeared, potentially leaving behind a black hole shrouded in dusty gas. What do new infrared and X-ray observations tell us about this event?

Failure to Explode?

SN 2023ixf in Messier 101

This image from Gemini North shows the supernova SN 2023ixf along one of the spiral arms of Messier 101. Though less showy than core-collapse supernovae, “failed” supernovae may be another way to form stellar-mass black holes. [International Gemini Observatory/NOIRLab/NSF/AURA; CC BY 4.0]

Massive stars are famous for going out with a bang. After swelling into supergiants, these stars bring their brief lives to a close in a cosmic fireworks show: a supernova explosion that creates a neutron star or a black hole.

But a splashy supernova explosion may not be the only way for a massive star to go out. As observations have begun to suggest, some massive stars may — for reasons not yet fully understood — quietly collapse in upon themselves, going out with a whimper rather than a bang. Though less spectacular and far harder to spot, these “failed” supernovae highlight a possible pathway for the formation of stellar-mass black holes.

A Newborn Black Hole in a Dense, Dusty Blanket

Just last month, researchers reported their discovery of a potential failed supernova. The supergiant M31-2014-DS1 suddenly brightened in 2014 before fading to obscurity, leading researchers to suspect that the star had experienced a failed supernova and created a black hole. The event shared similarities with another failed supernova candidate, NGC6946-BH1.

JWST data of M31-2014-DS 1

Left: A JWST image of M31-2014-DS 1. Right: Spectral energy distributions of the progenitor star/the remnant using data from 2005 to 2012 (empty circles), 2022 to 2023 (filled circles), and 2024 (colored lines). Click to enlarge. [De et al. 2026]

In a new article published in the Astrophysical Journal Letters, Kishalay De (Columbia University; Flatiron Institute) and coauthors have used JWST and the Chandra X-ray Observatory to learn more about what remained after M31-2014-DS1 disappeared. The JWST observations show an extremely red object that has faded considerably in just a few years, with a deep absorption feature from silicate dust and numerous narrow molecular absorption lines. Chandra did not detect any X-rays.

Together, these observations support a model in which the collapsing star ejected its outer layers in a relatively low-energy event (somewhere in the broad ballpark of one hundred-millionth to one ten-thousandth the energy of a typical core-collapse supernova) and formed a black hole. Some of the ejected material fell backward and fed the newborn black hole, while the remainder created a dense blanket that blocked the light from the accreting black hole from view.

Continued Evolution

schematic of the gas surrounding a newborn black hole

Cartoon showing the authors’ model and the model parameters inferred from the observations. Click to enlarge. [De et al. 2026]

In observations from 2024, the source had faded to just 7% of the progenitor star’s luminosity, supporting the conclusion that the star has disappeared and what lies beneath the dusty ejecta is a black hole. De’s team expects that as time goes on, the gas that obscures the accretion-powered luminosity of the newborn black hole will expand and become less dense along our line of sight. At some point — estimated to be no sooner than about 27 years from now — the X-ray emission from the accreting black hole should pierce the dusty shroud and become visible to sensitive X-ray observatories. In the infrared, JWST will be able to track the source’s evolution caused by continued dust creation in the expanding shell.

Thanks to its location in a neighboring galaxy, M31-2014-DS1 has granted astronomers a front-row seat to a poorly understood pathway for black hole formation. Going forward, we can expect further insights into M31-2014-DS1 and, hopefully, the discovery of many more sources like it.

Citation

“Fading into Darkness: A Weak Mass Ejection and Low-Efficiency Fallback Accompanying Black Hole Formation in M31-2014-DS1,” Kishalay De et al 2026 ApJL 999 L25. doi:10.3847/2041-8213/ae468d

Ursa Major III/UNIONS 1

The smallest satellite of the Milky Way, Ursa Major III/UNIONS 1, has posed a mystery since its discovery. Is this smattering of stars a dwarf galaxy or a star cluster?

The Milky Way’s Mysterious Companion

In 2024, researchers discovered a small group of stars orbiting the Milky Way at a distance of about 33,000 light-years in data from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). This collection of stars amounts to just 16 solar masses and shines with only 11 times the Sun’s luminosity, making it 4–5 times less massive than the next faintest Milky Way satellite.

Considering its small stellar population and small size (its half-light radius is 10 light-years, which is typical for a globular cluster), this satellite appeared at first glance to be an ordinary star cluster. But early data also suggested that the stars had an unusually high velocity dispersion, which could mean that they are nestled within a dense and massive dark matter halo — making the satellite not a star cluster but a dwarf galaxy.

Reflecting this uncertainty, the satellite has two names: Ursa Major III (its dwarf galaxy designation) and UNIONS 1 (its star cluster classification). Can new data clear up the confusion?

New Dispersion Data

In a recent research article, William Cerny (Yale University) and collaborators described new observations of 16 stars in Ursa Major III/UNIONS 1 with the Keck II telescope.

plots of velocity dispersion

Normalized likelihoods for the velocity dispersion based on the initial data (left) and the new data (right). The initial velocity dispersion measurement of 3.7 km/s is ruled out by the new measurements. Click to enlarge. [Cerny et al. 2026]

Crucially, these measurements enabled a new estimation of the system’s velocity dispersion. In contrast to earlier work that found a high velocity dispersion of 3.7 km/s, the new data favor a value of just 0.1 km/s and rule out values above 2.3 km/s. While these results do not unequivocally preclude the possibility of dark matter, they strongly favor little to no dark matter. This goes against the initial evidence in favor of the dwarf galaxy hypothesis.

These observations also helped to confirm one candidate binary pair and identify three new binary candidates. (This large binary fraction, though interesting, isn’t diagnostic, as both star clusters and dwarf galaxies are known to host high proportions of binary stars.)

Further Evidence

Cerny’s team conducted a further test of the Ursa Major III/UNIONS 1’s identity by measuring the metallicities of its 12 brightest stars. They found that the stars are metal poor and have little metallicity dispersion. This finding again points to a star cluster, in which stars form in a single epoch, rather than a dwarf galaxy, which may feature multiple generations of stars with different degrees of chemical enrichment.

metallicity dispersion vs velocity dispersion for faint Milky Way satellites

Upper limits on velocity dispersion and metallicity dispersion for Ursa Major III/UNIONS 1 compared to other faint Milky Way satellites. There are no confirmed dwarf galaxies for which the metallicity dispersion and velocity dispersion are both lower than for Ursa Major III/UNIONS 1. Click to enlarge. [Cerny et al. 2026]

Based on the newly collected evidence, Cerny and collaborators concluded that Ursa Major III/UNIONS 1 is likely a star cluster, though the possibility that it’s instead a dwarf galaxy could not be ruled out. If the star cluster classification holds, Ursa Major III/UNIONS 1 would be the least luminous metal-poor star cluster known. This classification may also imply the presence of a large number of compact objects — white dwarfs, neutron stars, or black holes — to keep the cluster intact, filling the role that dark matter would have played.

Going forward, further chemical abundance measurements of Ursa Major III/UNIONS 1 may still sway the argument one way or another. And as facilities like the Vera C. Rubin Observatory and the Euclid space telescope continue to collect data, other extreme Milky Way satellites may come to light.

Citation

“No Observational Evidence for Dark Matter Nor a Large Metallicity Spread in the Extreme Milky Way Satellite Ursa Major III/UNIONS 1,” William Cerny et al 2026 ApJL 999 L8. doi:10.3847/2041-8213/ae29b8

AT 2024wpp

Across the universe and across the electromagnetic spectrum, flares, bursts, and waves are making their mark on the sky. Today, we’re giving an update on three exceptional transients that have been discussed previously on AAS Nova: AT 2024wpp, GRB 250702B, and GW231123.

The Most Luminous Known Fast Blue Optical Transient, AT 2024wpp

Discovered in September 2024, AT 2024wpp burst onto the scene as the most luminous example of a fast blue optical transient (FBOT) — a growing class of events that are more luminous, more rapidly evolving, and bluer than supernovae. In October 2025, astronomers published their findings on AT 2024wpp’s X-ray and radio evolution. Just last month, a research team led by Natalie LeBaron (University of California, Berkeley) described the results of a complementary investigation of the transient’s ultraviolet, optical, and near-infrared behavior.

AT 2024wpp light curves

The ultraviolet through infrared light curve of AT 2024wpp. Results from different filters are offset vertically for clarity. [LeBaron et al. 2026]

LeBaron and collaborators collected ultraviolet through infrared photometry as well as optical and infrared spectroscopy to characterize the event. These efforts led to the first-ever collection of ultraviolet data during the rising period of an FBOT. Overall, the data reveal an almost featureless spectrum from the optical through the near-infrared and a persistent, weeks-long thermal continuum, both of which seem to be common for FBOTs.

The observations point to a luminous central energy source that continuously injects energy into its surroundings, powers multiple outflows, and ionizes the ejected material. These features can be explained by the rapid accretion of material by a neutron star or a black hole, in which the accreted material forms a disk and is ejected in both a fast-moving polar outflow and a slower-moving equatorial outflow.

Though the exact nature of the central object powering AT 2024wpp’s intense radiation isn’t known — the extreme luminosity of this event points to a stellar-mass black hole as the central object, but a neutron star cannot be ruled out — the overall picture of this event matches what has been found for previous FBOTs. Upcoming instruments and surveys will discover more FBOTs and spot them earlier in their evolution, transforming our understanding of these rare transients.

An Ultra-Long X-Ray and Gamma-Ray Transient, GRB 250702B

GRB 250702B is an exceptional gamma-ray burst: an intense flash of gamma rays lasting anywhere from a few milliseconds to several hours. GRB 250702B is unprecedented among events in this class because it features several bursts arising over the course of roughly 7 hours, plus X-ray precursor emission going back 24 hours before discovery. We’ve previously covered JWST follow-up observations of GRB 250702B, and today we’re taking a look at its high-energy evolution.

Swift and Chandra observations of GRB 250702B

Swift (black circles) and Chandra (blue circles) observations of GRB 250702B. Click to enlarge. [Adapted from O’Connor et al. 2025]

Brendan O’Connor (Carnegie Mellon University) and collaborators collected observations of GRB 250702B from the Neil Gehrels Swift Observatory, the Nuclear Spectroscopic Telescope Array (NuSTAR), and the Chandra X-ray Observatory, stretching from just half a day after the transient was discovered to 65 days later. The team aimed to determine whether GRB 250702B fit the profile of an ultra-long gamma-ray burst accompanying a star’s collapse into a black hole or a relativistic tidal disruption event, in which a star is ripped apart by a massive black hole.

O’Connor’s team found that GRB 250702B didn’t fit neatly into either category. While many of the event’s properties closely resembled those of ultra-long gamma-ray bursts, the hours-long gamma-ray emission and the precursor X-ray emission were outliers. Certain features are more easily reconciled with the tidal disruption of a main-sequence star by an intermediate-mass black hole, but others are similarly hard to account for. Finding the smoking gun for a tidal disruption event — an abrupt shutoff to the X-ray emission — will require long-term X-ray monitoring.

Because GRB 250702B has features that align with both ultra-long gamma-ray bursts and tidal disruption events, O’Connor and coauthors proposed scenarios that combine aspects of both possibilities: either a stellar-mass black hole tidally disrupting a star or the merger of a helium star with a stellar-mass black hole.

The Most Massive Gravitational Wave Source, GW231123

When compact objects like neutron stars and black holes merge, their collisions produce ripples in spacetime that are accessible to detectors on Earth. One of the most exciting events so far from the fourth gravitational wave transient catalog is GW231123, which appears to be caused by the merger of two black holes, each at least 100 times the mass of the Sun. The LIGO, Virgo, and KAGRA (LVK) collaborations reported a total mass of 238 solar masses for the event, which would make it the most massive binary black hole merger yet observed.

In a recent article, Ilya Mandel (Monash University and OzGrav) urged caution when interpreting seemingly extreme events like GW231123. Mandel’s argument is a statistical one, highlighting how certain aspects of the Bayesian techniques used to estimate GW231123’s mass could have skewed that estimate toward an artificially high value.

posterior probability distributions for maximum mass of GW231123 and all gravitational wave events seen by LVK

The posterior probability distribution on the maximum mass as derived from the total LVK catalog (blue), compared to the posterior for GW231123 (orange; multiplied by 10 for visibility). Click to enlarge. [Mandel 2026]

In general, Mandel points out, events that appear extreme are simply more likely to be statistical fluctuations. As an example, consider a large group of people having their heights measured, but with very large measurement uncertainties. The highest reported height in this situation is likely to be associated with a reasonably tall person whose height had a large upward measurement error, not necessarily someone who is exceptionally tall. Similarly, the reported most massive gravitational wave source is likely not quite as massive as claimed.

Mandel also demonstrated how considering the population of black hole mergers detected by LVK as a whole can provide a different perspective on GW231123’s likely mass. By analyzing this entire sample together, Mandel showed that the maximum mass for the population of merging black holes appears to be considerably lower than the mass estimated for GW231123, again raising the possibility that the true mass of the event is lower than the current estimate.

Ultimately, Mandel pointed out that at present it’s impossible to distinguish between three possibilities: that GW231123 is a true outlier among the population of merging black holes, representing a member of a new, unexplored population; that it’s a member of the same population as other black hole mergers seen by LVK but inhabits an extreme high-mass tail; or its high mass estimate is a statistical artifact.

Citation

“The Most Luminous Known Fast Blue Optical Transient AT 2024wpp: Unprecedented Evolution and Properties in the Ultraviolet to the Near-Infrared,” Natalie LeBaron et al 2026 ApJL 997 L10. doi:10.3847/2041-8213/ae2910

“Comprehensive X-Ray Observations of the Exceptional Ultralong X-Ray and Gamma-Ray Transient GRB 250702B with Swift, NuSTAR, and Chandra: Insights from the X-Ray Afterglow Properties,” Brendan O’Connor et al 2025 ApJL 994 L17. doi:10.3847/2041-8213/ae1741

“What Is the Most Massive Gravitational-Wave Source?” Ilya Mandel 2026 ApJL 996 L4. doi:10.3847/2041-8213/ae278d

illustration of a tidal disruption event

Where do the mysterious cosmic messengers called neutrinos come from? Researchers search for a connection between neutrinos and accretion flares from black holes.

Seeking Connections

A photograph of the IceCube Neutrino Observatory

The IceCube Laboratory at the Amundsen-Scott South Pole Station in Antarctica. [Felipe Pedreros, IceCube/NSF]

Neutrinos — neutral, nearly massless elementary particles — are generated by natural particle accelerators across the universe. Facilities like the IceCube Neutrino Observatory, which employs detectors buried a mile deep in Antarctic ice, spot neutrinos from distant astrophysical sources, but matching these neutrinos to their sources is no easy task.

Thus far, researchers have discovered a compelling connection between neutrinos and distant active galactic nuclei, which are powered by the accretion of gas onto supermassive black holes. The active galactic nucleus phase of a black hole tends to be long lived, with activity lasting hundreds of thousands of years or more, making these nuclei lasting sources of neutrinos.

Sampling Flares

Recently, researchers have found tentative evidence for coincident neutrinos from short-lived accretion events like tidal disruption events, in which a star is torn apart and accreted by a black hole. These studies have uncovered a potential connection between neutrinos and tidal disruption events with mid-infrared echoes, signaling the presence of obscuring dust.

map of sampled black hole flares and neutrinos

A map of the locations of the neutrinos (yellow diamonds) and black hole flares (blue circles) on the sky. Click to enlarge. [Wang et al. 2026]

To investigate whether there is any connection between neutrinos and short-lived accretion events, Megan Wang (Massachusetts Institute of Technology) and collaborators first compiled a sample of 99 black hole flares observed by NEOWISE. These flares were selected for their strong mid-infrared emission, location in the nuclei of galaxies, and a fast rise and slow decline. The team applied additional selection criteria to remove other bright transients, such as supernovae, and retain only tidal disruption events and active galactic nucleus accretion flares.

For the neutrino side of the equation, Wang’s team turned to IceCube’s most recent catalog of neutrino events from astrophysical sources. In total, they compiled a sample of 68 events for which the odds of the source being astrophysical was greater than 50%, and for which the directional uncertainty was below 50.

No Coincidences

For a flare and a neutrino to be considered spatially and temporally coincident, the neutrino must arrive within one year of the flare, and the flare must fall within the 90% certainty contours of the neutrino. Ultimately, Wang’s team found no neutrinos that fulfilled both criteria and only one that was spatially but not temporally coincident with a black hole flare.

This result appears to be at odds with previous studies that found several neutrinos that were coincident with tidal disruption events. Wang and collaborators noted that while this work finds no association between neutrinos and black hole flares, it’s possible that the team is studying a different population of accretion events.

infrared fluxes of black hole flares

Infrared fluxes of the flares sampled in this study (magenta), optically identified flares (gray), and the tidal disruption events thought to have coincided with neutrino detections (green and gold dashed lines). [Wang et al. 2026]

For example, while the previous neutrino-associated flares were flagged for having strong mid-infrared emission, they differ from the current sample in that they were detected at optical wavelengths. The flares sampled in this work were detected in the mid-infrared and have little to no optical emission, potentially highlighting intrinsic differences compared to previous samples.

Future work, including studies that leverage upcoming datasets from the Vera C. Rubin Observatory, the Roman Space Telescope, and the NEO Surveyor, can address this question further and search for connections with specific populations of black hole flares.

Citation

“Testing the Association of Supermassive Black Hole Infrared Flares and High-Energy Neutrinos,” Megan Wang et al 2026 ApJL 998 L29. doi:10.3847/2041-8213/ae3f90

LMC

Small galaxies are expected to have even smaller companion galaxies, according to current cosmological models. A recent study takes a look at one dwarf galaxy and finds more tiny companions than anticipated. 

Cosmology Predicts Satellite Galaxies

Critical to our understanding of dark matter and galaxy formation, satellite galaxies are small, faint galaxies surrounding a larger host galaxy. The leading cosmological model, known as lambda cold dark matter (ΛCDM), predicts dark matter halos of all masses to host smaller dark matter subhalos whose luminous counterparts make up satellite galaxies. 

While the satellite systems of Milky Way-like galaxies are well characterized, the companions of dwarf galaxies are less well studied. Despite being significantly smaller, dwarf galaxies should also host even smaller satellites galaxies around them. These satellites push the limits of observations — extremely faint and hard to distinguish from background sources, dwarf galaxy satellites require innovative imaging techniques to discover. 

In recent years, new surveys have identified a number of satellites around dwarf galaxies that appear broadly consistent with ΛCDM predictions, though the sample size is still small. Continuing the search for little galaxies’ little galaxies is critical to testing the ΛCDM framework and will further our understanding of galaxy formation on the smallest scales. 

DDO 161 satellite map

Satellite candidates around DDO 161 as they appear on the sky. Click to enlarge. [Li et al 2026]

Searching for Satellites Around DDO 161

Diving into a specific system, a team led by Jiaxuan Li (李嘉轩) from Princeton University began an in-depth search for satellites around the dwarf galaxy DDO 161. Prior to this study, DDO 161 was known to have one companion, UGCA 319, that is about 10 times smaller than its host. 

The authors used data from the Legacy Surveys to search for satellites within about 400,000 light-years of DDO 161 — the expected reach of DDO 161’s gravitational influence. After removing bright sources and smoothing the image to bring out very faint objects, they found eight satellite galaxy candidates including UGCA 319.

Images of the satellite candidates of DDO 161 from the Legacy Surveys. The top row shows the confirmed satellites, and the bottom row shows the rejected satellites. Click to enlarge. [Li et al 2026]

To confirm if these candidates were actual satellites of DDO 161, the team performed follow-up observations of the seven unconfirmed sources with the Magellan 6.2-meter telescope. Using the surface brightness fluctuation technique — measuring how a galaxy’s light fluctuates from pixel to pixel to determine its distance — the authors confirmed that three of the candidates are located at a similar distance to DDO 161 and are likely satellites. With the previously confirmed companion UGCA 319, DDO 161 has four satellite galaxies with stellar masses above 250,000 solar masses, making it the most satellite-rich dwarf galaxy known to date. 

Too Many Satellites

How does DDO 161 compare to theoretical predictions for galaxies of its size? The authors ran cosmological simulations to quantify the expected satellite populations around galaxies of similar size to DDO 161. Just as it is an outlier observationally, DDO 161 is also an outlier when compared to the simulation results that predicted fewer satellite galaxies in similarly massive host systems. 

DDO 161 Satellite Population Comparison

Number of satellites versus host galaxy stellar mass for DDO 161 (red circle) compared to other observed low-mass galaxy satellite systems (data points) and predicted ranges from simulations (blue regions). DDO 161 is a clear outlier, having a high number of satellites for the host galaxy’s stellar mass. Click to enlarge. [Modified from Li et al 2026]

Making sure to cover all bases, the authors considered their observational uncertainties, limitations of the simulations, different relationships between stellar and dark matter halo mass, and possible environmental effects. None of these checks could, at this point, sufficiently explain the satellite overabundance in DDO 161, introducing a possible “too-many-satellites” problem for this galaxy. A larger sample of satellites around dwarf galaxies is necessary to better understand this discrepancy, test our current cosmological framework, and give new insight into galaxy formation on small scales. 

Citation

“A Possible ‘Too-many-satellites’ Problem in the Isolated Dwarf Galaxy DDO 161,” Jiaxuan Li et al 2026 ApJL 998 L24. doi:10.3847/2041-8213/ae3ddd

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