What factors impact how long it takes for a supermassive black hole binary to merge? New research investigates the influence of orbital inclination on the population of merging black holes in our universe.

One Merger Leads to Another

When galaxies merge, the supermassive black holes at their centers are thought to link up in a binary system and eventually merge as well. Evidence for these supermassive black hole pairs and their collisions is piling up: researchers have identified a small but growing number of galaxies containing two accreting supermassive black holes, and the compelling evidence for the gravitational wave background points to the existence of an immense population of supermassive black hole binaries.

With observational evidence for supermassive black hole mergers taking shape, theorists are exploring the details of these mergers. One aspect of this process that hasn’t yet been examined fully is how the relative inclination of the merging galaxies impacts the time it takes for their central black holes to merge — a factor that has the potential to shape the demographics of merging supermassive black holes across our universe.

Ready, Set, Spiral

Sena Ghobadi (Georgia Institute of Technology) and collaborators used 3D dynamical models to explore how the angle at which galaxies collide — and therefore how the black holes’ orbits are tilted relative to one another — impacts the time it takes the supermassive black holes at their centers to merge.

In the simulations, Ghobadi’s team placed a 10⁶–10⁸-solar-mass black hole at the center of a disk galaxy. Then, they sent a second, smaller black hole spiraling toward it from a distance of 1 kiloparsec (3,300 light-years).

The team varied the inclination of the incoming black hole’s orbit relative to the disk of the target galaxy from 0 to 75 degrees. They also explored the impact of changing the black hole masses and the galaxy’s central gas density, disk rotation speed, and gas mass fraction. For each simulation, they recorded how long it took for the incoming smaller black hole to get within 10 parsecs of the larger black hole.

(Dis)inclined to Merge

The simulations showed a clear trend with changing inclination: a black hole with an orbital inclination greater than 20 degrees took longer to merge than those with inclinations of 0–20 degrees. For black holes approaching the merger on slightly inclined orbits, dynamical friction between the black hole and the stars and gas of the galaxy worked to drag the black hole down into the disk, decreasing its inclination over time and guiding it toward a merger. Aside from the influence of inclination, the team also found that higher-mass supermassive black holes and more rapidly rotating galactic disks tended to lead to faster mergers.

For inclinations greater than roughly 45 degrees, a dramatic transition took place, with the orbits of the incoming black holes becoming more inclined over time rather than settling down into the disk. These simulated black holes failed to merge in the allotted time of 14 billion years.

With highly inclined binaries failing to merge within the lifetime of the universe, this suggests that the dual active galactic nuclei and the supermassive black hole binaries that produce gravitational waves in the universe today are the result of binaries with initial inclinations of 20 degrees or less.

Citation

“Evolution of Supermassive Black Hole Pairs on Inclined Orbits in Postmerger Galaxies,” Sena Ghobadi et al 2026 ApJ 999 131. doi:10.3847/1538-4357/ae40bc

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Prior to exploding, massive stars advance through action-packed evolutionary stages that are exciting but often difficult to fully decipher. A recent study focuses on a unique late-stage star to better understand its properties and evolution.

To B[e] a Supergiant

After depleting their cores of hydrogen, massive stars journey off the main sequence through a series of evolutionary phases before ending in dramatic supernova explosions. A massive star’s post-main-sequence evolution is intense, often sloughing off layers of material that can settle into a circumstellar disk. The B[e] supergiant phase describes a blue supergiant star surrounded by a cool, dense circumstellar disk where oxygen is more abundant than carbon. These environments often lock up carbon atoms in carbon monoxide molecules, leaving excess oxygen available to form other molecules including SiO, TiO, and water vapor. These molecules can survive under different physical conditions, making them key tracers of the environments around B[e] supergiants.

One particularly interesting B[e] supergiant, LHA 115-S 18, has been the subject of numerous studies but remains poorly constrained. The star exhibits significant variability both in brightness and emission features on timescales of days to years, and previous works have suggested that the star may have an as-yet unconfirmed binary companion. Molecular line emission from CO and TiO have been identified in LHA 115-S 18, indicating a circumstellar disk, but to date, water vapor emission has not been identified in any B[e] supergiant. Identifying more complex oxygen-bearing molecules like water will further reveal the physical properties and possible binary companion of LHA 115-S 18.

Identifying Emission in LHA 115-S 18

To characterize the emission around LHA 115-S 18, María Laura Arias (Institute of Astrophysics La Plata; National University of La Plata) and collaborators obtained high-resolution near-infrared spectroscopy of the star using the Immersion Grating Infrared Spectrometer on the Gemini South telescope. From the high-resolution spectra, the team identified several atomic emission lines indicative of the star’s gaseous envelope, revealing the structure of material around the star: an inner hot disk of ionized material and an outer cool disk of neutral material. Looking at molecular emission from carbon monoxide — a tracer of both gas motions and stellar age — the authors confirmed the presence of a rotating ring of gas around LHA 115-S 18 and determined that the star is an evolved, post–red supergiant object.

Through computing a synthetic spectrum to compare to the observed spectrum, the authors identified many water vapor emission lines in the circumstellar environment around LHA 115-S 18. While water vapor has been detected in other evolved late-type stars, this is the first detection of water vapor from a B[e] supergiant star. For water vapor to survive the star’s intense radiation, it must exist in a dense, cool, heavily shielded disk around the B[e] supergiant.

Sustaining a cool disk around such a hot star indicates that the circumstellar environment around LHA 115-S 18 is complex and likely shaped by binary interaction. With emission features characteristic of both cool and hot evolved stars, this particular B[e] supergiant presents itself as a unique object holding critical insights into the late-stage evolution of massive stars. Further multiwavelength observations of LHA 115-S 18 will constrain its potential binarity and reveal a possible evolutionary link between B[e] supergiants and other late-stage evolutionary phases of massive stars.

Citation

“High-resolution Near-Infrared Spectroscopy of the B[e] Supergiant LHA 115-S 18: Discovery of Hot Water Vapor Emission,” María Laura Arias et al 2026 ApJL 1000 L49. doi:10.3847/2041-8213/ae524a

The post Water Vapor Around a Sizzling Supergiant: High-Resolution Spectroscopy of B[e] Supergiant LHA 115-S 18 appeared first on AAS Nova.

Researchers have recently discovered a number of ultra-long-period pulsars that are difficult to explain with typical pulsar models. A new article explains how these pulsars might arise from massive stars in close binary systems.

Strangely Slow Sources

When massive stars go supernova, the explosion can leave behind the condensed stellar core in the form of a neutron star. Neutron stars that spin rapidly become pulsars, whose fast rotation powers beams of radio emission along the star’s poles.

Until not too long ago, pulsars appeared to have periods no longer than 12 seconds. Above that “sluggish” rotation rate, researchers suggested, pulsars could no longer produce electron–positron pairs and channel them along polar magnetic field lines to generate their characteristic radio signals.

So it seemed, until unexpectedly slow pulsars began to crop up. The first trend-breaking pulsars had periods of a few dozen seconds, and objects with pulse periods of minutes or hours have now been found. What could these strange objects be?

Neutron Star Origin Story

In a recent article led by Savannah Cary (University of California, Berkeley), researchers proposed that these strange radio sources are pulsars, but pulsars that rotate slowly and have magnetically powered (rather than rotationally powered) radio emission.

The proposed origin story for these ultra-long-period pulsars begins with a binary system containing a massive star and a close stellar companion. As the massive star evolves, it transfers its outer layers to the companion and expires as a stripped-envelope supernova that births a rapidly spinning neutron star. As the supernova ejecta crashes into the companion star, the star heats up and expands to 5–100 times its original radius.

Meanwhile, the explosion delivers a ferocious kick to the newborn neutron star, likely unbinding the binary system. This sets the neutron star on a new course that could take it through the puffed-up outer layers of its former companion, where it may collect gas from the star and ejected material from the supernova into an accretion disk.

Slowing Down

Cary’s team explored the outcomes of this scenario with a trio of simulations: hydrodynamical models for the shock heating of the companion star, numerical models for the formation of the disk around the neutron star, and analytical models for the interaction between the disk and the neutron star.

For a binary system containing stars of 6.4 and 4.0 solar masses and a separation of 20 solar radii — values drawn from observations and simulations — the resulting unbound neutron star collects a disk in 8–10% of cases. The neutron star’s mass, radius, rotation rate, and magnetic field strength determine whether the neutron star and the disk will interact. If they don’t, the neutron star remains rotating rapidly like a typical pulsar. If they do, the neutron star’s rotation will slow to a relative crawl over about a million years. For modest magnetic fields between 10¹³ and 10¹⁴ Gauss, this means rotation periods in the realm of 10² seconds, whereas stronger magnetic fields, above 10¹⁵ Gauss, yield periods of roughly a day.

This work suggests that close binary systems could be the source of ultra-long-period pulsars, of which Cary’s team estimates there may be 10–1,000 in the Milky Way. Future work will explore this possibility further, examining broader swaths of parameter space to learn more about how this scenario could contribute to the population of ultra-long-period pulsars in our galaxy.

Citation

“Accretion from a Shock-Inflated Companion: Spinning Down Neutron Stars to Hour-Long Periods,” Savannah Cary et al 2026 ApJ 996 141. doi:10.3847/1538-4357/ae1d43

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Flashing on and fading quickly, recurrent novae are captivating astronomical phenomena. A recent study identifies one system that challenges our current understanding.

Rapid Recurrent Novae

When a white dwarf — the white-hot remnant of a dead star — has a close binary companion, the white dwarf can pull material from the companion star onto its boiling surface. Ignited in a bright flash, the accreted material is blown out from the white dwarf in a nova explosion that gradually expands and fades over time. Some novae, known as recurrent novae, repeat on observable timescales, creating a “new” star in the sky on periods ranging from one to one hundred years.

Our nearest galactic neighbor, Andromeda, hosts the most rapid recurrent novae observed to date, including the shortest-period known recurrent nova, M31N 2008-12a. M31N 2008-12a has erupted once a year for millions of years and will eventually meet its fate in a supernova explosion. 

Discovered in 2017, the second-shortest-period nova M31N 2017-01e has an outburst about every 2.5 years. This nova has sparked intrigue among researchers due to its low-amplitude outbursts and rapid evolution compared to other recurrent novae. While M31N 2017-01e exhibits some emission features typical for recurrent novae, recent studies have suggested that the system’s companion star may be a moderately young, blue B-type star. Most novae occur in systems where the white dwarf’s companion is a late-type main-sequence, subgiant, or giant star, making M31N 2017-01e an unusual case requiring further investigation. 

Dialing In on M31N 2017-01e

Aiming to constrain the nature and companion star of the nova, Shatakshi Chamoli (Indian Institute of Astrophysics and Pondicherry University) and collaborators performed a multiwavelength analysis using ultraviolet and optical observations of M31N 2017-01e.

In monitoring the nova during and in between outbursts, the authors identified a source at the reported location of M31N 2017-01e that exhibited variability and color consistent with previous observations of the system. Through a detailed photometric analysis, the authors found that the color and emission properties of the source are consistent with a hot, early-type star as was previously suggested. Though all the observational signs point toward a B-type companion, there’s one glaring obstacle to that scenario: such a massive star would typically be unable to transfer the amount of mass necessary to fuel the nova’s frequent eruptions without the accretion becoming unstable.

Be a Companion

What else, then, could the companion be? The authors considered another stellar companion known as a Be star — a rapidly rotating, early-type star that occasionally hosts a disk of loose stellar material. With a blue color and spectral features similar to B-type stars, a Be star could match the observational properties of the nova’s companion while solving the problem of its accretion. Outbursts of M31N 2017-01e likely arise due to the white dwarf lying very near or within the Be star’s circumstellar disk, siphoning material and adequately fueling the system’s recurring eruptions. To confirm this hypothesis, researchers will need to perform follow-up infrared observations to search for the tell-tale signs of a dusty disk around the companion star. 

This system is rare and challenges the assumed properties of recurrent novae. From this study, it is clear that nova progenitors are potentially quite diverse and require further multiwavelength observational programs to identify more systems like M31N 2017-01e.

Citation

“Challenging Classical Paradigms: Recurrent Nova M31N 2017-01e, a BeWD System in M31?” Shatakshi Chamoli et al 2025 ApJ 991 174. doi:10.3847/1538-4357/adf843

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Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Gap–Giant Association: Are Planets Hiding in the Gaps?
Authors: Caleb Lammers and Joshua Winn
Authors’ Institutions: Princeton University
Status: Published in ApJ

The Kepler space telescope played an almost decade-long game of hide-and-seek. After nine and a half years of operation, Kepler detected more than 2,700 planets outside of our solar system primarily by using the transit method, which is particularly efficient at detecting planets that orbit close to their host stars. Astronomers have also been detecting planets with the radial velocity technique, which is better at finding larger planets at longer orbital periods. By looking for planets with both the transit and radial velocity techniques, astronomers can look for both close-in and far-out planets to paint a more complete picture of other solar systems in our galaxy.

Using the Kepler Giant Planet Survey, the authors of today’s article found that among the 26 systems with three or more transiting planets, four were found to also have an outer giant planet. They quickly noticed that instead of an evenly spaced out inner system of planets, every single one of these four systems with the outer giant planets revealed a notable gap between two of their inner planets. They dubbed this phenomenon the “gap–giant association” — in other words, when an outer giant planet exists in a planetary system, there is evidence of a large gap between two of the inner planets, as shown in Figure 1. This pattern has been observed before, but there has been little theoretical work to explain the effects of outer giants on orbital spacings. In today’s article, the authors ask the question: are there planets inside the gap hiding from us? To address this, they conduct simulations to see if (a) these systems can host a hiding planet in their gaps without becoming dynamically unstable and (b) if such a planet can remain hidden when looked for with the transit method.

Close Your Eyes and Count to N

The authors of today’s article model these four systems (Kepler-48, Kepler-65, Kepler-90, and Kepler-139) with a planet added to their gaps and use an N-body simulation (i.e., fancy physics calculations that track how planets gravitationally interact) to evolve the systems over time and evaluate their long-term stability. They find that each of these injected planets has a high survival rate, which means that each of these four systems could stably host an additional ~2–20 Earth-mass planet in its gap for billions of years without falling apart. The authors then calculated whether the outer giants could “hide” these theoretical gap planets by tilting the planets’ orbits enough so that they would not be detectable with the transit method. In order to do this, the outer giant would have to exert a large enough gravitational force on the gap planet to cause it to precess at a rate independent of its neighbors, allowing its orbital inclination to grow. However, they found that the outer giant planets are either too far away or not massive enough to do the job.

Only one system, Kepler-90, has an outer giant that could potentially tilt the gap planet’s orbit enough for it to fly under the radar if the giant were also on a modestly inclined orbit. However, previous detections of Kepler-90’s outer giant suggest that its orbit is well-aligned with those of the inner system, making this hypothesis implausible. They therefore concluded that it is unlikely that planets between ~2 and 20 Earth masses are hiding within these gaps.

Seeking Out Another Option

Before completely abandoning this hypothesis, the authors propose that maybe the gaps do contain planets, but they’re just too small for Kepler to detect. They calculate the detection efficiencies as a function of planet radius and orbital period for each of the four systems and find that planets smaller than about ~0.5–1 Earth radius could have gone unnoticed (as shown in Figure 2). While this would make these systems slightly unusual (most multiplanetary systems have relatively uniform planet sizes), it’s not completely impossible. However, they don’t rule out the possibility that these gaps are truly empty, and that the presence of an outer giant could prevent planets from forming in certain regions or disrupt their orbits after formation. While it is hard to draw any firm conclusions with a sample size of four systems, this study highlights how much we still don’t understand about planetary system architecture, and therefore planetary formation and evolution.

Though the Kepler mission has come to an end, ongoing radial velocity surveys will continue to expand the sample of systems like these four so astronomers can get a clearer picture of what’s behind this gap–giant association.

Original astrobite edited by Annelia Anderson.

About the author, Tori Bonidie:

I am a 5th-year PhD candidate studying exoplanet atmospheres at the University of Pittsburgh. Prior to this, I earned my BA in astrophysics at Franklin and Marshall College where I worked on pulsar detection as a member of NANOGrav. In my free time you can find me cooking, napping with my cat, or reading STEMinist romcoms!

The post Planetary Hide and Seek appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Formation and Evolution of Compact Binaries Containing Intermediate-Mass Black Holes in Dense Star Clusters
Authors: Seungjae Lee et al.
First Author’s Institution: Seoul National University
Status: Published in ApJ

A delicate dance goes on in astronomy between theory and observation. Most astronomy research, by and large, fits into one of two categories. In one category, you start with a theory, model, or simulation, and attempt to figure out what observations you might expect given those initial assumptions. The other approach begins with raw data and attempts to determine what kinds of theories, models, and fundamental astrophysical assumptions might give rise to it. Accepted knowledge usually happens when these two approaches agree and reinforce each other, but a lot of the actual science occurs when one side dances ahead of the other — either we have theories and models that make predictions that cannot yet be observationally tested, or we have data that defy attempts at fitting underlying models.

Another dynamical dance goes on in globular clusters between binary black holes. Today’s article investigates intermediate-mass black holes, or IMBHs, a pristine example of an astronomical topic in which theory has danced ahead of observations. IMBHs fill the gap between stellar mass and supermassive black holes. We can observe black holes in multiple ways, usually from the electromagnetic radiation of the gas disks surrounding some black holes (either stellar or supermassive), or the gravitational waves of two orbiting or merging black holes. Unfortunately, each method requires some extra component — either gas to accrete or a companion to orbit — and IMBHs are far more challenging to observe through these methods. Hence, we have a lot of ideas about how they might form, and scarce observational evidence for their existence.

That hasn’t stopped many researchers from trying to make better models and predictions. One of the most commonly proposed mechanisms for forming IMBHs is in dense globular or nuclear star clusters (up to a million times denser than our region of the Milky Way). In these dense environments, stars and the black holes they produce might have a chance to find and run into each other often enough to reach the masses of the IMBH regime through mergers. Today’s authors use a series of N-body simulations to study these dense stellar clusters, into which they embed IMBHs. Their study investigates the detectability of gravitational waves from intermediate-mass-ratio inspirals (IMRIs). Because so many mergers occur in dense clusters, it allows for more mergers between objects with greater mass differences. The mass ratio between the two objects (usually denoted as ‘q’) is one of the most important parameters for characterizing a merger, and a high q value creates a gravitational wave signature that is different from a low q, even if the total mass of the system is the same.

To investigate the formation and detectability of IMRIs, the authors conducted a suite of direct N-body simulations of star clusters with total masses between 5×10⁴ and 10⁵ solar masses, containing ~10⁵ particles. They include initial conditions for how densely packed the particles should be, given how far they are from the cluster’s center (called a Plummer profile). They also include a rule for how the initial masses of the stars are distributed (called a Kroupa initial mass function). Then, they use a code called Stellar EVolution for N-body (SEVN) to follow the evolution of the stars in the system for 100 million years.

In that time, massive stars in the simulation can evolve into neutron stars and stellar-mass black holes. In addition to simulating the clusters’ stars, the authors embed an IMBH of 300–5,000 solar masses into the clusters. The key to this article is that some of the stellar-mass binary black holes will interact with the IMBH, creating an IMRI, which gives off gravitational waves. This article’s primary goal is to characterize these IMRIs and to test how well current and future gravitational wave detectors will measure them.

In the simulation, when a stellar-mass binary black hole and an IMBH come close, the authors must create a “merger criterion” — essentially, how it is decided whether the two objects merge, and what the subsequent “gravitational wave” would look like. Once they model the wave itself, they turn to five current or future detectors to see how well they will measure those waves if they were at particular astronomical distances from Earth.

One of the primary relationships the authors look at in the cluster is the half-mass radius velocity dispersion of the cluster and the number of IMRI encounters between the IMBH and stellar-mass objects. One can think of the velocity dispersion as a more complicated “average velocity” of the stars in the cluster (a high velocity dispersion means, on average, the stars are moving faster, the half-mass radius is the distance from center that separates the inner half of the stars and outer half of the stars, and we use it because it is more representative of stellar motion in the overall cluster than the packed center or the mellow edge). Figure 1 shows the relationship between the number of IMRI events (y-axis) vs. the velocity dispersion (x-axis) of the cluster where the IMBH sits. Note that the y-axis is logarithmic, so a small increase in velocity dispersion actually results in a large increase in the number of IMRIs. This tells us that the number of events is much more dependent on the properties of the larger cluster than the IMBH itself.

In addition to analyzing the dynamics of the clusters and the mergers within them, the authors go further and attempt to characterize what specific detectors will see. They chose five detectors. The first is advanced LIGO (aLIGO), the current incarnation of the LIGO detector network. Next is the Einstein Telescope (ET), a future proposed European ground-based detector. Following that is the Laser Interferometer Space Antenna (LISA), a space-based detector set to launch in the 2030s that is designed to observe supermassive black hole mergers. Next are two detectors that are less established, the advanced Superconducting Omni-directional Gravitational Radiation Observatory (aSOGRO) and the Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO). aSOGRO is a proposed Earth-based detector that could theoretically measure gravitational waves at lower frequencies than the current LIGO-Virgo-KAGRA (LVK) detectors, and DECIGO is a proposed space-based detector designed to observe gravitational waves in the frequency range between the LVK network and LISA.

How do the authors translate the gravitational wave signal from the IMRI into what might be observed on Earth? First, they must pick a signal-to-noise ratio threshold, which is the ratio of the gravitational wave signal to the background noise in the detector. In this case, they chose a signal-to-noise ratio of 8, a commonly chosen threshold in the gravitational wave literature. From the signal-to-noise ratio, the authors can calculate a horizon distance. The horizon distance is the distance that a detector can measure a specific gravitational wave event with a given set of properties at or above the threshold chosen. Figure 2 shows the horizon distances (y-axis) for each detector for sets of gravitational wave sources with different properties vs. the mass of the IMBH generating the gravitational wave (x-axis). The solid and dotted lines represent two chosen masses of the IMBH’s stellar-mass companion. The figure shows that aLIGO (blue circles) can detect only low-mass IMBHs (≲ 300 solar masses) at a redshift of z ≲ 0.05. ET (red triangles) is sensitive to lower-mass IMBHs, and LISA (green diamonds) excels at detecting higher-mass IMBHs. aSORGO (purple triangles) covers a broad mass range but at lower redshifts. DECIGO (yellow Xs) offers the broadest mass range at the highest redshift.

Today’s authors offer a glimpse into globular clusters as dynamic environments where we might see IMRIs. They also predict how those signals might be received by our current, future, and proposed gravitational wave detectors. As next-generation observatories prepare to listen for these cosmic mergers, this work helps chart a path for where and how we might first detect the elusive IMBH.

Original astrobite edited by Kasper Zoellner.

About the author, William Smith:

Bill is a graduate student in the astrophysics program at Vanderbilt University. He studies gravitational wave populations with a focus on how these populations can help inform cosmology as part of the LIGO Scientific Collaboration. Outside of astrophysics, he also enjoys swimming semi-competitively, music and dancing, cooking, and making the academy a better place for people to live and work.

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