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SDSS J0849+1114

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: Episodic Feedback in Triple Active Galactic Nucleus Candidate SDSS J0849+1114 Revealed by Extended Ionized Gas
Authors: Xiaoyu Xu (许啸宇) et al.
First Author’s Institution: Nanjing University
Status: Published in ApJ

When Galaxies Collide

Galaxies are social creatures; they interact and merge (more astrobites talking about this are here and here)! When galaxies collide, the gravitational chaos acts like a funnel, driving massive amounts of cold gas toward the center. This gas rush has two major consequences: it triggers intense bursts of star formation (known as starbursts), and it feeds the central supermassive black holes, activating them as active galactic nuclei (AGNs).

But the story doesn’t end there. These powerful AGNs don’t just sit and feast on the gas. They launch high-velocity winds or jets that push back against the incoming gas, a process known as AGN feedback. (Read more about it in Astrobites here and here.) This feedback is thought to be the key mechanism by which supermassive black holes regulate their host galaxies, either by heating and expelling the gas (negative feedback, which starves star formation) or, in some cases, by compressing it (positive feedback, which promotes star formation).

While binary AGNs (two AGNs in one merging system) are rare, finding systems with three AGNs in one system is fascinatingly rare. The galaxy SDSS J0849+1114 (J0849+1114) is one such system, featuring three Seyfert 2 AGNs, a type of active galaxy with a bright, compact nucleus whose spectrum shows only narrow emission lines, within a tight region of about 5 kiloparsecs (kpc) or 16,000 light-years. Studying this system gives us a front-row seat to how multiple black holes interact and regulate their host environment during a complex merger. Very Large Array observations reveal that nucleus A (see Figure 1) contains two jets, inner and outer. In contrast, nucleus C has one jet, providing further evidence for the presence of an AGN.

Hubble and Very Large Telescope images of J0849+1114

Figure 1: Left: Hubble Space Telescope image taken in ultraviolet light. Right: Optical image from the VLT/MUSE instrument. The three black holes, nuclei A, B, and C, are marked with black crosses. The white contours are from Hubble, like on the left, and the yellow contours are of the MUSE instrument. We can observe the complex and disturbed morphology resulting from the ongoing merger. [Adapted from Xu et al. 2025]

Peering into the Triple Core with VLT/MUSE

To understand the gas dynamics in J0849+1114, the authors used the Very Large Telescope (VLT) and its Multi-Unit Spectroscopic Explorer (MUSE) instrument. MUSE is an integral-field spectrograph, meaning it provides spectra for every single spatial pixel (or “spaxel”) across the field of view. This allows astronomers to map not just where the light is coming from, but how the gas is moving and what is causing it to glow, all resolved spatially across the galaxy.

The main technique employed was two-component Gaussian fitting of key emission lines like hydrogen alpha (Hα) and ionized oxygen ([O III]λλ4959,5007), which can be seen in Figure 2. The width of a Gaussian line (or its velocity dispersion) in a spectrum tells us how fast the gas is moving. A narrow line means the gas is relatively calm, with most of it moving at similar speeds. A broader line, on the other hand, means the gas velocities are more spread out — some parts are racing toward us, others away — indicating turbulence or outflows. By comparing the widths of different components, astronomers can separate quiet, rotating gas from the high-speed winds launched by the active black holes.

spectra of J0849+1114

Figure 2: Zoomed-in spectra showing the Hβ and [O III] (left) and Hα, [N II], and [S II] (right) emission lines from the spot marked with an “X” in the MUSE image in Figure 1. The blue line shows the observed data, the orange line shows the best-fit model, and the two colored curves (light blue and red) represent the two components of the gas. The pink line shows the leftover differences between the data and the model. The First Component is narrow, representing gas that is relatively settled, often showing signs of rotation or slow movements associated with gravitational disturbances, like tidal tails (low velocity dispersion, σ1 ≤ 50 km/s). The Second Component is broad, representing highly turbulent or fast-moving gas, characteristic of powerful outflows or winds driven by the central AGNs (high velocity dispersion, σ2 > σ1). [Adapted from Xu et al. 2025]

What They Found: Gas Tails and Outflows

The VLT/MUSE observations successfully characterized both the undisturbed (First Component) and turbulent (Second Component) gas across the system.

1. Galactic-Scale Tidal Tails

The slow-moving gas (First Component) revealed extended structures of ionized gas stretching over 10 kpc (33,000 light-years), and in some directions, even more than 15 kpc (49,000 light-years) away from nucleus A. These large, low-velocity gas clouds align well with features known as tidal tails: the stretched-out arms of gas and stars pulled away by the violent gravitational forces of the merger.

2. Two Distinct Outflows Driven by Radio Jets

The fast-moving gas (Second Component) clearly showed two distinct sites: outflows originating from nucleus A and nucleus C.

  • Outflow A: This outflow extends over 5 kpc (16,000 light-years) around nucleus A. The gas kinematics and geometry strongly suggest that this outflow is being driven by nucleus A’s radio jet. This finding is key, as the measured kinetic power of the outflow is about 10 times stronger than what star formation alone could supply, and the current luminosity of the AGN is also insufficient to power it.
  • Outflow C: A smaller but detectable outflow extends about 5.9 kpc (19,000 light-years) around nucleus C, with a lower kinetic power compared to Outflow A. But, like Outflow A, the energetics and velocity gradients suggest this outflow is also linked to nucleus C’s radio jet.

A Black Hole That’s Recently Gone Quiet

The most striking implication of this study relates to the timing of nucleus A’s activity. The presence of extended ionized gas far from the nucleus (in the tidal tails, >10 kpc or 33,000 light-years away) provides a fascinating glimpse into the AGN’s recent past.

The physical conditions of this distant gas were determined using emission line ratios ([O III]/Hα and [N II]/Hα) on the Baldwin, Phillips, and Terlevich (BPT) diagram. A BPT diagram uses emission line ratios to diagnose the energy source that ionizes the gas: star formation, AGN, or shocks. The BPT diagram of J0849+1114 indicates that an AGN currently photoionizes the gas.

By running sophisticated photoionization models, the scientists calculated how luminous nucleus A must have been to ionize the gas currently found 10–15 kpc (33,000–49,000 light-years) away. They discovered that this required nucleus A to be 20–100 times more luminous than it currently is! Since light takes time to travel, and the ionized gas quickly recombines (on timescales of less than 100 years for this gas), this luminous phase must have ended very recently, approximately 30,000–50,000 years ago. This is a long time for us, but just a blink of an eye on cosmic timescales.

The Episode of Self-Regulation

By integrating the findings across different wavelengths and timescales, the current faint luminosity state, the past luminous state inferred from the distant gas, and the presence of radio jets of different ages, the authors propose a model of episodic AGN feedback in nucleus A:

  1. Past Activity (150,000 years ago): An active phase likely launched an outer radio jet, which subsequently drove the large-scale ionized gas outflow observed today.
  2. Peak Ionization (30,000–50,000 years ago): A subsequent burst of high accretion reached its peak, ionizing the distant tidal tails.
  3. Fading and Quenching (Today): The energy released by the jet and/or outflow during the active phase likely expelled or heated the surrounding gas (negative feedback), causing the central accretion disk to run out of fuel. The AGN has since faded rapidly to its current low-accretion state, marked by the appearance of a young inner radio jet.

A Quiet Ending After a Loud Beginning

J0849+1114 is not just a statistical anomaly as a triple AGN candidate; it serves as a crucial case study demonstrating the powerful and rapid effects of AGN feedback. High-resolution observations confirm that violent galaxy mergers trigger both powerful outflows and episodic bursts of extreme luminosity. Crucially, these outflows clear the gas and cause the central supermassive black hole to quickly fade from a luminous quasar phase to a quiet, low-accretion state within tens of thousands of years. This system provides strong, spatially resolved evidence that AGN feedback rapidly suppresses accretion onto the supermassive black hole and shapes the host galaxy on kiloparsec scales during the chaotic drama of galactic mergers.

Original astrobite edited by Lindsey Gordon.

About the author, Sowkhya Shanbhog:

I am currently a first-year PhD student at Scuola Normale Superiore in Pisa, Italy, where I am focusing on studying high-redshift quasars. Prior to this, I completed a dual BS-MS degree at the Indian Institute of Science Education and Research in Pune, India. Now, I am eager to expand my involvement in science communication and outreach initiatives. I have recently developed an interest in cooking, particularly since moving to a new city. I find solace in listening to music during my leisure time.

illustration of a planet getting uncomfortably warm

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: Near the Runaway: The Climate and Habitability of Teegarden’s Star b
Authors: Ryan Boukrouche, Rodrigo Caballero, and Neil T. Lewis
First Author’s Institution: Stockholm University
Status: Published in ApJL

To find life on other planets, we start our search for planets like Earth. (It’s the best example of a habitable planet that we know of!) Earth’s distance from the Sun allows for liquid water to exist on the surface, which is vital for life as we know it. This region, called the “habitable zone,” describes the right atmospheric conditions and proximity to its host star that a planet can have to support liquid water. Earth is also special because of its atmosphere, which contains greenhouse gases such as water vapor and carbon dioxide (CO2); these help keep the average global temperature moderate enough for life to exist.

A few factors govern the habitable zone for a given planetary system: for example, the strength of a star’s radiation, the composition of the planet’s atmosphere, and a planet’s orbital period around its star. This is because a planet’s “energy budget” is dictated by these factors. For a planet to have a “stable” climate, the energy it receives from its star must balance the energy it re-emits. For instance, in a given planetary system, the closer a planet’s orbit is to its host star, the more sunlight it receives and therefore the more energy it must radiate away. This balance is between outgoing longwave radiation, which is heat emitted by the planet, and absorbed/incoming shortwave radiation, which is the sunlight the planet receives from its host star.

Some Earth-like analog planets have piqued the interest of astronomers because observations suggest that they lie within the habitable zones of their respective stars. One system, named after its discoverer, Bonnard Teegarden, features an M-dwarf star with three orbiting Earth-mass planets. It exists about 12.5 light-years away, making it one of the closest planetary systems to us. One of the planets, Teegarden’s Star b, is believed to have very similar characteristics to Earth: a radius of 1.02 REarth and a mass of 1.16 MEarth. Because the planet orbits an M-dwarf star, which is much cooler (temperature-wise) and less massive than our Sun, the planet’s orbit is much, much smaller than Earth’s, with a semimajor axis of 0.0259 au (compared to 1 au for Earth) and an orbital period of 4.9 days (compared to 365 days for Earth).

The authors of today’s article explore how close Teegarden’s Star b is to its habitable zone based on current observations, and how future observations could lead to vastly different interpretations of its habitability.

Exoplanets are small and faint, making it difficult for current technology to study their atmospheres accurately. Modern simulations, known as general circulation models, can model how a planet’s atmosphere might behave under various assumptions. Isca, the climate and atmospheric dynamics model used in this article, has been used to model exoplanetary atmospheres in previous studies. Modeling a planet around an M-dwarf, the authors tried matching the planet’s properties as closely as possible to Earth, using a simple cloud model (which is challenging to model), moist physics (since our atmosphere contains water vapor), and a radiative transfer scheme. In their simulation, the authors also assumed that Teegarden’s Star b is tidally locked, meaning its dayside always faces its sun. For example, our Moon always shows the same side to us on Earth and is considered tidally locked.

They also assigned Teegarden’s Star b a similar atmospheric composition to Earth: 78% nitrogen (N2), 28% oxygen (O2), and a CO2 concentration of 400 parts per million by volume (ppmv). Unlike Earth, they did not include an ozone layer; however, they state that including one did not significantly change the results. Combinations of these different factors were run for 8–41 Earth years, until the Teegarden’s Star b model either reached a stable state (where the energy balance equals 0 W/m²), or it exceeded the runaway greenhouse gas threshold — the point at which the energy balance exceeds zero, and the planet’s surface temperature increases uncontrollably.

The average amount of radiation that a planet receives is called its instellation, or sometimes “insolation” when talking about our Sun. For reference, Earth has an insolation value of around 1365 W/m². Different studies of Teegarden’s Star b suggest an instellation of 1481 W/m² or 1565 W/m², so the authors multiple values in that range (see Fig. 1).

plots of modeled energy balance over time for Teegarden's Star b

Figure 1: For a surface albedo of 0.07 (ocean-dominated), the energy balance for each tested instellation (ISR; 1481–1540 W/m²) is shown. As time increases, a stable atmosphere would consistently reach an energy imbalance value of 0 W/m². In contrast, an unstable (runaway greenhouse) atmosphere would have a value greater than 0 W/m², indicating that it is receiving more energy than emitting. [Boukrouche et al. 2025]

The biggest difference in results comes from the two different instellation values of 1480 and 1565 W/m² (see Fig. 2). For 1480 W/m², Teegarden’s Star b lies just within the habitable zone, but admittedly not by much. It’s only about 20–40 W/m² away from becoming uninhabitable. This would give Teegarden’s Star b a mean surface temperature about 18K higher than that of present-day Earth. Since a difference of one Kelvin is equivalent to a difference of one degree Celsius, this corresponds to a temperature of 18℃ (32.4℉) above your average day on Earth — pretty hot and unlikely to be comfortable for lifeforms similar to ours. The precipitation would also be similar to the wettest regions of the Sahara Desert, so overall, Teegarden’s Star b would be a relatively dry planet. For 1565 W/m², Teegarden’s Star b crosses the runaway greenhouse threshold and cannot be considered habitable for life similar to ours.

global mean surface temperature and planetary albedo of Teegarden's Star b

Figure 2: The global mean surface temperature (top) and planetary albedo (bottom) of Teegarden’s Star b are shown with various instellation values. The filled markers represent models that reached an equilibrium, or stable, state, whereas the empty markers are models that reached a runaway, uninhabitable state. The two vertical black lines represent the two main instellation estimates of 1481 and 1565 W/m². Once the models exceed the ~1520 W/m² instellation value, they start to fail and reach a runaway greenhouse effect. [Boukrouche et al. 2025]

Our best guess of the habitability of Teegarden’s Star b is highly dependent on future observations and measurements. A slight change in the estimate of the planet’s instellation could have drastic consequences for its habitability. The assumptions of the authors’ model — its atmospheric composition and how heat and moisture are transported through the atmosphere — might also affect this habitability. We don’t quite know why Earth has its particular atmospheric composition, especially 78% N2, so why might Teegarden’s Star b have the same? A CO2 concentration of 400 ppmv also aligns with present-day Earth values, but pre-industrial CO2 levels on Earth were approximately 280 ppmv. As for future observations, it might be possible to measure the potential presence of N2 through indirect ways using the Large Interferometer For Exoplanets (LIFE). Teegarden’s Star b is still a promising Earth analog, but we’ll need better information in the future to constrain some of these assumptions.

Original astrobite edited by Joe Williams.

About the author, Mckenzie Ferrari:

I’m currently a PhD student in the geophysical sciences program at the University of Chicago. While I now study the atmosphere and oceans of Earth, most of my previous research focused on simulations of Type Ia supernovae and galaxy formation and evolution. In my free time, I foster cats for a local organization, enjoy cooking, and can often be found running along Lake Michigan.

artist's impression of a quasar

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: X-Ray Investigation of Possible Super-Eddington Accretion in a Radio-Loud Quasar at z = 6.13
Authors: Luca Ighina et al.
First Author’s Institution: Harvard–Smithsonian Center for Astrophysics and Italian National Institute for Astrophysics
Status: Published in ApJL

Quasars are some of the most extreme objects in the entire universe. Despite being as far as tens of billions of light-years away, they nonetheless can appear as bright as some stars in our own Milky Way. To put their brightness in perspective, one just has to consider the Sun. Compared to puny human scales, our star is a truly gargantuan object. It’s a fully functional fusion reactor, churning hydrogen into helium in its core and outputting an unfathomable amount of energy in the process. This glowing furnace is so bright that it can cook us with the heat of an oven during the day, even at a distance of more than 90 million miles (about 8 light-minutes). That distance dramatically dilutes the radiation of the Sun by the time it reaches us, as we only receive a small sliver of its total output, and yet we can still feel its heat pounding down on us when we stand beneath a clear summer sky.

However, if one were to move the Sun about 30 light-years away, it would appear completely unremarkable. You’d need to be under relatively dark skies to even spot it! The vastness of space would simply crush the output of our solar engine; however, if you place a luminous quasar at that same 30 light-year distance, its searing radiation would seem to effortlessly cross the cosmic gulf looming between the stars, scorching us relentlessly with the same heat as the Sun does today. The power of a quasar simply puts our star to shame.

Still, even quasars have limits. The source of power for these objects is a supermassive black hole. The supermassive black hole that lives at the center of our Milky Way is currently dormant. However, many supermassive black holes (especially in the early universe) actively gulp down matter, releasing tremendous energy that escapes in the form of radiation across the electromagnetic spectrum. The escaping photons bump into particles on their way out, exerting an outward pressure. If enough light is unleashed by the quasar, this pressure will actually balance against the pull of gravity, cutting off the food supply for the black hole. This negative feedback loop means that a given quasar has an upper limit to its brightness, called the Eddington luminosity, and to the speed at which it accretes matter, called the Eddington rate. The authors of today’s bite examine a particularly misbehaved quasar that seems to violate even these extreme limits.

It’s a Bird! It’s a Plane! It’s a Jet?

The authors observed the quasar RACS J032021.44−352104.1 (RACS J0320−35 for short) with several radio observatories, including the Giant Metrewave Radio Telescope, the Australia Telescope Compact Array, and the Australian Large Baseline Array. Combining this data with publicly available observations, they find that the source is “radio-loud,” or bright at very long wavelengths. Typically, this kind of emission is expected to be generated by powerful jets that are ejected from the poles of a quasar. For example, the authors compare this radio-loud quasar to similar sources, which show significant variability in the X-ray part of the spectrum. This is a tell-tale sign that the X-ray emission is also generated by these jets (which might come out in fits and spurts and thus cause fluctuations in brightness over time).

However, when the authors examine X-ray data of RACS J0320−35 taken by the Chandra X-ray Observatory, they find surprising results. The quasar is extremely luminous in X-rays, making it one of the brightest in the early universe. But despite the fact that it pumps out a huge number of these energetic photons, it seems to preferentially emit only the lower-energy band of X-rays and completely lacks the highest-energy emission that characterizes similar sources. In technical terms, the X-ray spectrum of RACS J0320−35 is incredibly “soft.” Moreover, this X-ray emission seems to be constant on the timescale of months, though further observations will be required to test if it varies on longer timescales. Still, the softness of the spectrum and weak variability of this system mean that its X-ray emission is unlikely to be produced by a jet.

The authors carefully consider a particular variety of jet — one that is sharply angled towards us. Since quasar jets often travel at incredible speeds, the special theory of relativity kicks in and causes strange behaviors. In particular, an effect called relativistic beaming can cause a source to appear much brighter if traveling towards the observer at extremely high speeds. However, they find that although relativistic beaming can explain the mysterious X-ray properties of RACS J0320−35, such a scenario is incompatible with its observed radio emission. Moreover, a lack of gamma-ray emission and weak variability are further pieces of evidence against the jet origin of the X-ray emission.

Limits Are Meant to Be Broken

The article presents one more fascinating possibility for the origin of the X-rays in this source. Some theoretical work and simulations show that if a black hole breaks the Eddington limit and starts accreting matter at “super-Eddington rates,” the X-ray spectrum that results can be incredibly soft. The authors find excellent agreement between the observations and the predictions from a particular model that simulates a very slowly spinning black hole (see Figure 1). This scenario seems to be a promising explanation for the X-ray emission in RACS J0320−35, which is extremely exciting for several reasons.

plot comparing the observed energy spectrum of RACS J0320−35 and the theoretical spectra predicted by several models

Figure 1: A plot showing the observed energy spectrum of RACS J0320−35 and the theoretical spectra predicted by several models. In particular, the green circles (visible light and ultraviolet (UV) emission), blue diamonds (X-ray emission), and red squares (radio emission) represent the real observations. The shaded pink region represents the predictions of a model that simulates a black hole spinning very slowly and accreting at super-Eddington rates. The model seems to match the observed X-ray and visible/UV emission excellently. However, explaining the observed radio emission might require conditions that violate the assumptions behind this scenario. [Adapted from Ighina et al. 2025]

Many astronomers are considering super-Eddington accretion to explain the masses of early quasars and their fainter counterparts, called active galactic nuclei. Because supermassive black holes are supposed to have a cap on their accretion rate (the Eddington limit), the earliest black holes in the universe should only have been able to reach large sizes after sufficient time had passed. Astonishingly, observations from JWST are finding massive active galactic nuclei everywhere in the early universe, which seems to violate this — these black holes appear astonishingly massive despite existing for only a fraction of the universe’s current age. However, if early black holes could grow faster than expected by undergoing super-Eddington accretion, this tension might be resolved.

Despite the promising initial results of this model, there are a few caveats to the results presented in today’s article. For example, the radio jets observed from this source require a rapidly spinning black hole, which conflicts with the model assumptions. The authors note that a radio jet can be very far from a black hole, tracing its past activity. In fact, this jet may have actually spun down the central supermassive black hole by extracting energy from it, a fascinating possibility that will require both further observations and simulations to explore. Today’s bite shines a brilliant light on this possibility by analyzing RACS J0320−35, a fascinating quasar in the early universe and a stunning example that cosmic limits are meant to be broken.

Original astrobite edited by Veronika Dornan.

About the author, Ansh Gupta:

I’m an astronomy graduate student at the University of Texas at Austin working with Steven Finkelstein. I use data from JWST to study the formation and growth of the first galaxies and black holes in the universe. In my spare time, I enjoy playing piano, reading, and making YouTube videos.

Illustration of stellar-mass black holes embedded within the accretion disk of a supermassive black hole

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 Fate of EMRI-IMRI Pairs in Active Galactic Nucleus Accretion Disks: Hydrodynamical and Three-body Simulations
Authors: Peng Peng et al.
First Author’s Institution: Peking University
Status: Published in ApJ

Active galactic nuclei (AGNs) are the extremely luminous central regions of some galaxies, powered by gas accreting onto their supermassive black holes and often outshining the entire galaxy in which they reside. One reason they are so studied in astronomy is that they connect many pieces of physics and astronomy in one cosmic place. This is especially true for AGNs as potential gravitational wave sources. Gravitational waves are observed when two compact objects, usually black holes, orbit each other. Black holes span a massive range of masses, but they are typically categorized into one of three categories: stellar-mass black holes, or sBHs (tens to hundreds of times the mass of the Sun), intermediate-mass black holes or IMBHs (hundreds to thousands of times the mass of the Sun), and supermassive black holes or SMBHs (millions to billions of times the mass of the Sun). AGNs are special because they are among the very few places where black holes across this entire mass spectrum might be found in the same place at the same time. Not only do they host SMBHs at their centers, but their gas disks are ideal nurseries for capturing and growing sBHs and IMBHs.

Current-generation gravitational wave detectors like the LIGO–Virgo–KAGRA (LVK) network can observe stellar-mass to lite-IMBH black hole mergers. Future detectors like the Laser Interferometer Space Antenna (LISA) will be able to observe black holes in the intermediate-mass to supermassive mass range. In addition to the mass range a detector can detect, it is also valuable to know the mass ratio (usually denoted by q) that a detector might detect. Unequal-mass-ratio mergers can tell us a lot about general relativity that more equal-mass mergers cannot because the smaller object orbits the more massive one many times right before the merger, essentially providing a gravitational wave measurement of spacetime around the larger black hole. (See this video for an example of black hole orbits with a large mass ratio, and imagine the spacetime observations around the larger black hole that could be possible with orbits like that.)

One of the most promising advances in gravitational wave detection with LISA will come with the observation of extreme-mass-ratio inspirals (EMRIs), usually defined as involving a smaller black hole that is at least 10,000 times less massive than the massive black hole it orbits (though the exact mass ratio defining an EMRI is a matter of convention and may vary somewhat). In addition, LISA will be able to observe intermediate-mass-ratio inspirals (IMRIs), usually defined as when the smaller black hole is 100–10,000 times less massive than the larger one.

This article uses multiple techniques to address the question of what happens when an AGN disk hosts both an IMBH and an sBH at the same time. The authors began with a hydrodynamic simulation of an AGN gas disk around a 106-solar-mass SMBH. They add a 103-solar-mass IMBH into the gas disk. Because the IMBH orbits within a gas disk, the gas exerts a force on it, causing its orbit to shrink toward the SMBH (a process called migration). Additionally, the IMBH carves out a path through the gas. Once the IMBH carves out enough of a path in the gas, the authors add an sBH of 20 solar masses into the simulation near the IMBH.

The authors test the sBH outcome for two initial conditions of the gas disk. The first, which I will refer to as “InnerDisk,” is when gas already exists inside the IMBH’s orbit (see Figure 1). The other, which I will refer to as “NoInnerDisk,” is when the simulation begins with gas only outside the IMBHs orbit, with no gas initially between the IMBH and SMBH. In this case, gas crosses the IMBH’s gap after the simulation starts. In the InnerDisk case, the sBH initially gets pushed inward from the presence of the inner gas, but that gas steadily drains into the SMBH and is only partly refilled, so the gas’s push on the sBH weakens over time. In the NoInnerDisk case, the IMBH’s direct pull on the sBH becomes more important. The amount of gas that leaks across the IMBH orbit into the inner disk gradually settles into a steady state that is less dense than the InnerDisk case. With a weaker gas push, the sBH stays closely tied to the IMBH and migrates inward at nearly the same rate. In both setups, the IMBH carves a gap in the gas and keeps moving inward, but the presence and evolution of inner gas chiefly determine how closely the sBH can keep up.

Simulated gas disk around an SMBH with an implanted IMBH

Figure 1: Simulated gas disk around an SMBH with an implanted IMBH at 0 days (left), 10.3 days (middle), and 155 days (right). Brighter orange indicates higher gas density, and darker orange/red indicates lower gas density. An sBH was added at 100 days and is seen in the right panel. These three panels represent the “InnerDisk” scenario. [Adapted from Peng et al. 2025]

Once the sBH and IMBH migrate close enough to the SMBH, gravitational waves are responsible for more and more of the energy loss and orbital decay of the system compared to the gas. To account for that, once the sBH and IMBH migrate close enough to the SMBH, instead of using a hydrodynamic simulation of a gas disk, the authors switch over to a “three-body problem” solver. Because these are black holes emitting gravitational waves, regular old Newtonian mechanics is insufficient, so they add post-Newtonian terms to correct for this. Additionally, though they no longer model the gas hydrodynamically, they do include terms for a gas “force” acting on the black holes to mimic the gas disk.

Once the IMBH and sBH were in the gravitational regime, their outcomes became much more chaotic. As seen in Figure 2, a slight change in the initial phase angle of the sBH can lead to drastically different consequences for the system. In some cases, the sBH is ejected entirely. In other instances, it merges with the IMBH soon after the simulation begins. Yet in others, it first merges with the SMBH. This leads to one overall message of this article: the orbits of the IMBH and sBH tend to be regular with some gentle variation when they are farther out in the gas disk, but they become highly chaotic once they shrink into the gravitational wave regime closer to the central SMBH.

Figure showing the outcomes of 100 different simulations

Figure 2: Post-Newtonian simulation outcomes for different values of sBH initial phase angle from 0 to 2π in increments of 0.02π, while all other initial conditions were kept fixed. Green (binary formation) represents a merger of the IMBH and sBH before reaching the SMBH; red (EMRI after ejection) represents the sBH being ejected from the system, but not before some of its orbits can be observed as an EMRI event; EMRI-IMRI (orange) represents the sBH merging with the SMBH, followed by the merger of the IMBH with the SMBH; and blue (ejection) represents the sBH being ejected from the system entirely before entering a gravitational wave EMRI regime. [Peng et al. 2025]

AGN disks may provide a natural setting for interactions among stellar-mass, intermediate-mass, and supermassive black holes. The authors of this article demonstrated that gas can keep an IMBH and an inner sBH migrating together until gravitational waves dominate, after which slight differences in their orbital phases can lead to a wide range of outcomes. While uncertainties remain, this study provides more evidence that LISA could identify. Until LISA flies, continued simulations like this one will help refine the spectrum of EMRIs and IMRIs we can expect to see.

Original astrobite edited by Maggie Verrico.

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.

illustration of K2-18b

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: Soot Planets Instead of Water Worlds
Authors: Jie Li et al.
First Author’s Institution: University of Michigan
Status: Published in ApJL

They Might Be Planets with a Lot of Water…

Since the discovery of PSR B1257+12 c and d in 1992 and  51 Pegasi b in 1995, we have found evidence for thousands of planets in other star systems. One of the most striking things (aside from how common planets seem to be) is how many of them are so unlike anything we had imagined we would find. Our growing list of exoplanets includes a truly remarkable variety of types, from cold rocky planets smaller than Earth to scorching hot giants bigger than Jupiter.

However, one category of planets is particularly interesting: the sub-Neptunes. These planets, smaller than Neptune but larger than Earth, are characterized by their low densities, which suggests they could be dominated by water or volatile-rich atmospheres. What makes sub-Neptunes so intriguing is that we don’t have a clear counterpart for them in our own solar system. As such, we don’t really know a lot about them other than there seem to be a lot of them out there.

Although these planets are sometimes theorised to be rocky worlds with large hydrogen–helium envelopes, they have alternatively been considered as water worlds, i.e., worlds with giant planet-wide oceans thousands of kilometres deep. The thinking goes that since water ice appears to be abundant beyond the snow line, water worlds would be a natural consequence of planet formation. And if these planets exist, some might host a temperate liquid ocean with the conditions for life.

…or They May Just Be CHON(ky)

However, today’s article suggests that these supposed water worlds may not be as wet as we think they are. They may instead be rich in what are called refractory carbonaceous materials. This term describes solids rich in carbon, hydrogen, oxygen, and nitrogen, or CHON. It is a bit of a mouthful and is often just referred to as “soot,” but it is important to remember that it is different from the black stuff you would find in ye old chimney. Soot, in this case, is a major component of comets. We know that this type of material is present around planet formation, as protoplanetary dust contains not just silicates (rock) and water ice but also a significant amount of CHON, and comets are leftovers from this dust.

Soot is stable and remains in the solid state to much greater temperatures (∼500K) than water ice (∼160K), so the authors argue that there should be regions in the protoplanetary disk where planets accrete both rock and soot but little water. They define a “soot line” akin to the snow line and look at three archetypical planets that may form, shown in Figure 1.

illustration of rocky planets, soot planets, and soot-water planets based on their formation location

Figure 1: Illustration of a protoplanetary disk and three chemically distinct planet types that may form as the distance from the host star increases. Close in, the temperature in the disk is too high for volatiles to exist in the solid state, but farther out, the temperature drops to allow for water to freeze into ice beyond the water ice line, also known as the snow line. Between the two regions lies another where it’s cool enough for carbonaceous materials or “soot” to avoid destruction via thermally driven reactions. Depending on where planets form, they may contain a varying amount of astrophysical soot. [Li et al. 2026]

Inside the soot line, planets would be rock-rich worlds with low carbon or water content (e.g., terrestrial planets) because it is just too hot for any soot to stay together. Beyond the soot line but before the snow line, you would find carbon-rich rocky worlds (soot planets). They have low water content, as it is still too hot for water ice to exist, but these planets are rich in CHON. Beyond the snow line, a combination of rock/carbon/water worlds becomes possible, here labelled as soot-water worlds. The authors note that even though the last one has a significant fraction of water, it is distinct from traditional “water worlds” because it includes a significant component of hydrocarbon-rich material. Again, it is also important to remember that a soot world wouldn’t mean a black powder ball hanging in space, but rather a world that is composed of a lot of CHON, like Saturn’s moon Titan.

What Do the Models Say?

The authors got down to modelling planet compositions based on both observations of protoplanetary disks and the distribution of solid materials found in comets.

They considered two model planets. One is fully stratified, i.e., with a metallic core enclosed in a silicate mantle overlain by a hydrocarbon-rich layer and then a water-ice surface layer. The other, a single-layer mixed planet, is a hypothesised scenario where iron, silicate, soot, and water are fully mixed throughout the planet as a result of exotic chemistry from the high temperature inside the sub-Neptune planet. They expect any potential real planets to lie somewhere between the two extremes. The mass–radius relations for these models can be seen in Figure 2, where they are also compared to a number of known exoplanets, of which several fall within the models’ parameter spaces.

Mass–radius relations for model Earth-like rocky planets

Figure 2: Mass–radius relations for model Earth-like rocky planets (black curves), soot planets (gray bands), and soot-water worlds (blue bands). On the left are multi-layer planets, while on the right are single-layer planets. Overlaid are a number of exoplanets along with their respective uncertainties. Also shown as dashed lines are models for Earth-like planets with 50% rock and 50% water. These fall squarely within the same region as soot-water worlds, making the two indistinguishable from each other. [Li et al. 2026

A particularly interesting result that the authors note is that the predicted mass–radius relationship for the water worlds, which incorporates soot, is similar to that predicted previously for a 50% water planet with no carbon. That is, if you base your interpretation on the mass–radius relationship alone, it is impossible to distinguish between a world made of rock and water and a water-rich planet that incorporates a significant amount of soot.

We Might Have a Telescope That Can Help

How might we break through this impasse? Well, because significant fractions of methane and other simple hydrocarbons are expected to be released from the interior, the soot-rich planets may feature methane-rich atmospheres. These may naturally lead to the formation of hydrocarbon hazes, akin to the tholins in Titan’s atmosphere.

Looking at the atmospheres of exoplanets is one of the main mission goals of JWST. Many of the spectra from sub-Neptunes have so far been featureless, which may indicate the presence of clouds or photochemical haze. The telescope also has the ability to detect carbon-bearing species in the atmospheres of other sub-Neptunes, like with the discovery of CO2 and CH4 in the atmospheres of K2-18b and TOI-270d. Although these planets currently orbit interior to the soot lines of their respective stars, they may have originally formed farther out and later migrated inward during their evolution. Of particular interest is TOI-270d. Aside from also showing signs of water, it has a carbon-to-oxygen ratio that is moderately high for the planet, hinting that it could be a world with a considerable amount of soot.

The presence of soot may have significant implications when it comes to habitability. The planet’s core may be rich in diamond, which would impede the movement of volatiles in the mantle. This would make it challenging for the planet to generate a magnetic field and thus leaving any potential life vulnerable to cosmic radiation. However, they could also be abundant in methane and other volatile organic compounds, substances thought to be crucial for the development of prebiotic chemistry. Regardless, it is interesting that there might be something out there that is not so unlike something we know from our own solar system, a hazy supersized Titan. While the frigid moon is unlikely to show signs of life, a temperate soot-water world might be one place to look in the future.

Original astrobite edited by Sowkhya Shanbhog.

About the author, Kasper Zoellner:

I have a Master of Science in astronomy and I am currently working towards a PhD in physics and educational science. My greatest passion is the search for exoplanets and how stellar variability may influence the possibility of life. I am also interested in science outreach, education and discussing what sci-fi novel to read next!

galaxies known as little red dots

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 Discovery of Little Red Dots in the Local Universe: Signatures of Cool Gas Envelopes
Authors: Xiaojing Lin et al.
First Author’s Institution: Tsinghua University and University of Arizona
Status: Published in ApJ

Astronomers are using JWST to search the early universe for strange and captivating objects, and the results so far have exceeded all expectations. For every question we definitively answer, several deeper ones seem to emerge. The study of active galactic nuclei (AGNs), black holes that are actively consuming matter at the centers of galaxies across the universe, is no exception. Our pre-JWST understanding of AGNs has been completely shattered by the discovery of an astounding population of objects called little red dots (LRDs).

We know these objects host active black holes for several reasons. For one, they’re incredibly small despite being extremely energetic (that’s the “little” in their name). The smallest LRD we’ve seen appears to be less than 100 light-years across — for reference, the nearest star to the Sun is 4 light-years away — and yet is still as bright as a galaxy. The most plausible explanation for such brightness and compactness is a supermassive black hole that’s actively consuming matter. As further evidence for this hypothesis, we observe broad Balmer emission lines, spectral features that serve as a clear signature of AGN activity, in the vast majority of LRDs.

However, that’s where the similarities between LRDs and more typical AGNs seem to end. First off, astronomers have been unable to detect any significant X-ray emission from LRDs. This is shocking, since X-rays are a telltale signature of previously known AGNs. Another classic way to identify AGNs is by looking for changes in their brightness over time, since they can show drastic variability in their emission on the scale of years or even days. LRDs buck the trend by showing little to no fluctuation. Other classic AGN signatures that LRDs lack include radio emission and an abundance of hot dust. Explaining all of these unusual features has been incredibly challenging; nevertheless, coming up with a solution is crucial because there appear to be orders of magnitude more LRDs than any other kind of AGN in the early universe!

Wishing on a Star

One leading explanation for LRDs has recently gained significant traction. The hypothesis suggests that early black holes may have been cocooned in shrouds of extremely dense gas. This scenario helps explain the absence of many typical AGN signatures in LRDs. For example, this dense gas could absorb the X-rays generated by the central black hole. The gas-enshrouded model also explains a particular feature in the spectra of many LRDs known as a Balmer break. This feature is usually interpreted as coming from old stars with relatively cool atmospheres. The cocooned AGN hypothesis implies that Balmer breaks could actually be made by a black hole embedded in a cloud of gas, which somewhat resembles an enormous star!

This model helps explain the properties of some puzzling LRDs, including the most distant one we currently know of. However, their enormous distance makes them difficult to study in detail. It would be much easier to examine LRDs if they existed in the nearby universe, since they would appear much brighter and we could probe them on small spatial scales. Unfortunately, previous studies with JWST suggest that LRDs started becoming a lot less common around 12 billion years ago (corresponding to a redshift of about z = 4).

Luckily, less common doesn’t mean extinct! The authors of today’s article leverage data from the Sloan Digital Sky Survey (SDSS), which has taken millions of spectra of objects across the night sky over the past few decades. By mining this rich and diverse dataset, they identify a handful of LRDs in the nearby universe. Although they’re still far beyond our own Local Group, they’re practically in our backyard compared to the distant LRDs we’ve seen before. The authors followed up the new sources with some of the most powerful ground-based telescopes, enabling a detailed study of their properties.

A Chick That Forgot to Hatch

The first major result is that these local LRDs really do resemble many of the distant ones identified by JWST. They show a characteristic “V”-shaped spectrum in the ultraviolet and visible, a key feature in the definition of LRDs. They’re also quite compact. Using existing data from the Hubble Space Telescope, the authors measured the size of one source and found a tiny core embedded within a galaxy only a few thousand light-years across. For comparison, the Milky Way spans about 100,000 light-years.

Using spectroscopy from the Large Binocular Telescope, one of the Magellan Telescopes, and the MMT (formerly Multiple Mirror Telescope), the authors of this article find a rich variety of spectral features. This is where the proximity of these objects is extremely important. Taking spectra of the distant LRDs in this astounding level of detail would be at best prohibitively expensive (multiple full days of JWST observations) and at worst be completely impossible. In contrast, the nearby LRDs appear much brighter than their distant cousins. Their spectra are like intricate tapestries, with each thread revealing new details about their properties.

The authors find numerous broadened emission lines, signaling the presence of an active black hole. They also detect several intriguing absorption lines. These are most striking in one source, J1025+1402, nicknamed “The Egg.” In this object, there are clear absorption features from sodium, potassium, iron, and calcium. These elements appear in the observed states only under specific physical conditions. The authors interpret these findings as implying the presence of an envelope of cool gas surrounding the central black hole (see Figure 1). Such a scenario closely resembles the gas-cocooned AGN model proposed to explain distant LRDs!

schematic of a little red dot model

Figure 1: A schematic showing the authors’ interpretation of the nature of LRDs. This scenario is built up based on spectroscopic observations from several ground-based telescopes. Although the local sources differ slightly from the ones in the early universe, this model is strikingly similar to the ones invoked to explain some of the most extreme LRDs. The fact that such objects exist in the nearby universe is intriguing and raises further questions as to how black holes enter such a phase. [Lin et al. 2026]

These observations provide direct evidence for a potentially new kind of AGN. These local objects do differ slightly from their more distant counterparts; for example, they show some emission from hot dust, which is absent in early LRDs. Still, this adds another piece to the puzzle of this cosmic mystery. The authors hope that future observations will further illuminate the inner workings of LRDs across cosmic time. For example, studies may reveal what objects “hatch” from a source like the Egg, offering new insight into the life cycles of these unusual AGNs.

Original astrobite edited by Niloofar Sharei.

About the author, Ansh Gupta:

I’m an astronomy graduate student at the University of Texas at Austin working with Steven Finkelstein. I use data from JWST to study the formation and growth of the first galaxies and black holes in the universe. In my spare time, I enjoy playing piano, reading, and making YouTube videos.

Fomalhaut debris disk

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: ALMA Reveals an Eccentricity Gradient in the Fomalhaut Debris Disk
Authors: Joshua B. Lovell et al.
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Published in ApJ

Step 1: Understanding How to Carve Your Debris Disk

Let’s start with our solar system: the Kuiper belt, a large ring of icy asteroids, is believed to have been sculpted into its current shape by Neptune. Neptune may have previously scattered objects in the Kuiper Belt through gravitational interactions, but some of them (like Pluto) remain in an orbital resonance with Neptune. In the same way that Neptune shapes the Kuiper Belt, today’s authors believe a planet could be shaping an exo-Kuiper Belt around the star Fomalhaut.

What on Neptune is an orbital resonance, though? A planet orbiting a star has an orbital period, and the gravitational forces between nearby (astronomically speaking) objects can push these objects into a state where their orbital periods are multiples of each other. For example, Pluto and Neptune have a 2:3 orbital resonance, meaning Pluto completes two orbits for every three that Neptune does. The same can happen for the asteroids and planetesimals in the Kuiper Belt, so the same should happen in other star systems!

Step 2: Make Your Observations

If we understand how debris disks carved by exoplanets look — and we think we do — then we should be able to infer the existence of exoplanets! Today’s authors have used observations from the Atacama Large Millimeter/submillimeter Array (ALMA) of the debris disk around Fomalhaut and made some very clever calculations. We’ve known about this disk for a while, which is why today’s authors have studied it with a new analysis technique they developed.

Planeteismals — basically big rocks from a few to hundreds of kilometers across — that orbit in this disk do so with a certain eccentricity, and typically things in the same orbit would have the same eccentricity. But today’s authors were clever — they checked if there was an eccentricity gradient, meaning the planetesimals’ eccentricities depend on their semi-major axis (i.e., the mean orbit radius); we would typically not expect any eccentricity gradient for bodies orbiting a star unperturbed. The authors discovered that the gradient for planetesimals around Fomalhaut is negative, which implies the presence of a planet when you look at the maths behind gravitational interactions between planets and planetesimals.

A negative eccentricity gradient means the planetesimals gather up at the point on the orbit farthest from the star (the apocenter), and since there are more planetesimals in that region, they appear brighter in the ALMA data (see Fig. 1 left); the ring also appears slightly wider. If the eccentricity gradient were positive, the same thing would happen at the point on the orbit closest to the star (the pericenter). The authors term this phenomenon the “eccentric velocity divergence.”

observed and modeled brightness of the Fomalhaut debris disk

Figure 1: Left: The observed intensity of the Fomalhaut debris disk with ALMA. Middle: the authors’ model that fits the ALMA data the best. Right: The residual (data – model) between model and data. White means there is a close match to the data (which is better). [Lovell et al. 2025]

When the authors ran their eccentric velocity divergence calculations for the Fomalhaut disk model, they compared it to observations using a Markov Chain Monte Carlo algorithm. Figure 1 shows their best-fitting model, which fits remarkably well, based on the residual (i.e., the difference between model and data) you can see on the right — including the slightly wider ring at the apocenter!

The authors tested other scenarios with different gradients and allowed for the planetesimals to oscillate their eccentricity around their orbit, but they didn’t find a better-fitting scenario.

Step 3: Find a Carving Planet

Okay, so those were the details. The authors investigated a few scenarios to see what could be causing the observed debris disk and its negative eccentricity gradient, as well as an intermediate ring sitting between the main disk and the star that recently was seen with JWST. The authors tested two scenarios: one where a planet sits between the rings and evacuates the nearby region, and another where a planet is interior to the inner ring and clears the gap through orbital resonances (kind of like Neptune!). An illustration can be seen in Figure 2.

Illustration of possible planet-based scenarios that could create the observed debris disk around Fomalhaut

Figure 2: Illustration of possible planet-based scenarios that could create the observed debris disk around Fomalhaut. One features a planet between the observed debris disk rings, and another is where the planet is interior to both and carves the gap with orbital resonances. [J. Williams]

A planet was previously thought to exist around Fomalhaut, but it is now accepted there is not one we can currently observe. The authors point out that the possible planet sculpting this debris disk could be the same planet we thought existed previously, but at a lower mass (1–16 Earth masses; almost a Neptune mass). We can’t observe a planet with these parameters yet, but maybe with future observing facilities!

Finally, the authors stress, however, that it might not be a planet causing the observed structure — it could instead be the gravity of the planetesimals in the disk. Unfortunately, existing models are not equipped to explore this scenario, which is why the authors are planning to develop tools to investigate this next.

Original astrobite edited by Sandy Chiu.

About the author, Joe Williams:

I’m a third-year PhD student at the University of Exeter in the UK, and I study protoplanetary discs — mainly the tiny dust grains and their ices! In my spare time, I’m a climber, crocheter, and reader of sci-fi and fantasy books. My favourite sci-fi series is The Expanse!

Hubble image of a supernova in the galaxy NGC 2525

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: Supernova-Induced Binary-Interaction-Powered Supernovae: A Model for SN2022jli
Authors: Ryosuke Hirai (平井遼介) et al.
First Author’s Institution: RIKEN Pioneering Research Institute, Monash University, OzGrav: The ARC Centre of Excellence for Gravitational Wave Discovery
Status: Published in ApJ

There are thousands of dying stars that go bang in the night. These explosions are known as supernovae, and they are classified primarily on what spectral lines they show in the days and weeks following their initial explosion. Most broadly, they are separated into Type I (spectra that show little or no evidence of hydrogen) and Type II (spectra that show hydrogen). The physics leading to each type of supernova is different, and astronomers use all of the information at their disposal to learn about these cosmic fireworks.

One of the most useful tools for monitoring and learning about a supernovae is its light curve — the plot of the explosion’s brightness over time. This light curve usually appears as a steep incline up to a peak in brightness several days after the initial detonation, and then a shallow decline in brightness over the following weeks. There are, however, a few examples of supernovae that break this mould.

Today’s article seeks to understand the curious case of supernova SN2022jli. Unlike the typical gradual fade in brightness, the light curve of this supernova shows a periodic bump of brightness every 12.5 days (see Figure 1). The proposed mechanisms to explain this include the blown-apart supernova material interacting with concentric and regularly spaced shells of circumstellar matter, or a binary companion that interacts with the supernova remnant each orbital period. The authors of today’s article investigate the latter scenario by running 3D hydrodynamical simulations of an interacting, post-supernova binary to try to recover SN2022jli’s periodic light curve and constrain the orbital characteristics of the system.

light curve of SN2022jli

Figure 1: The light curve of SN2022jli shows periodic oscillations in the brightness, superimposed on top of the typical fading luminosity of a supernova. The left plot shows the light curve across various colours, and the associated bottom panel shows the oscillations with the typical fading supernova signal subtracted. The right plot shows the light curve of the authors’ proposed mechanism, where the different coloured lines are for different viewing angles on the system at the heart of the supernova. [Moore et al 2023 (left) and Hirai et al. 2025 (right)]

The authors only have the supernova’s light curve to go off of, so they explore a range of orbital scenarios to explain SN2022jli. They run a series of models evolving a neutron star — the degenerate core left over after a low-mass supernova — together with a main-sequence stellar companion, varying the orbital eccentricity and companion mass, while fixing the orbital period (at the observed 12.5-day oscillatory period) and neutron star mass to 1.4 solar masses — a typical mass for a newborn neutron star.

Why a neutron star? SN2022jli is a sub-class of Type I supernovae involving a “stripped” massive star which has lost its hydrogen — most likely through interactions with a close companion. These interactions typically circularise the pre-supernova orbit, and the subsequent explosion most commonly yields neutron stars. To explain the necessarily highly eccentric post-supernova orbit that may cause the periodic oscillation in SN2022jli, there was most likely a “natal kick” — a sudden and asymmetric burst during the supernova that rockets the neutron star in a random direction — which widened the orbit and de-circularised it, giving further evidence for a neutron star remnant.

Immediately following the supernova explosion, ejecta interacting with the main-sequence companion star would heat the star and cause it to swell up. This inflated companion then feeds matter to the orbiting dense neutron star, and the accretion is most intense when the stars are closest in their orbit (called the periastron). During accretion, however, mass is not perfectly absorbed onto the neutron star, and there is some feedback onto the surrounding gas due to the complex thermal and magnetic physics around the neutron star. In this study, the authors model cases where there is no feedback, feedback from thermal radiation due to accretion, or feedback from bipolar outflows via jets or disk winds. Snapshots from the simulation involving a 5-solar-mass companion on an orbit with eccentricity of 0.5 and bipolar feedback is shown in Figure 2.

simulations of a neutron star with an inflated main-sequence companion

Figure 2: Several density snapshots of the 3D hydrodynamic simulations show how the neutron star (blue point) in the binary accretes and sculpts matter from the inflated main-sequence companion star (bright diffuse blob in the centre of each panel). The xy panels are a top-down look onto the orbital plane, while the xz and yz panels show a cross section through the plane. [Hirai et al. 2025]

The authors evolve the simulation for several orbits and show that periodic bursts of accretion onto the neutron star are capable of producing the oscillatory behaviour seen in SN2022jli’s light curve. The scale of the brightness oscillations are best recreated if the plane of the binary’s orbit is aligned with our line of sight (shown with the +x red line on the right side of Figure 1). Still, the profile of the undulations is not exactly reproduced by the model, and the outcome is very sensitive to our viewing angle of the binary orbit.

SN2022jli is just one of an emerging class of oscillatory supernovae, with more and more being discovered with highly sensitive modern all-sky surveys. Today’s authors rigorously modelled one proposed scenario to explain this periodic oscillation in brightness and showed that it is viable. More modelling using this interacting-binary mechanism on other oscillating supernova light curves — those with longer periods and different supernova classifications — will show whether this explanation is ubiquitous, or if the population of dancing supernova light curves is due to some other process!

Original astrobite edited by Anavi Uppal.

About the author, Ryan White:

I am a masters student at Macquarie University in Australia, working mainly on binary/multiple systems with massive stars (Wolf–Rayets in particular!). Outside of study, I’m a novice film buff, baking sourdough all the time, probably drinking coffee, and trying to get more into reading and frisbee/squash. You can also find me procrastinating on Bluesky @astroryan.bsky.social.

field of stars containing RR Lyrae variables

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: Search for the Blazhko Effect in Field RR Lyrae Stars Using LINEAR and ZTF Light Curves
Authors: Ema Donev and Željko Ivezić
First Author’s Institution: XV. Gymnasium (MIOC)
Status: Published in AJ

RR Lyrae stars are a class of pulsating variable stars — similar to better-known Cepheid variables — that sit on the horizontal branch of the Hertzsprung–Russell diagram. Because of the regularity with which they pulsate, these stars are useful for a number of scientific applications, including standard-candle distancing (helping astronomers set the scale of distances in the universe) and as probes of very old star formation in nearby populations (because most RR Lyrae stars are at least 10 billion years old).

Today’s article studies the Blazhko effect in RR Lyrae stars. Simply put, the Blazhko effect is a long-term change of the duration (period) or strength (amplitude) of pulsation in some RR Lyrae variables. Fig. 1 shows an example from today’s article. While this effect was first observed as early as 1907, the physical mechanism for Blazhko modulation is still formally unknown, as is the percentage of RR Lyrae stars that exhibit it. Broadly speaking, there are three explanations for this effect: 1) nonlinear resonance between a star’s primary pulsation mode and some higher-level pulsation, 2) magnetic influence, or 3) cycles in the convection activity.

example of the Blazhko effect

Figure 1: An example of the Blazhko effect. Each panel shows data from ZTF for the same source for different seasons (at different times). The best-fit pulsation model for the total data set is shown in red. Over time, the actual pulsation of the source (black data) varies significantly from the average best fit due to Blazhko modulation. [Adapted from Donev and Ivezić 2025]

Today’s article searches for and identifies a population of Blazhko stars that may be used for future research into the Blazhko effect. Using data from the Lincoln Near-Earth Asteroid Research (LINEAR) asteroid survey and the Zwicky Transient Facility (ZTF) survey, the authors analyze around 2,857 RR Lyrae stars found in both data sets. The LINEAR survey was taken over a period of about 6 years, and the ZTF survey over about 5 years. On average, there is a 15-year difference between the LINEAR and ZTF observations. Using both, therefore, allows the authors to search for Blazhko modulation in each survey individually, as well as to compare between the two over the 15-year period. They additionally require a source to have at least 150 data points in both surveys to be considered.

From this initial set of RR Lyrae stars, today’s article identifies 531 potential Blazhko star candidates that are moved on to a visual inspection step. In order to identify the candidates for visual inspection, the authors establish two pre-selection methods based on the direct light curve and periodogram for each source:

  1. Light curve selection works by algorithmically assigning a score to each source, with higher scores indicating a greater expectation that the source is a Blazhko RR Lyrae star. The scores are associated with best-fit pulsation models. One way a given source could earn points was by having a very high reduced χ2 statistic in one or both data sets. Blazhko modulation changes the characteristics of the pulsations over time, meaning the best-fit model will be a poor fit to many of the pulsations within one or both data sets. Generally, poorer fits mean higher reduced χ2 values. In addition, candidates could earn points by having a moderately high reduced χ2 statistic in one or both data sets, as well as a significant change in pulsation characteristics of the best fit model from one data set to the next. Such a change between data sets is an indication of long-term Blazhko modulation. From this 479 of the 531 candidates are identified.
  2. Periodogram selection works by looking for interactions between the primary pulsation and Blazhko frequencies. First, the authors create a periodogram for the time-series data. In short, a periodogram plots a number of possible frequencies (or periods) of variability in the data versus the “power” associated with that frequency (or period), where higher power means the data vary more strongly at that frequency. When periodic data have only one associated frequency, the periodogram will show a single peak with high power. In the case where there are two effects of variation (in this case, the pulsation of the star and the Blazhko modulation) with disparate frequencies, a single, large peak will occur at the average frequency, with a smaller peak appearing to either side. Fig. 2 shows an example using simulated data. By identifying the location and strength of these side peaks, the authors are able to identify a handful of additional Blazhko sources (29), as well as estimate the frequencies of Blazhko modulation.
simulated Lomb–Scargle periodogram

Figure 2: A simulated Lomb–Scargle periodogram made using the sum of two sine functions with similar, but different, frequencies. Note the primary peak at the frequencies’ mean, and the smaller side peaks indicating the difference. [Adapted from Donev and Ivezić 2025]

From here, the authors visually inspect the 531 candidates and confirm that 228 of them exhibit convincing evidence of the Blazhko effect. They are able to place a lower limit on the percentage of RR Lyrae stars that are Blazhko sources at 11.4 ± 0.8%. In addition, they report that for a certain subclass of RR Lyrae stars, those that show the Blazhko effect have pulsation periods 5% shorter on average but no significant difference in amplitude. But a less common subclass of RR Lyrae stars shows no significant difference in period or amplitude when comparing Blazhko sources to the general population. Finally, the authors highlight that some sources show Blazhko modulation in one data set, but not the other, indicating that the modulation itself may change over time. Further research into this finding may help us better identify the most likely physical mechanism(s) for the Blazhko effect.

Original astrobite edited by Kylee Carden.

About the author, Catherine Slaughter:

Catherine is a PhD candidate in astrophysics at the University of Minnesota. Her research primarily deals with stellar population astrophysics in local dwarf galaxies, with particular focus on the intersection between computational and observational research methods. Prior to moving to Minnesota, she completed her BA in Physics and Astronomy, and MSc in Astronomy Research at Leiden University.

Illustration of a supermassive black hole enveloped in gas

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: Massive Black Hole Seed Formation in Strong X-Ray Environments at High Redshift
Authors: Kazutaka Kimura, Kohei Inayoshi, and Kazuyuki Omukai
First Author’s Institution: Tohoku University
Status: Published in ApJ

Environments of Massive Black Holes

Astronomers have found that nearly every observable galaxy hosts a central supermassive black hole (SMBH) — a black hole more than a million times more massive than the Sun. Using JWST, they have peered back into the early universe to observe some of the earliest galaxies, and these early galaxies also contain SMBHs. In fact, many of these galaxies seem to contain SMBHs that are more massive than we expected them to be this early in the universe.

We believe that most black holes form at the ends of the lives of massive stars, which collapse under their own gravity when they run out of fuel. We expect that this happened to the first stars too, forming “light” (as in “lightweight”) seeds of SMBHs with masses up to about 100 solar masses. Astronomers also think that the right conditions in the early universe could lead to the formation of a “heavy” seed with a mass on the order of 104 solar masses or more; this kind of black hole is called a direct-collapse black hole (DCBH).

In order to form a DCBH, a gas cloud needs to collapse without fragmenting to form star clusters and stars — if stars form, there won’t be enough mass funneled to the center of the cloud to form the DCBH. One important criterion for this to happen is the absence of coolants, like metals and molecular hydrogen (H2), which cause fragmentation. One way to eliminate H2 is with a particular type of ultraviolet light called Lyman-Werner radiation, which can destroy H2 molecules. However, recent results from the Hydrogen Epoch of Reionization Array (HERA) suggest that early galaxies emitted more X-rays than expected, which would increase the amount of H2 and make it less likely for DCBHs to occur. Today’s authors investigate the feasibility of forming DCBHs in these X-ray-bright environments.

Modeling Black Hole Formation

Today’s authors model the collapse of gas in dark-matter halos that have a nearby neighboring halo. This neighboring halo emits radiation like ultraviolet light and X-rays that impact the formation of molecules like H2, which affects the gas chemistry. This radiation is typically emitted by hot gas in star-forming regions. The authors also track quantities like the gas density and temperature as the gas collapses. Once the gas collapses enough, a star forms at the center and continues to accrete surrounding material. Once this star evolves and reaches the end of its life, it becomes a black hole.

The authors considered different intensities of X-ray radiation relative to a critical value of Lyman-Werner radiation believed to destroy enough H2 to form a DCBH, which is called J21. They explore values of the X-ray intensity Jx relative to J21, considering ratios of 0, 10-6, 10-5, and 10-4.

Another important consideration was the baryonic streaming velocity. This is the relative bulk velocity between baryonic (i.e., “normal”) matter and dark matter in the early universe. Higher streaming velocities can delay the collapse of gas in dark-matter halos, which affect gas properties like density, temperature, and chemistry. The streaming velocity depends on the redshift, z, which is given by σbsm = 30 km/s (1+z)/(1+z0), where z0 = 1,100. This “standard” relationship has been calibrated with measurements from observations. The authors consider two cases for the streaming velocity: vbsm = 0 and vbsm = 1σbsm, which are denoted in the figures below.

X-Rays and Seed Suppression

The first main results are shown in Figure 1. The authors find that a large number of intermediate-mass seeds form from H2 cooling, shown in blue, regardless of the X-ray intensity. A smaller number of higher-mass seeds form from H–H2 and H–H cooling, but these cooling processes become suppressed as the X-ray intensity increases. This is because the increased X-ray intensity converts more H to H2. Interestingly, accounting for the streaming velocity all but wipes out black hole formation via H2 cooling except at very high X-ray intensities.

histograms of seed black hole mass

Figure 1: Mass distributions of seed black holes. The top row uses a baryonic streaming velocity of 0 and the bottom row uses the standard streaming velocity. Columns represent increasing X-ray intensities (left to right). Colors represent the dominant cooling mechanisms: H2 (blue), atomic hydrogen H transitioning to H2 (orange), and solely H (green). [Kimura et al. 2025]

Another main set of results are the black hole mass functions (that is, the number density of black holes for a given black hole mass), shown in Figure 2. Note that this differs from Figure 1 because we are now accounting for the number density of black holes, not just the number, and we are no longer categorizing black holes based on their cooling mechanism. As before, we see that most massive black holes are eliminated with a high X-ray intensity and no streaming velocity, but a higher streaming velocity tends to produce more massive black holes regardless of the X-ray intensity. These black hole mass functions provide a useful comparison to observational results from telescopes like JWST.

histograms of seed black hole number density

Figure 2: Seed black hole mass functions for low and high X-ray intensities (left and right columns, respectively). Rows are the same as in Figure 1. [Kimura et al. 2025]

In summary, today’s authors find that a high X-ray intensity leads to less-massive black holes due to the increased amount of H2, but accounting for the baryonic streaming velocity can produce massive black holes even with a large amount of X-rays. The number densities of these black holes appear to agree with observations from JWST, meaning that this could be a promising formation mechanism for the observed SMBHs in the early universe.

Original astrobite edited by Sparrow Roch.

About the author, Brandon Pries:

I am a graduate student in physics at Georgia Institute of Technology (Georgia Tech). I do research in computational astrophysics with John Wise, using machine learning to study the formation and evolution of supermassive black holes in the early universe. I’ve also done extensive research with the IceCube Collaboration as an undergraduate at Michigan State University, studying applications of neural networks to event reconstructions and searching for signals of neutrinos from dark matter annihilation.

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