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What is the source of radio transients? Astronomers still aren’t sure, but that’s not stopping them from modeling their observations of these mysterious flashes.

The Challenge of Modeling a Mystery

Imagine a radio astronomer in the first moments following an alert that their telescope just recorded something strange in the sky. Their computer informs them that for barely a millisecond, it spotted a mighty flash of radio waves (aptly called a fast radio burst), but before any humans knew something was happening, the flash had already faded. What was that?

In some fields of astronomy, the next steps would be obvious. The scientist would need to write a model that simulates the physics of some known system, then fiddle with the input parameters of that model until its outputs resemble their data. This radio astronomer is not so lucky, though. That’s because although there are many good ideas and many theorists actively working on it, scientists still do not know the source of fast radio bursts.

So, what is the radio astronomer to do? How can you fit a model and learn anything about what you just saw when you don’t know what caused it? That’s where the authors of a recent publication led by Emmanuel Fonseca, West Virginia University, come in.

A New Tool

Observed data of a fast radio burst (left), the best-fitting fitburst model (center), and the residuals to the fit (right). Click to enlarge. [Fonseca et al. 2024]

Fonseca and collaborators created a flexible model that is able to reproduce a wide range of different pulse shapes and sizes, then coded it all up as an open-source Python package called fitburst. Some of their input parameters, like the dispersion measure, correspond to physical quantities, and they include every bit of realistic physics that they can. Other input parameters, however, are just heuristics. Fitting all of the parameters in their model won’t tell you why certain frequencies remained dim while others flared, but it will tell you the relationship between frequency and peak brightness.

That’s a crucial intermediate step towards developing a more complete theory of fast radio bursts, since it allows scientists to classify the population of observed bursts even without a full understanding of their underlying cause. Already astronomers have noted that there seem to be at least a few distinct types of fast radio bursts, and with a tool like fitburst, they can begin to quantify the differences between these populations.

Careful and Complete Implementation

A fit to a different fast radio burst, which arrived at several staggered, frequency-dependent times. Click to enlarge. [Fonseca et al. 2024]

Fonseca and the team also derived analytic expressions for the derivatives of each of their input parameters, which unlocked a powerful family of model-fitting algorithms that rely on this extra information to find the best values. In a series of comparisons with real observations of fast radio bursts, they convincingly demonstrate both that these algorithms can find best-fitting solutions, and also that these solutions closely resemble the observed data.

Excitingly, the researchers also noted that the fitburst model is flexible enough to fit other types of pulses as well. Although designed primarily for fast radio bursts, it can also be used to analyze observations of pulsars and other radio transients. The team encourages all radio astronomers to take fitburst for a spin, and they themselves already list four distinct projects underway. The future of fitburst is bright, much like the mysterious flashes it models.

Citation

“Modeling the Morphology of Fast Radio Bursts and Radio Pulsars with fitburst,” E. Fonseca et al 2024 ApJS 271 49. doi:10.3847/1538-4365/ad27d6

projection of the gamma-ray sky

Where do the highest-energy particles in the universe come from? New research suggests that the sources of ultra-high-energy cosmic rays aren’t necessarily the sources of ultra-high-energy photons as well.

Cosmic Rays Across the Universe

Across the universe, extremely energetic charged particles called cosmic rays zoom through space. These particles are usually protons or the bare nuclei of helium atoms, but they can also be electrons, the nuclei of atoms heavier than helium, or other particles like positrons.

Exactly where these particles are accelerated to nearly the speed of light is an open question. One clue to the origin of the most energetic cosmic rays — ultra-high-energy cosmic rays — is that these particles are not distributed evenly across the sky. Any proposed source of ultra-high-energy cosmic rays, like supernovae, gamma-ray bursts, or other highly energetic cosmic beacons, must be able to explain this distribution.

Distribution of resolved extragalactic gamma-ray sources used in this work

Distribution of resolved extragalactic gamma-ray sources used in this work. The color bar indicates the gamma-ray flux of each source. [Partenheimer et al. 2024]

From Energetic Photons to Energetic Particles

Angelina Partenheimer (University of Wisconsin) and collaborators investigated the possibility that the highest-energy cosmic rays and the highest-energy photons have the same source. To test this hypothesis, the team constructed a sample of resolved gamma-ray sources with energies between 50 megaelectronvolts and 1 teraelectronvolt. They then modeled the distribution of ultra-high-energy cosmic rays that might be emitted by this collection of gamma-ray sources.

They used two scenarios to model the cosmic-ray distribution. In the first scenario, the cosmic-ray flux scales with the gamma-ray flux, meaning that sources that are brighter in gamma rays also produce more cosmic rays. In the second scenario, each gamma-ray source appears equally bright in cosmic rays from our vantage point. While this doesn’t reflect reality — it would imply that more distant gamma-ray sources produce more cosmic rays — this treatment may help correct for the fact that catalogs of gamma-ray sources are increasingly incomplete at larger distances.

projection of the modeled cosmic-ray distribution

The modeled cosmic-ray flux for the scenario in which the sources are uniformly bright. The resulting dipole for this scenario is roughly five times larger than what has been observed. [Partenheimer et al. 2024]

Cosmic-Ray Bright, Gamma-Ray Dim?

In both scenarios, the modeled cosmic-ray distribution is far more uneven than what has been observed. Much of the unevenness comes from the extremely bright gamma-ray source Markarian 421, which helps to produce a dipole in the cosmic-ray distribution 5–10 times larger than what has been observed. This suggests that resolved gamma-ray sources alone cannot be the sources of ultra-high-energy cosmic rays; many more sources are needed to balance out the few extremely luminous objects.

Partenheimer’s team found that roughly 80,000 “missing” sources are needed to match the observed distribution of ultra-high-energy cosmic rays. This is far larger than the known population of resolved gamma-ray sources, which could mean that the producers of the highest-energy cosmic rays are unresolved gamma-ray sources. Alternatively — and perhaps surprisingly — the sources of ultra-high-energy cosmic rays might not produce gamma rays at all!

Citation

“Ultra-High-Energy Cosmic-Ray Sources Can Be Gamma-Ray Dim,” Angelina Partenheimer et al 2024 ApJL 967 L15. doi:10.3847/2041-8213/ad4359

photograph of Uranus

Uranus is thought to possess a core of rock and ice beneath its vast frosty atmosphere. Just how much rock lies at the center of this giant world is unknown, but a newly proposed technique could provide a way to find out.

Core Concerns

photograph of Uranus and its rings from JWST

This JWST Near-Infrared Camera image of Uranus shows the planet’s faint ring system and 9 of its 27 moons. [NASA, ESA, CSA, STScI]

When the Voyager 2 spacecraft whizzed past Uranus in January 1986, it revealed the planet’s dark, delicate rings and its pale cyan atmosphere. Precisely what lies beneath the ice giant’s thick atmosphere is unknown, though researchers expect that the planet’s core is made of rock and ice.

But just how much of the core is made of rock is unknown, and it’s likely to be challenging to measure. Space-based measurements of gravitational pull are are often used to infer a planet’s interior density structure. However, if some of Uranus’s atmospheric gas is mixed into the rock, the mixture will have a density similar to that of ice, making it impossible to differentiate between rock and ice using gravity measurements. How, then, can we tell how much rock is in Uranus’s core?

Noble Gas Method

Francis Nimmo (University of California, Santa Cruz) and collaborators proposed that the amount of rock in Uranus’s core could be calculated by measuring the concentration of argon-40 in its atmosphere. Argon-40, a form of the noble gas argon containing 40 neutrons, is the most common type of argon in Earth’s atmosphere.

Argon can be produced through the radioactive decay of potassium, which clings to silicate-rich materials like the rocks thought to be present in Uranus’s core. As potassium slowly decays to argon with a 1.25-billion-year half-life, the newly produced argon diffuses into the planet’s atmosphere. By measuring the amount of argon in Uranus’s atmosphere, Nimmo’s team suggests, researchers can infer the amount of potassium — and rock, by extension — in the planet’s core.

plot of calculated argon-40 concentration

Concentration of argon-40 as a function of the core rock mass in units of Earth masses, ME, and the transport factor, f. [Nimmo et al. 2024]

A Complex Measurement

Nimmo and coauthors find that if the transport of argon from Uranus’s core to its atmosphere is efficient, an atmospheric probe could easily measure the concentration of argon-40. But because there appears to be a trade-off between the mass of the rock core and how efficiently it propels argon-40 into the atmosphere, a spacecraft would need to measure the total mass of the core through gravity measurements or seismology to get a final estimate of how much rock is in the core.

There are other possible complications: some argon in Uranus’s atmosphere was likely already present when the planet was swirled together from the nebula that birthed the Sun and the planets. To disentangle the argon produced in the core from the argon present since the planet’s birth, a visiting spacecraft would need to measure the ratio of argon-40 to argon-36, a form of the element that is produced in supernovae. This ratio would then need to be compared to the primordial ratio of the two forms of argon, which is not known precisely.

The opportunity to test the authors’ theory may lie ahead: a Uranian orbiter and probe was the top priority in the 2023–2032 Planetary Science and Astrobiology Decadal Survey. With two decades or more until the possible arrival of such a spacecraft, scientists have time to contemplate how to measure the makeup of Uranus’s core.

Citation

“Probing the Rock Mass Fraction and Transport Efficiency Inside Uranus Using 40Ar Measurements,” Francis Nimmo et al 2024 Planet. Sci. J. 5 109. doi:10.3847/PSJ/ad3b93

illustration of a white dwarf collecting gas from its stellar companion

With T Coronae Borealis expected to have an outburst any day now, recurrent novae are in the news. Recently, researchers reported their investigation of a recurrent nova that brightens every year.

Recurring Stellar Characters

light curves from M31N 2008-12a's eruptions from 2013 to 2022

Vertically offset light curves from M31N 2008-12a’s 2013–2022 eruptions. [Basu et al. 2024]

Recurrent novae are periodic outbursts that happen when a white dwarf — the exposed core of an evolved star with a mass of about 8 solar masses or less — snags some gas from a puffy red giant companion. Heated by the blisteringly hot surface of the white dwarf, this accreted gas ignites in a flash of nuclear fusion. This process can recur for millions of years, creating with each outburst a “guest” star that fades until the next eruption.

Known recurrent novae have outbursts anywhere from every year to every 98 years. The nova with the most recorded appearances is M31N 2008-12a, which hails from our galactic neighbor, Andromeda. Researchers have witnessed the star brighten 15 times since its discovery in 2008, and a dive into the archives dredged up three previous eruptions in 1992, 1993, and 2001. What can this collection of eruptions tell us about M31N 2008-12a’s past, present, and future?

Light curve showing the overall behavior as well as the "cusp" feature

Light curve in the i’ band, showing the overall behavior as well as the “cusp” feature. Click to enlarge. [Adapted from Basu et al. 2024]

Characterizing Outbursts

Judhajeet Basu (Indian Institute of Astrophysics and Pondicherry University) and collaborators examined optical, ultraviolet, and X-ray data to examine the behavior of M31N 2008-12a during its annual outbursts from 2017 to 2022. Their investigation showed that each outburst was roughly the same — rising rapidly to its peak in about a day, then declining sharply for 2–4 days before fading more gradually.

In some wavelength bands, the light curves show a “cusp” feature rising above the expected curve. The “cuspy” look of the light curve at certain wavelengths could be evidence for outflowing jets emerging from the poles of the star. These types of jets have been seen for other recurrent novae, like the Milky Way’s RS Ophiuchi.

From Nova to Supernova

histogram showing the frequency of days since last eruption

Demonstration of the possible increase in time between eruptions in the last few years. [Adapted from Basu et al. 2024]

Basu’s team found that while each recent outburst has looked mostly the same, the time between eruptions has gotten longer, on average, over the last seven years. The slowly increasing time between eruptions could mean one of two things: the mass of the white dwarf is decreasing over time, reducing the star’s ability to siphon gas from its companion, or the accretion rate is slowing. Calculations show that the star’s mass is increasing with time, so a decrease in the accretion rate must be responsible. This could point to anything from a change in the orbital dynamics of the system to the donor star running out of gas.

Researchers estimate that M31N 2008-12a has been experiencing nova eruptions every year for the past million years. Despite the repeated eruptions that remove mass from the white dwarf’s surface, the star is gaining more mass than it’s losing, creeping ever closer to the Chandrasekhar limit. Once the star hits this mass limit in another 20,000 years or so, it will be too massive to support itself against gravity and will undergo one final outburst as a supernova.

Citation

“Multiwavelength Observations of Multiple Eruptions of the Recurrent Nova M31N 2008-12a,” Judhajeet Basu et al 2024 ApJ 966 44. doi:10.3847/1538-4357/ad2c8e

A photograph of stars and long, horizontal, bright streaks caused by satellites.

As construction continues on the Vera Rubin Observatory, the skies above its mountaintop home grow more and more crowded following every rocket launch. Astronomers, conscious of the plans for mega-constellations of new satellites in the next few years, are rightfully worried: will these satellites and the tiny bits of debris that come with every deployment and collision affect the new telescope’s long-awaited, gigantic survey?

Threats to Ambitious Plans

After several decades inhabiting only the dreams and blueprints of astronomers, the Vera Rubin Observatory is finally a real, physical place. Now a building and construction site near the summit of Cerro Pachón in Chile, its concrete and steel structure already houses most of what’s needed to begin one of the most ambitious surveys of the sky ever conceived. The Legacy Survey of Space and Time, or LSST, promises to revolutionize every sub-field of astronomy from cosmology to planetary science, and scientists around the world are eagerly awaiting its kickoff.

photograph of Vera Rubin Observatory

The Vera Rubin Observatory. [RubinObs/NSF/AURA/H. Stockebrand; CC BY 4.0]

The plan is to use the largest camera ever built to photograph the entire night sky, repeatedly, for a decade. Unfortunately, though, stars and galaxies aren’t the only objects that will show up in these wide-angle images. Anything placed in orbit around Earth will blunder through the pictures as well, potentially reflecting sunlight towards the telescope as they zip along their looping trajectories. This will cause streaks and flashes in some of the images, which, without careful filtering, could either obscure or mimic the subtle signal of a fleeting astronomical event.

Tiny Pieces, Potentially Large Impact

Astronomers have known this might be a problem for a while now, and the LSST team has spent considerable time figuring out how to handle satellites and large chunks of space debris. While challenges remain and the correction techniques won’t ever be perfect, the community is prepared to handle anything large enough to be tracked by ground-based radar, or about 10 cm. But, what about smaller objects, like the bits of debris created when two satellites collide?

In a February Research Note, one astronomer voiced concerns that these tiniest pieces of space junk could overwhelm LSST’s transient detection algorithms. This prompted a team led by J. Anthony Tyson, University of California, Davis, to model more thoroughly how glints from small, nearby objects would appear in LSST images.

Closer and Faster Than The Stars

An illustration of how a nearby, moving satellite would be blurred out compared to an equally bright but faraway and stationary star. [Adapted from Tyson et al. 2024]

Thankfully, the researchers concluded that there likely isn’t much cause for alarm. While they point out that it should be possible to build filters for these events, they also point out a more important and ironic conclusion: because the objects are so close to the telescope, they’ll actually appear fainter than you might initially expect. Since the observatory is designed to concentrate light from objects that are effectively infinitely far away, objects as close as a few thousand kilometers will appear blurry and out of focus. This means a flash that otherwise would have occupied just a few pixels will be smeared out across many, and in most cases will become lost in the noise.

The authors conclude that “In general… the large population of [low Earth orbit] debris below a few centimeters in size may pose little challenge for LSST transient science.” While there are still hurdles to overcome and challenges to solve before LSST can deliver on its extraordinary promises, thankfully, dealing with tiny bits of space junk likely won’t be one of them.

Citation

“Expected Impact of Glints from Space Debris in the LSST,” J. Anthony Tyson et al 2024 ApJL 966 L38. doi:10.3847/2041-8213/ad41e6

An illustration of an exoplanet being engulfed by its home star, as 8 UMi b somehow has not been

New research may have revived the mystery of 8 Ursae Minoris b, a seemingly doomed exoplanet that shouldn’t exist.

The Planet That Shouldn’t Be: 8 UMi b

When first discovered, the exoplanet 8 Ursae Minoris b (8 UMi b; also called Halla) puzzled astronomers. The planet should have been engulfed by its host star as the star swelled into a red giant, but there was no question that the planet was there, resolutely tugging on its star as it completed each 93-day orbit.

Previously, researchers explained away this impossibility by suggesting that 8 UMi was once a lower-mass star with a close-in stellar companion. As 8 UMi began its expansion into a red giant, it swallowed its companion. The subsequent shakeup of 8 UMi’s interior changed its evolutionary path and halted its expansion, saving 8 UMi b from a fiery fate.

The key to testing this hypothesis is determining 8 UMi’s age: if the star is old — 9 billion years old or so — then the binary merger scenario is feasible. If the star is young, that would make a merger quite unlikely — and the mystery of 8 UMi b will live on.

Age Estimation

Plot of theoretical isochrones showing the position of 8 UMi

The position of 8 UMi on theoretical isochrones of various ages. This analysis yielded an age of 1.9 billion years for this star. [Adapted from Chen et al. 2024]

A team of stellar sleuths led by Huiling Chen (Peking University) set out to determine 8 UMi’s age. The team used position information and photometry data from the Gaia spacecraft as well as a high-resolution spectrum of the star from a 1.93-meter telescope at the Haute-Provence Observatory. These measurements allowed the team to determine the star’s temperature, surface gravity, and chemical composition.

Using these data, Chen’s team estimated 8 UMi’s age with three different methods: stellar isochrones (theoretical relations between brightness and temperature for stars with different masses but the same age), kinematics, and chemical abundances. The three methods yielded age estimates in the range of 1.9–3.5 billion years — far younger than the nearly 9 billion years estimated for the binary merger scenario.

plots demonstrating age estimation methods using chemical abundances

Age estimates from two chemical abundance methods. These two methods yielded age estimates of 3.3 and 3.5 billion years. Click to enlarge. [Chen et al. 2024]

A Mystery Once Again

The newly calculated age for 8 UMi would make it extremely unlikely for a merger with a binary companion to be responsible for saving 8 UMi b from engulfment. How, then, does this planet exist?

While Chen and collaborators emphasize that more work is needed to solve the mystery once and for all, one of the newly derived stellar properties could provide an explanation: Chen’s team estimated 8 UMi’s mass to be 1.7 solar masses, which is about 13% larger than previous estimates. This larger mass could mean that 8 UMi is slightly more compact than expected, and it would mean that 8 UMi b’s orbital period corresponds to a slightly larger orbital distance — just large enough, perhaps, for the planet to eke out survival on the edge of its star.

Citation

“The Kinematic and Chemical Properties of the Close-in Planet Host Star 8 UMi,” Huiling Chen et al 2024 ApJL 966 L27. doi:10.3847/2041-8213/ad3bb4

Artist's impression of a supermassive black hole in a galaxy

For the first time, researchers have examined unexpectedly massive black holes during a time period called cosmic noon. These black holes may fill in the gap between over-massive black holes in the early universe and those present today.

Black Holes Then and Now

illustrations showing how a large black hole can form from the direct collapse of a massive cloud of gas

Illustration of the formation of a massive black hole seed from the collapse of a gas cloud in the early universe. Click to enlarge. [NASA/STScI/Leah Hustak]

Exactly how and when our universe’s supermassive black holes grew to their impressive size is a topic of intense debate. Using JWST, researchers discovered that some black holes in galaxies less than a billion years after the Big Bang are astonishingly large given the universe’s young age. Many of these over-massive black holes sit at the centers of low-mass galaxies, meaning that they’re also unexpectedly large given the mass of their host galaxies. These findings imply that early black holes either grew from “seeds” that were already quite massive or gained mass rapidly.

Observations of black holes in the universe today support the hypothesis that many supermassive black holes grew from massive seeds, and low-mass galaxies with over-massive black holes are common in the local universe as well as in the early universe. But what’s missing from this tale of black hole growth is what happened in between these two time periods: where’s the link between over-massive black holes in the early universe and today?

picture and spectrum of a low-mass galaxy with an active galactic nucleus

Observations of a low-mass galaxy with an active galactic nucleus. Click to enlarge. [Adapted from Mezcua et al. 2024]

Activity at Cosmic Noon

To connect these two epochs, Mar Mezcua (Institute of Space Sciences, Spain) and collaborators looked toward a period of the universe’s history known as cosmic noon. This period, when the universe was just 2–3 billion years old, is marked by high star-formation rates and fast black hole growth. To study black holes during this time period, Mezcua’s team searched for galaxies containing actively accreting supermassive black holes, also called active galactic nuclei.

Starting from a sample of more than a thousand galaxies with active galactic nuclei, the team selected 12 low-mass galaxies with high-quality data and redshifts that placed them at cosmic noon. Measurements of emission-line widths revealed that these black holes were roughly 100–1,000 times more massive than the black holes in similarly sized active galaxies in the local universe. They are also more massive than expected given the typical ratio of black hole mass to stellar mass.

Drawn from the Same Population

plot of black hole mass versus galaxy stellar mass

Black hole mass versus galaxy stellar mass for the black holes in this study (red squares), over-massive black holes in the early universe (dark purple squares), and other populations of black holes. Click to enlarge. [Mezcua et al. 2024]

When Mezcua’s team compared the over-massive black holes in their cosmic noon sample to those seen in the early universe with JWST, they found that both samples showed the same relationship between black hole mass and stellar mass. The luminosities and accretion rates were also similar. This suggests that these two groups of black holes, both of which are overly massive compared to other black holes present in their respective time periods, belong to the same population.

The two groups of black holes may have different reasons for being overly massive, though: in the early universe, the presence of too-massive black holes is thought to mean that these black holes grew from massive “seed” black holes. Later, at cosmic noon, black hole feedback has had time to disrupt and heat star-forming gas, and interactions between galaxies have stripped away star-forming material. Both of these processes could cause a black hole to remain large compared to its host galaxy.

This marks the first time researchers have studied over-massive black holes during cosmic noon, and there’s much more to learn about black holes in this time period. An investigation into outflows and mergers may help researchers understand how these outsize black holes formed and grew.

Citation

“Overmassive Black Holes at Cosmic Noon: Linking the Local and the High-Redshift Universe,” Mar Mezcua et al 2024 ApJL 966 L30. doi:10.3847/2041-8213/ad3c2a

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

Researchers estimate that the accretion disks of supermassive black holes could host millions of stars. When these stars evolve into black holes, they may reshape the observational properties of the disks they call home.

Stellar Extremophiles

Countless stars across the universe have taken up residence in the vicinity of supermassive black holes, including in the dusty disks that surround black holes that are actively accreting gas, otherwise known as active galactic nuclei. Some stars are born in these black hole disks, condensing out of the dusty gas on the outskirts of the disk, where the gas is cooler and feels less of the black hole’s tidal pull. Others may be trapped there, the friction of passing through the disk eventually wearing their orbits down until the stars settle within the disk.

Whether born there or captured, many of these stars will evolve into stellar-mass black holes. Researchers estimate that in the 10–100-million-year lifetime of an active galactic nucleus, its disk may host anywhere from 100 to 100 million stellar-mass black holes. How can we tell if a supermassive black hole’s accretion disk is home to stellar-mass black holes?

Heating Up the Outskirts

plot comparing the temperature of an accretion disk with and without embedded stellar-mass black holes

Modeled temperature of an accretion disk with (blue line) and without (red line) embedded stellar-mass black holes. [Adapted from Zhou et al. 2024]

A team led by Shuying Zhou (Xiamen University) searched for the signs of stellar-mass black holes in an accretion disk by modeling an active galactic nucleus with a disk containing 1,000–100,000 black holes. The team found that because the stellar-mass black holes alter the surrounding disk by accreting some of the gas, they can potentially change the observational properties of the disk.

As stellar-mass black holes orbit within the supermassive black hole’s accretion disk, they accrete some of the gas. This accreted gas becomes extremely hot and emits X-rays that warm the gas that’s nearby. In the outskirts of the disk, where the temperature is lower, this process can heat the disk a potentially measurable amount.

comparison spectral energy distributions for models with and without black holes embedded within the accretion disk

Comparison of the model output for a static standard disk (SSD) and a disk in which stellar-mass black holes are embedded (SSD with sBHs) and a composite active galactic nucleus spectrum. Click to enlarge. [Zhou et al. 2024]

Spotting Black Holes in Black Hole Disks

Zhou’s team compared the spectral energy distributions — how energy output is distributed across different wavelengths of light — for accretion disks that host stellar-mass black holes and those that do not. For a supermassive black hole with a mass of 100 million solar masses, the presence of stellar-mass black holes in the accretion disk greatly boosts the disk’s energy output at wavelengths greater than 470 nanometers (nm) and slightly suppresses the disk’s energy output at shorter wavelengths. For more massive black holes, the energy-boosting effect happens at longer wavelengths, above about 800 nm.

In addition to altering the spectral energy distribution, the presence of stellar-mass black holes may also increase the accretion disk’s half-light radius, or the radius within which half of the disk’s light is emitted. This change is potentially measurable through microlensing of active galactic nuclei by foreground galaxies. In fact, it may have already been measured — some microlensing measurements suggest that the half-light radii of distant active galactic nuclei are 2–4 times larger than expected for a typical accretion disk.

Citation

“Stellar Black Holes Can ‘Stretch’ Supermassive Black Hole Accretion Disks,” Shuying Zhou et al 2024 ApJL 966 L9. doi:10.3847/2041-8213/ad3c3f

nebula Pa 30

In astronomy, sometimes 1 + 1 = 1. That’s the case when white dwarfs collide, creating a single massive remnant that sheds mass through powerful magnetic winds.

When Stellar Remnants Collide

Nearly all of the stars in the Milky Way, including the Sun, are fated to become white dwarfs. These Earth-sized objects are the super-hot crystallized cores of stars that have lost their outer layers after ballooning into red giants. When two white dwarfs collide, the collision can trigger a supernova, create a neutron star — an object even denser than a white dwarf — or merge the two white dwarfs into one.

In a recent research article, Yici Zhong (University of Tokyo) and collaborators modeled the properties of post-merger white dwarfs, focusing on their fast-moving magnetized winds. The results may be applicable to an unusual class of supernovae that are faint, fade quickly, and fail to fully explode — leaving behind a white dwarf remnant.

Magnetic Outflows

plot of radial wind speed versus latitude

Radial velocity of the white dwarf’s wind as a function of latitude. The wind is fastest near the star’s equator. [Adapted from Zhong et al. 2024]

When two white dwarfs merge into one, the resulting white dwarf is expected to rotate rapidly and be highly magnetized. This combination of speedy spinning and strong magnetic field launches a thick and powerful wind from the star’s surface and carries mass away from the star. To learn more about the magnetic winds of white dwarfs, Zhong’s team carried out numerical magnetohydrodynamics simulations of a post-merger white dwarf.

Zhong’s team found that the wind doesn’t emerge from the entire surface of the star equally and is instead fastest and most luminous near the star’s equator. Nor does the wind blow steadily: some of the gas launched from the star’s surface gets trapped in the magnetic field, and a periodic rearranging of the magnetic field ejects bubbles of this trapped gas.

The Winds of WD J005311

These findings may give researchers new ways to study a recently discovered white dwarf called WD J005311, which appears to be the remnant of a collision of white dwarfs that triggered a supernova. Unlike most supernovae caused by colliding white dwarfs, the explosion didn’t destroy the stars completely, and the surviving star’s 16,000-kilometer-per-second winds pummel the supernova remnant from within.

Modeled torque, luminosity, and mass-loss rate of the wind over time

Modeled torque, luminosity, and mass-loss rate of the wind over time. Click to enlarge. [Zhong et al. 2024]

Zhong’s team used their model to estimate the star’s properties, which have been challenging to measure directly. They estimated the star’s mass at 1.1–1.3 solar masses and its magnetic field at 20–50 million Gauss (about 20–50 million times stronger than Earth’s).

The team’s model also predicts that the star’s wind blows outward 20–40% faster at its equator than its poles, and this asymmetry could be detected through optical spectroscopy. The uneven speed means that the wind exerts more pressure on the surrounding nebula along its equator, possibly creating an uneven shock that could be seen in future X-ray observations.

Citation

“The Optically Thick Rotating Magnetic Wind from a Massive White Dwarf Merger Product. II. Axisymmetric Magnetohydrodynamic Simulations,” Yici Zhong et al 2024 ApJ 963 26. doi:10.3847/1538-4357/ad1f5c

illustration of a tidal disruption event

Is the gamma-ray burst GRB 191019A a typical long-duration gamma-ray burst from a dying massive star, an anomalously long burst from colliding objects, or something else entirely?

Powerfully Mysterious

illustration of a neutron star merger

An illustration of a neutron star merger, which is one way to create a gamma-ray burst. [ESA 2002/Medialab]

Gamma-ray bursts are among the most mysterious phenomena in the universe. These powerful flashes of gamma rays have conventionally been divided into two categories according to their length: “long” bursts are those that last more than about two seconds, while “short” bursts are those that are shorter than two seconds.

With time and accumulating data, these length-based classifications have become associated with different sources: long gamma-ray bursts seem to arise from core-collapse supernovae, when the curtain closes on stars substantially more massive than the Sun, and short gamma-ray bursts seem to happen when two extraordinarily dense objects like neutron stars collide. However, as the tally of gamma-ray bursts has grown, so has the list of events that fail to fall neatly into these two categories.

artist's impression of GRB 191019A

This illustration depicts GRB 191019A as resulting from the collision of two stars within the dense environment of a galactic nucleus. Today’s article suggests that the event took place far closer to the galaxy’s supermassive black hole. [International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick/M. Zamani; CC BY 4.0]

An Alternative Hypothesis

In October 2019, researchers discovered the gamma-ray burst GRB 191019A, which lasted just over a minute and appeared at first to be a standard long burst arising from a supernova. Later, this interpretation was called into question: no associated supernova emission was spotted, the burst was slightly fainter than expected for its type, and its host galaxy wasn’t as vigorously star forming as the galactic hosts of long gamma-ray bursts tend to be. Together, these findings suggested that GRB 191019A may be part of an emerging class of gamma-ray bursts that arise from colliding objects despite lasting longer than two seconds.

In a recent article, a team led by Robert Eyles-Ferris (University of Leicester) explored another possibility: that GRB 191019A isn’t a gamma-ray burst at all. Instead, Eyles-Ferris’s team proposed that rather than a supernova or a cosmic collision, what caused the event was a star that wandered too close to a supermassive black hole and was summarily torn apart — a tidal disruption event.

Examining a Rare Phenomenon

Some aspects of GRB 191019A, such as its location near the center of a galaxy, naturally align with this alternative hypothesis. Other factors, such as the event’s length, seem at odds with this explanation — tidal disruption events tend to play out over the course of months rather than minutes. Eyles-Ferris and collaborators hypothesized that GRB 191019A isn’t just any tidal disruption event but an ultra-deep one, in which the doomed star is so stretched out by the black hole’s tidal forces that the star wraps all the way around the black hole and collides with itself, launching a relativistic jet in the process.

Using a mathematical model, Eyles-Ferris and coauthors showed that GRB 191019A’s luminosity and time scale are consistent with what’s expected for an ultra-deep tidal disruption event. If this hypothesis is correct, GRB 191019A is the first example of an ultra-deep tidal disruption event and just the fifth known jetted tidal disruption event. As rare as these events are likely to be, other instances will crop up somewhere in the vastness of the universe, and Eyles-Ferris’s team has a way to pick them out: a flash of low-energy X-rays that erupts the moment the star collides with itself.

Citation

“Ultradeep Cover: An Exotic and Jetted Tidal Disruption Event Candidate Disguised as a Gamma-ray Burst,” R. A. J. Eyles-Ferris et al 2024 ApJL 965 L20. doi:10.3847/2041-8213/ad3922

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