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multi-wavelength image of the Sun

For those of us in the Northern Hemisphere, the winter solstice is rapidly approaching — so today, let’s make up for the lack of sunshine by basking in the warmth of some solar physics research! This instance of the Monthly Roundup gives a quick overview of four recent research articles that share new findings from our home star.

Tracing Threads in a Solar Prominence

Prominences are massive arcs of cool, dense plasma that appear high above the solar surface along the edge of the Sun’s disk. (These same features, when seen on the Sun’s disk rather than silhouetted against space, are called filaments.) Occasionally, prominences erupt in massive explosions that eject solar plasma into the solar system. Other times, they gently dissipate after weeks or months.

The advent of high-resolution solar imaging has revealed new features in solar prominences, including threads that are 3,500–28,000 kilometers long and just 210 kilometers wide. These threads are thought to form when magnetic flux tubes become filled with cool and dense plasma. Recently, Yuxiang Song (Chinese Academy of Sciences; University of Science and Technology of China) and collaborators used the New Vacuum Solar Telescope to examine the behavior of 35 threads in a solar prominence.

displacement and intensity oscillations in a thread in a solar prominence

Example of a displacement oscillation (top row) and an intensity oscillation (bottom row) in a solar prominence thread. Click to enlarge. [Song et al. 2024]

They found that 29 of the 35 threads exhibited displacement oscillations, meaning that the threads wiggled around in a sinusoidal way. The average period of these spatial oscillations was 26 minutes. Eight of the 35 threads showed intensity oscillations; their brightness varied with a typical period of 7.7 minutes. Seven of the threads showed both displacement and intensity oscillations. These values are consistent with observations of threads in other solar prominences.

What causes the displacement and intensity oscillations in the threads? The thread oscillations appear to originate with oscillations deeper in the Sun’s atmosphere, which propagate outward and excite waves in the solar magnetic field. The waves, in turn, generate the oscillations in the threads. However, the authors raised the possibility that the two types of oscillations examined in this work have different causes. Future work is needed to disentangle the question of solar threads.

Studying the Sun from the Moon

Gone are the days when astronomers could observe the Sun only from Earth’s surface. Now, a spacecraft fleet scrutinizes the Sun from almost every angle. Among the spacecraft with an eye on the Sun is Chang’E-2: a lunar orbiter that carries an instrument called the Solar X-ray Monitor

A team led by Man-Hei Ng and Chi-Long Tang (Macau University of Science and Technology and China National Space Administration) used data from the Solar X-ray Monitor to study conditions in solar flares emerging from two active regions on the Sun. Solar flares are brief, brilliant flashes of light that are powered by the rearrangement of solar magnetic fields. Ng and Tang’s team aimed to determine which elements and ions were present in these solar flares. These data give clues as to how matter and energy are transported from the lower, denser regions of the Sun’s atmosphere to the superheated solar corona and beyond.

plot of elemental abundances relative to the photospheric abundances

Examples of elements experiencing the inverse first ionization potential effect (iron; top) and the first ionization potential effect (calcium; bottom). The plots show the ratio of the measured elemental abundance with respect to the abundance of that element within the solar photosphere. Click to enlarge. [Adapted from Ng et al. 2024]

Previous research has shown that elements that require relatively little energy (<10 electronvolts) to ionize and elements that require a relatively large amount of energy to ionize behave differently. Generally, easily ionized elements are more prevalent in flaring regions than at the denser, cooler solar surface. This effect is called the first ionization potential effect. This behavior sometimes flips in regions where the solar magnetic field is especially complicated; the reversed behavior is called the inverse first ionization potential effect.

In this study, Ng and Tang’s team observed the inverse first ionization potential effect in iron for the first time, and they confirmed the existence of this effect for silicon. They also noted another curious effect for the first time: heavy elements like iron took longer to return to their pre-flare levels than lighter elements did, showing that inertia plays an important role. Intriguingly, one of the solar flares showed yet another rare effect: a post-flare change in the elemental abundances from values typical of the solar corona to those more similar to deeper layers of the Sun’s atmosphere, suggesting that this plasma was sourced from below, in the chromosphere or photosphere.

High-Resolution Imaging of a Solar Current Sheet

Many of the explosive events that take place in the Sun’s atmosphere owe their existence to magnetic reconnection: the process of rearranging magnetic fields in a way that releases pent-up magnetic energy, providing a way to accelerate particles to high speeds. Pankaj Kumar (American University; NASA Goddard Space Flight Center) and coauthors recently reported on high-resolution solar images that reveal this process in great detail.

extreme-ultraviolet images of the Sun, showing the location of a solar current sheet

Top: A broad view of the Sun, including the active region studied in this work, indicated in white. Bottom: A zoomed-in view of the active region, showing the location of the current sheet (CS). [Adapted from Kumar et al. 2024]

The images, which come from the Solar Dynamics Observatory spacecraft and the 1.6-meter Goode Solar Telescope, have an incredible resolution of just 50 kilometers. They show a current sheet — a surface that separates regions of oppositely directed magnetic fields, such as those found in active regions on the Sun. (Active regions are areas on the Sun’s surface where the magnetic field is particularly strong and complex; these regions are the sites of solar activity like solar flares.) While short-lived current sheets have been seen before, this study marks the first time a current sheet has been directly imaged over such a long duration; the current sheet in this study persisted for about 20 hours and covered an area of roughly 2 arcseconds by 6 arcseconds.

Kumar’s team observed plasma flowing out from and in toward the current sheet at semi-regular intervals. These quasi-periodic flows provide evidence of particle acceleration via magnetic reconnection and show that reconnection helps to heat the active region plasma. Overall, the observations presented in this work provide evidence for the magnetic breakout model, a widely used model of solar eruptive events like flares and coronal mass ejections. The breakout model describes how magnetic field lines enclosing the dense plasma of a solar filament/prominence reconfigure and release the filament into space.

In addition to providing support for the breakout model, the new observations can serve as a jumping-off point for modeling and laboratory studies of magnetic reconnection. The results of this study even have implications for stars other than the Sun, helping to explain the quasi-periodic pulsations seen at X-ray wavelengths on other stars.

The Sun as a Particle Accelerator

Solar flares and coronal mass ejections — immense eruptions of solar plasma and magnetic fields — are the sites of particle acceleration and X-ray production. Understanding how much of the energy produced in the events goes toward accelerating electrons to breakneck speeds is critical to understanding the Sun’s role as a particle accelerator.

high-energy image of the Sun from the Solar Dynamics Observatory

A 17.1-nanometer image of the Sun from the Solar Dynamics Observatory. The annotations indicate the locations of the acceleration region, the hard (i.e., high-energy) X-ray sources, and the radio emission at various frequencies. Click to enlarge. [Adapted from James & Reid 2024]

Working toward this goal, Alexander James and Hamish Reid (University College London) analyzed data from a solar radio burst that took place in 2013. Solar radio bursts are brief periods of intense radio emission that are caused by fast-moving electrons and often accompany coronal mass ejections or solar flares. This particular burst was associated with a solar flare and a coronal mass ejection.

Using multi-wavelength observations of the flare, coronal mass ejection, and solar radio burst, James and Reid estimated the speeds of the electron beams they observed escaping from the acceleration site. This analysis showed extremely high speeds for electrons in the beams, ranging from 44% to 59% of the speed of light. The team then used simulations to learn about the properties of the region in which these beams were accelerated, finding that the acceleration occurred in a region that stretched some 15,000–100,000 kilometers in the direction tangent to the Sun’s surface, but just 1,000 kilometers vertically.

This work marks the first time researchers have estimated the properties of escaping electron beams from remote-sensing observations. A comparison of the results from remote-sensing data and those from the Solar Orbiter spacecraft, which ventures close to the Sun every 6 months, will provide further insight into the particle-accelerating abilities of our home star.

Citation

“Thread Displacement and Intensity Oscillations in a Quiescent Prominence,” Yuxiang Song et al 2024 ApJ 975 280. doi:10.3847/1538-4357/ad813c

“Unveiling Mass Transfer in Solar Flares: Insights from Elemental Abundance Evolutions Observed by Chang’E-2 Solar X-Ray Monitor,” Man-Hei Ng et al 2024 ApJ 972 123. doi:10.3847/1538-4357/ad5da3

“Direct Imaging of a Prolonged Plasma/Current Sheet and Quasiperiodic Magnetic Reconnection on the Sun,” Pankaj Kumar et al 2024 ApJ 973 74. doi:10.3847/1538-4357/ad63a2

“Estimating the Total Energy Content in Escaping Accelerated Solar Electron Beams,” Alexander W. James and Hamish A. S. Reid 2024 ApJ 976 128. doi:10.3847/1538-4357/ad7b38

MWC 758 protoplanetary disk

Did vortices sculpt the crescent-shaped clumps of dust around the young star MWC 758? Using data from the Atacama Large Millimeter/submillimeter Array (ALMA), researchers have mapped the motions of the dust clumps and weighed in on the vortex hypothesis.

Complicated Disks

How planets form is one of the most pressing questions in astronomy. Observations increasingly show that protoplanetary disks, the sites of planet formation, are complex objects. These disks feature rings, gaps, spirals, and vortices, any of which might signal the presence of baby planets.

Among the intriguing features seen in protoplanetary disks are crescents: asymmetric regions where the density of dust is enhanced. These regions are readily visible in observations by ALMA and have been found in several protoplanetary disks.

Researchers suspect that crescents are caused by swirling regions called vortices. Like debris caught in an eddy in a stream, dust could theoretically become trapped in a vortex, forming the clumps seen in images — and potentially creating a perfect dusty ecosystem for planets to form.

The Causes of Crescents

Recently, a team led by I-Hsuan Genevieve Kuo (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan; University of Arizona) investigated the causes of crescents in the protoplanetary disk around the star MWC 758, also called HD 36112. MWC 758 is a 1.5–2-solar-mass star that is about 3.5 million years old and less than 500 light-years away. Its disk sports several intriguing features, including two crescents and a spiral. Previous research has attributed these features to the presence of one or more planets.

continuum images of MWC 758

Images of the MWC 758 disk taken at a wavelength of 1.3 mm in 2017 (left) and 2021 (right). Click to enlarge. [Kuo et al. 2024]

Vortex theory predicts that vortices in a protoplanetary disk will revolve around the central star at the Keplerian velocity (i.e., following the predictions of Kepler’s laws of motion). To test this theory, Kuo’s team used ALMA data from 2017 and 2021 to measure the motion of the two crescents. They found that the crescents moved in the direction of the disk’s rotation, with the inner crescent moving 50% slower than expected for the vortex hypothesis and the outer crescent moving 33% faster than expected.

Spiral vs. Vortex

observed and expected azimuthal velocity of dust clumps

Observed azimuthal (i.e., around the disk) velocity of the dust clumps (green and dark blue triangles) and expected velocity for Keplerian rotation (aqua circles). Click to enlarge. [Kuo et al. 2024]

Does this finding necessarily rule out the vortex hypothesis? Kuo and coauthors first investigated and eliminated the possibility that imperfections in the disk, like warps or eccentricity, were the cause of the mismatch with theory. They then noted that the motion of the crescents matches the Keplerian velocity at a radius of about 0.46 arcsecond. If a planet were present at that radius, there’s a chance it could throw the system off-kilter and produce the observed behavior — but MWC 758’s putative planets are located well inside and outside this radius.

Instead, the non-Keplerian rotation of the dust clumps might be caused by the interaction of vortices and spirals. In this case, the vortices themselves, which are invisible to us, are moving in the way predicted by theory, but the spiral knocks the (visible) dust off course.

Luckily, this prediction is testable. If vortices and spirals are vying for control of the dust crescents in MWC 758’s disk, their power struggle would be less effective on large dust grains than on small dust grains. In other words, large dust grains should adhere more closely to the expected Keplerian velocity than small dust grains do. Future high-resolution observations at different wavelengths, which probe grains of different sizes, may provide an answer.

Citation

“ALMA Observations of Proper Motions of the Dust Clumps in the Protoplanetary Disk MWC 758,” I-Hsuan Genevieve Kuo et al 2024 ApJL 975 L33. doi:10.3847/2041-8213/ad86c1

Are there places in the galaxy that are better suited for habitability? A new study suggests that the Sun’s trek through the Milky Way may contribute additional complexity to the search for life in the galaxy. 

Galactic Habitable Zones

Illustration of the solar system habitable zone

Illustration of a star system’s habitable zone. Too close to the star, shown in red, the planet will be too hot to support life, and farther away, the planet is too cold. The zone in the middle, shown in green, is the distance from the star that is just the right temperature for a planet to host liquid water, a crucial ingredient of life. Click to enlarge. [NASA]

Earth sits in a comfortable seat, not too hot and not too cold, just the right distance from the Sun to maintain an environment suitable for life. This region, known as the habitable zone, is frequently investigated when astronomers search for life in other star systems. While a planet’s proximity to its host star heavily influences its properties, the star system’s overall location in the galaxy can also impact the chances for life. 

In crowded areas and active star-forming regions, energetic events like supernovae can subject their neighboring star systems to harmful radiation. In the galaxy’s outskirts, there has not been enough chemical enrichment to provide the materials necessary for life. This implies that our solar system resides in a galactic habitable zone, and this region may host other life-harboring systems.

But in the search for habitable planetary systems, it is not sufficient to only consider a star’s current position in the galaxy. As evidenced by its chemical makeup, the Sun likely formed closer to the galactic center and migrated outwards to its current orbital radius, experiencing a wide variety of conditions as it moved through the galaxy. While the Sun’s current orbit is suitable for life, could its migration history have influenced the solar system’s habitability? 

Probability density of stars' final locations after travel through the Milky Way in tested galaxy models

Simulation results showing how different models of galactic central bars and spiral arms transport stars over time. The top panel shows that for a steady model, stars are not significantly transported away from their birth location. The bottom panel shows that for models with dynamic changes in both the central bar and the spiral arms, this causes sufficient migration to explain the Sun’s migration pathway from an inner radius to its current location. Click to enlarge. [Baba et al 2024]

Migration Through the Milky Way

To understand the drivers and conditions of the Sun’s migration, Junichi Baba (Kagoshima University; National Astronomical Observatory of Japan) and collaborators perform simulations of various models of the galaxy. The authors find that dynamic changes in the Milky Way’s central bar and its spiral arms are sufficient to transport the Sun from its origin to its current position in the galaxy.

Though the Milky Way’s bar and arms drive the Sun’s migration through the galaxy, the specific path the Sun takes depends on whether or not it becomes trapped in corotation resonance with the central bar — orbiting the galactic center at the same speed as the bar’s rotation. The authors test a trapped and untrapped migration path and find that these scenarios produce significantly different migration pathways. In the trapped scenario, the Sun’s orbital radius oscillates more drastically, while in the untrapped case the Sun more gradually shifts to its current position. These pathways subsequently expose the solar system to different environments within the galaxy.

Sustaining Life

Histories of surrounding environmental changes along the sun's potential orbits

Histories of environmental changes surrounding the Sun along its potential orbits. The orange line shows the path of a trapped orbit, where the Sun experiences more dramatic environmental changes than an untrapped orbit (shown in green). Click to enlarge. [Baba et al 2024]

How does the Sun’s path through the galaxy impact the habitability of the solar system? The authors investigate the galactic environments the Sun would encounter on both migration pathways, including environmental factors like star formation, gamma-ray bursts, and interactions with comets and other stars. In the trapped case, the solar system encounters significantly more of each environmental component. On one hand, exposure to star formation and highly energetic gamma-ray bursts would expose any life forms to significant and deadly radiation. On the other hand, more exposure to comets and metal-enriched star-forming regions provides more opportunities for life-forming molecules to be deposited into the solar system. 

This study highlights the importance of considering how star systems have migrated through the galaxy over their histories. Their current galactic environment is not the only influence on their ability to harbor life. Thus, as the search for life continues in exoplanet systems, tracing the orbital paths these systems have trudged is crucial for understanding their habitability.

Citation

“Solar System Migration Points to a Renewed Concept: Galactic Habitable Orbits,” Junichi Baba et al 2024 ApJL 976 L29. doi:10.3847/2041-8213/ad9260

solar flare

Solar flares are one of the most common solar phenomena, but there’s still much to learn about them. Until now, researchers suspected that the high-energy radiation of a solar flare is only occasionally accompanied by a flare at visible wavelengths, but new work suggests that white-light flares aren’t so uncommon after all.

Classes of Solar Flares

extreme-ultraviolet image of the sun with a magnetic field model overlaid

Why study the causes of solar flares? Flares are just one example of how the Sun’s complex magnetic field interacts with its dynamic plasma environment. [NASA/GSFC/Solar Dynamics Observatory]

Flares are bright, localized flashes of radiation powered by the release of pent-up magnetic energy in a star’s atmosphere. The Sun launches anywhere from tens to hundreds of solar flares each year, and flares are common on other stars as well. While the radiation from a flare can span the entire electromagnetic spectrum, flares are categorized by the strength of their soft (i.e., lower energy) X-ray emission, ranging from relatively mild A-class flares to powerful X-class flares.

In addition to strong X-ray and ultraviolet emission, some flares also brighten across the visible portion of the spectrum. These white-light flares make up just a small fraction of all solar flares, though they appear to be more common on other stars. (The flare associated with the 1859 Carrington event — arguably the most famous solar flare — was the first white-light flare to be recorded.) To understand the origins of white-light flares, it’s critical to determine whether these events are truly as scarce as they seem.

white-light distribution during a solar flare

An example of the spatial and temporal distribution of white light during a solar flare. This particular flare is class X9.3, making it a very powerful flare. Click to enlarge. [Adapted from Cai et al. 2024]

New Search Methods

Recently, Yingjie Cai (Chinese Academy of Sciences; University of Chinese Academy of Sciences) and collaborators demonstrated a new way to find white-light solar flares. First, the team considered where current search methods might be failing. Typically, white-light flares are identified by searching for differences between optical images of the Sun taken at different times. The team’s new method, which was developed by analyzing the properties of previously discovered white-light flares, improves upon the existing method in two key ways: 1) searching for spatial and temporal clusters of brightening at visible wavelengths, which are characteristic of white-light flares, and 2) searching for increased emission relative to the local background level of emission, rather than an arbitrary threshold brightness. This second tactic may enable the discovery of faint white-light flares, which seem to be especially rare.

Cai and coauthors applied their method to a sample of 90 solar flares evenly split between C, M, and X-class flares. Ultimately, they found that 9 of 30 C-class flares, 18 of 30 M-class flares, and 28 of 30 X-class flares could be classified as white-light flares. For a given flare energy class, white-light flares are more common among confined flares — those that are not accompanied by an explosion of plasma into space — than eruptive flares. The team also found that the duration of a white-light flare is related to its energy, with less energetic flares being more fleeting than more energetic flares.

Promising Proportions

plot of the incidence of white-light and non-white-light flares

Proportion of flares that are white-light flares (WLF) versus non-white-light flares (NWLF) as a function of flare energy class and whether the flare is eruptive or confined. Click to enlarge. [Adapted from Cai et al. 2024]

At first glance, these results suggest that more powerful flares are more likely to emit strongly at visible wavelengths, in line with previous research. However, the fact that 30% of the C-class flares in this study were white-light flares — the largest proportion of C-class white-light flares found by any study so far — suggests that white-light flares may be common even among weaker flares, just more difficult to detect with current instruments. If this is the case, the development of increasingly sensitive solar telescopes should boost the number of known C-class white-light flares.

Looking forward, the authors plan to refine their identification model, helping to amass a sample of white-light flares that can be used for statistical studies and comparisons with flares on other stars. This should also help to disambiguate the origins of white-light flares; its not yet clear what powers the visible-light emission of a solar flare, nor is it clear if the visible-light emission always arises from the same source.

Citation

“Statistics of Solar White-Light Flares. I. Optimization and Application of Identification Methods,” Yingjie Cai et al 2024 ApJ 975 69. doi:10.3847/1538-4357/ad793b

Cas A supernova remnant as seen by JWST, revealing the structure known as the Green Monster

JWST images of the supernova remnant Cassiopeia A (Cas A) revealed for the first time a perforated, filamentary structure named the “Green Monster” for its resemblance to the home-run-thwarting wall at Fenway Park. Recently, researchers reported on this feature and the cause of its curious spots.

Studying a Star’s Life from Its Remains

Cas A supernova remnant

This perspective on Cas A comes from the Hubble Space Telescope. Each of the tiny blobs of gas near the top of the shell is tens of times wider than the solar system. [NASA and The Hubble Heritage Team (STScI/AURA); Acknowledgment: R. Fesen (Dartmouth) and J. Morse (Univ. of Colorado)]

Stars much more massive than the Sun end their lives spectacularly. In the finale of their brief, brilliant lives, these stars are no longer able to produce enough energy to stave off the crushing pressure of gravity, causing them to collapse. In an explosion seen galaxies away, their outer layers rebound off their shrunken cores, leaving behind an intricate and colorful supernova remnant that slowly expands to span hundreds of light-years.

Among the most famous and well-studied remnants of a core-collapse supernova is Cas A, which has posed for countless astronomical portraits since its discovery in 1948. Recently, JWST data revealed a never-before-seen feature stretching across Cas A. Because of its green appearance in representative-color images from JWST, this feature was dubbed the Green Monster.

How the Green Monster Got Its Spots

In a recent article, Ilse De Looze (Ghent University) and collaborators focused on one particularly intriguing aspect of the Green Monster: multiple complete and partial circles that appear to be torn in the fabric of the structure. The team measured more than two dozen of these circles, each with diameters of about 1–3 arcseconds. If the Sun were placed at the center of one of these circles, the boundary of the circle would lie close to the inner edge of the Oort Cloud.

closeup of the Green Monster holes

A closeup of the Green Monster, showing the small holes investigated in this study. The orange arrows indicate the location of an ejecta filament that might be associated with the hole labeled “P2.” Click to enlarge. [De Looze et al. 2024]

What could create these shapes? De Looze’s team proposed several scenarios, each of which draws on the complicated interactions between the fast-moving ejecta from the exploded star, the surrounding circumstellar material, and shock waves that propagate outward and inward as the various materials collide.

In the scenario deemed most likely, the Green Monster is made up of circumstellar material that was lost before the star exploded as a supernova. This material now sits in front of the supernova remnant, from our perspective, and was impacted by a shock wave. The holes are created where ejected material has poked through the Green Monster.

What’s not yet clear is the timeline: did the collision between the outward-moving ejecta and the Green Monster happen before or after the Green Monster was struck by the shock wave? As De Looze’s team showed, either order is possible, and more data and simulations are needed to explore the sequence of events and the timescales involved.

Mapping Mass Loss

Cas A with locations of quasi-stationary flocculi labeled.

Locations of quasi-stationary flocculi, outlined in cyan, relative to the position of the Green Monster. Click to enlarge. [Adapted from De Looze et al. 2024]

Combined with Cas A’s other intricate structures, the Green Monster gives researchers clues to the tumultuous final years of this exploded star’s life. Currently, evidence points to a massive asymmetric mass-loss episode roughly 30,000–100,000 years before the supernova — in astronomical terms, just briefly before the ultimate explosion. The similarities between the Green Monster and previously discovered structures called quasi-stationary flocculi — dense, slow-moving clumps of circumstellar material — suggest that they may have arisen from the same mass-loss episode.

JWST has already given astronomers an entirely new view of Cas A, but there are surely more discoveries yet to come: future observations will sample the spectra of multiple regions of the Green Monster and several quasi-stationary flocculi, helping to reconstruct the final years of Cas A’s progenitor star’s life.

Citation

“The Green Monster Hiding in Front of Cas A: JWST Reveals a Dense and Dusty Circumstellar Structure Pockmarked by Ejecta Interactions,” Ilse De Looze et al 2024 ApJL 976 L4. doi:10.3847/2041-8213/ad855d

neutron stars approaching a merger

In 2015, the first detection of gravitational waves changed astronomy forever, opening an entirely new window onto the physics of extreme objects like black holes and neutron stars. Today, we’ll take a look at three research articles that predict new gravitational wave sources, probe the prospects of detecting gravitational waves from ultra-light black holes, and compare measurements of the low-frequency gravitational wave background with predictions from the population of supermassive black holes in the local universe.

Disks of Collapsing Stars

So far, the LIGO, Virgo, and Kagra (LVK) detectors have spotted gravitational waves from a bevvy of inspiraling objects: pairs of black holes, pairs of neutron stars, and black hole–neutron star duos. Using vast collections of quickly spinning neutron stars called pulsars, researchers have even found evidence for the muddled hum of distant supermassive black hole binaries.

Recently, Ore Gottlieb (Flatiron Institute and Columbia University) and collaborators explored the possibility of detecting gravitational waves from an entirely different type of source: the accretion disk surrounding a black hole born from the collapse of a rapidly spinning massive star (i.e., a collapsar).

simulated accretion disks and calculated gravitational wave amplitudes

The accretion disk at early times, showing prominent spiral arms (panel a), and at later times when the disk has homogenized (panel b). The remaining panels show the predicted gravitational wave amplitude at different times and at different disk angles. Click to enlarge. [Gottlieb et al. 2024]

Using general relativistic magnetohydrodynamic simulations, the team modeled the assembly of an accretion disk around a newborn black hole. Initially, the accretion disk is puffy and tenuous, allowing small instabilities or inhomogeneities to evolve into large-scale spiral arms. In this phase, the disk produces a flat, weak gravitational wave spectrum. Just seconds later, the accretion rate ramps up, boosting the density of the disk and strengthening the gravitational wave signal.

Gottlieb’s team estimates that the gravitational waves from the accretion disk of a collapsing star 33 million light-years (10 megaparsecs) away could be detected with a signal-to-noise ratio of dozens by current instruments. Potential future instruments like Cosmic Explorer would see the event more clearly, with a signal-to-noise ratio of hundreds. What these estimates translate to is a potential detection of dozens to hundreds of collapsars each year by future observatories — and the possibility that several of these events are already hidden somewhere in the existing LVK data, awaiting the analysis that would extract their convoluted signals from the noise.

Overall, Gottlieb and coauthors show that collapsars are a promising but as-yet-undetected source of gravitational waves. Be on the lookout for future research from these authors examining the specifics of gravitational wave emission from collapsar disks.

Black Holes in the Mass Gap

Until recently, observations suggested that black holes could be no lighter than 5 solar masses. In addition, neutron stars are expected to top out out somewhere between 2 and 3 solar masses. This leaves a void between 2–3 and 5 solar masses called the mass gap. The reasons for this gap are unclear, and new observations have begun to populate the gap with candidate objects, suggesting that the “gap” may not be a gap at all. If these detections are confirmed, researchers must figure out how these mass-gap objects came to be.

In a recent article, Claire Ye (University of Toronto) and collaborators explored how black holes in the mass gap might form, leading to a prediction of how often we should expect to detect these black holes in gravitational wave observations. The team considered three main formation pathways, all of which involve a neutron star that is too massive to support itself, causing it to collapse into a black hole:

  1. a neutron star forms via the collapse of a massive star’s core and accretes stellar material that falls back on to the neutron star
  2. a neutron star forms through the collision of two white dwarfs or the accretion of material onto a white dwarf
  3. a neutron star gains mass through collisions or tidal interactions with another star
globular cluster NGC 1851

The densely packed globular cluster NGC 1851, as seen by the Hubble Space Telescope. [NASA, ESA, and G. Piotto (Università degli Studi di Padova); Processing: Gladys Kober (NASA/Catholic University of America)]

Because two of these scenarios involve close encounters between stars, Ye’s team modeled a site where stellar encounters are common: globular clusters. Globular clusters are roughly spherical collections of tens of thousands to millions of stars. The team modeled the evolution of 13 simulated globular clusters over 13.8 billion years. The simulated clusters were constructed to be similar to NGC 6752 — an especially dense “core-collapsed” globular cluster in which stellar interactions are expected to be more common than in a typical cluster — and NGC 1851, where a candidate mass-gap object was recently discovered.

The simulations show that mass-gap black holes can be produced in dense star clusters, with different simulation runs producing anywhere from 4 to 465 mass-gap black holes. The main formation pathway also varied between simulations, with merging neutron stars or white dwarfs generally being the most common, except in more massive clusters, where core-collapse supernovae produced nearly as many mass-gap black holes as mergers did.

plot of neutron stars and black holes formed in simulations

Neutron stars (left) and black holes (right) produced in globular clusters. The dividing line between neutron stars and black holes was set to be 2.5 solar masses. “Rapid” and “delayed” refer to the timescales over which supernova explosions play out. This factor has a large impact on the formation of mass-gap black holes. Click to enlarge. [Ye et al. 2024]

After their creation, some mass-gap black holes are ejected from their natal clusters through dynamical interactions or the force of the supernova that created them. A small number of the remaining black holes are captured into binary systems with other black holes or neutron stars, creating the conditions for gravitational waves to be emitted. The team calculated the rate of gravitational-wave-producing mergers to be less than one per cubic gigaparsec per year. Thus, while this work demonstrated that there are several ways to create black holes with masses in the mass gap, it’s not surprising that few candidates in this mass range have been detected so far.

Bringing the Background to the Foreground

The last of today’s research articles concerns supermassive black holes in the nearby universe. The slow rumblings of countless supermassive black hole binaries are thought to generate a subtle background hum of gravitational waves at nanohertz frequencies. In 2023, research collaborations across the globe announced the discovery of significant evidence for the presence of the nanohertz gravitational wave background, creating new opportunities to test our understanding of the population of supermassive black holes in the universe.

Cataloging the masses of supermassive black holes at different points in cosmic history is critical for testing models of galaxy formation and black hole growth. The masses and numbers of supermassive black holes can also be used to predict the strength of the nanohertz gravitational wave background; comparing this prediction against observations can help test whether the census of black holes is complete, especially at the high-mass end that is thought to dominate the gravitational wave signal.

To study the highest-mass black holes, Emily Liepold and Chung-Pei Ma (University of California, Berkeley) undertook a study of the most massive galaxies in the nearby universe. The team used the results of the MASSIVE survey, which produced images and spectra of nearby massive galaxies. Leveraging existing dynamical and stellar-population modeling of a subset of these galaxies, they calculated the stellar mass of each galaxy in the MASSIVE sample. Then, using established relations between a galaxy’s stellar mass and the mass of its central black hole — the more massive the galaxy, the more massive the black hole — they assigned a black hole mass to each galaxy in their sample.

plot of gravitational wave amplitude

Comparison of the characteristic gravitational wave strain, or amplitude, calculated in this work (purple circle) to values calculated by other teams (circles of other colors) and observed using pulsar timing arrays (triangles). [Adapted from Liepold & Ma 2024]

Finally, Liepold and Ma used their black hole mass function to calculate the strength of the gravitational wave background due to merging supermassive black hole binaries. The derived strength is consistent with observations, suggesting that the census of local massive black holes is close to complete. Curiously, the team’s measurements suggest a much higher mass density of black holes than what has been extrapolated from measurements of quasars — extremely luminous galactic centers that are powered by accretion onto a supermassive black hole. Future work will explore the reason behind this discrepancy.

Citation

“In LIGO’s Sight? Vigorous Coherent Gravitational Waves from Cooled Collapsar Disks,” Ore Gottlieb et al 2024 ApJL 972 L4. doi:10.3847/2041-8213/ad697c

“Lower-Mass-Gap Black Holes in Dense Star Clusters,” Claire S. Ye et al 2024 ApJ 975 77. doi:10.3847/1538-4357/ad76a0

“Big Galaxies and Big Black Holes: The Massive Ends of the Local Stellar and Black Hole Mass Functions and the Implications for Nanohertz Gravitational Waves,” Emily R. Liepold and Chung-Pei Ma 2024 ApJL 971 L29. doi:10.3847/2041-8213/ad66b8

illustration of a black hole floating through space

Could dark matter — the mysterious substance that makes up 85% of the mass of the universe — partially be black holes the size of planets? New research looks to the Milky Way’s neighbors for an answer.

Rogue Planets or Tiny Black Holes?

schematic of a gravitational microlensing event involving a black hole

Schematic of a black hole lensing the light from a background star as the black hole passes in front of the star from the vantage point of an observer on Earth. Click to enlarge. [NASA/ESA; CC BY 4.0]

Free-floating exoplanets and black holes once roamed the galaxy undetected. With the development of sensitive telescopes and attentive surveys, these objects are now revealed through gravitational microlensing. Microlensing surveys look for the short-lived increase in brightness caused by a foreground object passing in front of a background star and lensing, or focusing, the star’s light.

Thanks to surveys like the Optical Gravitational Lensing Experiment (OGLE), researchers have identified thousands of microlensing events. Most of these events are due to stars, brown dwarfs, and stellar remnants, but a small number seem to arise from objects the size of planets. While the majority of astronomers attribute these signals to free-floating exoplanets unmoored from their host stars, others have proposed a more speculative cause: primordial planet-sized black holes. Primordial black holes are theorized to have formed early in the universe when dense regions collapsed directly into black holes without forming stars first.

Previous research has suggested that if the observed microlensing signals are due to primordial planet-mass black holes rather than planets, as much as 10% of the mass attributed to dark matter is made up of tiny, ancient black holes. It’s an enticing theory, but a highly speculative one — how can we tell if the microlensing signals come from planets or planet-sized black holes?

OGLEing Microlensing Events

In their recent research article, Przemek Mróz (University of Warsaw) and collaborators showed that answering this question is all about looking in the right place. In the Milky Way’s disk and bulge, where stars and planets are common, it’s impossible to distinguish between microlensing signals from planets and planet-sized black holes. Away from these heavily populated areas, interference from stellar and planetary signals should be minimal, and microlensing signals could reveal primordial planet-mass black holes — should they exist — floating through the Milky Way’s dark-matter halo.

microlensing survey search area in the Magellanic Clouds

The survey’s search area overlaid on images of the Large (left) and Small (right) Magellanic Clouds. [Mróz et al. 2024]

From October 2022 to May 2024, Mróz’s team carried out the OGLE High-cadence Magellanic Clouds Survey, peering through the Milky Way’s sparsely populated halo toward the Large and Small Magellanic Clouds, two of the Milky Way’s satellite galaxies.

The resulting dataset included 17.6 million light curves from stars in the Magellanic Clouds, which the team whittled down using several criteria. First, they required that the light curves show an increase in brightness but be otherwise steady, eliminating variable stars and reducing the dataset to 2,538 light curves. Then, they removed false positives and light curves affected by imaging artifacts, leaving 308 light curves. Finally, they eliminated all light curves that weren’t fit well by a microlensing model — leaving just two possible microlensing events.

And Then There Was One

plots of candidate microlensing signals

The two microlensing candidates found in the authors’ survey. The top plot shows what is likely a bona fide microlensing signal, while the bottom plot shows what the authors believe is a stellar flare. [Mróz et al. 2024]

A close inspection of the remaining two candidates revealed that only one was likely to be a genuine microlensing event; the other appeared to be an interloping Milky Way star emitting a flare. The detection of just a single microlensing event in this expansive survey strongly limits the number of primordial planet-mass black holes in the Milky Way and, by extension, the contribution of these black holes to the overall mass of dark matter.

Extrapolating from their observations, Mróz’s team calculated that primordial planet-mass black holes — between roughly half the mass of the Moon up to the dividing line between planets and brown dwarfs — can make up at most just 1% of all dark matter. This in turn suggests that the microlensing signals previously attributed to primordial planet-mass black holes are instead due to the more mundane (though still fascinating) population of exoplanets without host stars.

Citation

“Limits on Planetary-Mass Primordial Black Holes from the OGLE High-Cadence Survey of the Magellanic Clouds,” Przemek Mróz et al 2024 ApJL 976 L19. doi:10.3847/2041-8213/ad8e68

illustration of an M-dwarf star and its planet

Which of the nearly 6,000 known exoplanets have atmospheres? With help from JWST, astronomers are inching closer to an answer, and new observations of a super-Earth planet around a low-mass star help to define the dividing line between planets with atmospheres and planets without.

How to Find an Atmosphere

With the number of known exoplanets growing steadily larger, a major challenge for astronomers is deciding how to allocate limited telescope time to study these planets further. Rocky planets with atmospheres make promising targets, but it’s not obvious which exoplanets should have atmospheres. Taking cues from the planets in our solar system and the subset of exoplanets that have been studied in detail, researchers have defined the concept of the cosmic shoreline, which separates planets with atmospheres from planets without on the basis of escape velocity — related to a planet’s mass and size — and the amount of starlight the planet receives.

While simple in concept, charting the cosmic shoreline is difficult, especially for planets circling M dwarfs: the smallest, coolest, and most common type of star. M dwarfs are notorious for their extreme space weather, which can strip away a planet’s atmosphere over billions of years. To learn more about the exact position of the cosmic shoreline and to guide our observations of M-dwarf exoplanets, researchers must search for atmospheres on planets subjected to the fierce conditions of an M-dwarf host star.

A Promising Planet

Discovered by the Transiting Exoplanet Survey Satellite (TESS), Gliese 486b (Gl 486b or GJ 486b) is a super-Earth exoplanet with a radius of 1.29 Earth radii and a mass of 2.77 Earth masses. Its host star is a 6.6-billion-year-old M dwarf that is known to explode with high-energy stellar flares. At just 26 light-years away, Gl 486b is one of the closest transiting terrestrial exoplanets known.

Last year, researchers published an investigation of Gl 486b’s atmosphere, finding that JWST transmission spectra suggested either a water-rich atmosphere or no atmosphere at all. If there is no atmosphere, the observed signal from water vapor must have come from cool regions on the host star rather than from the planet’s atmosphere.

Cartoon showing the geometry of a transit observation and a secondary eclipse observation

Cartoon showing the geometry of a transit observation and a secondary eclipse observation. Click to enlarge. [AAS Nova/Kerry Hensley]

Recently, Megan Weiner Mansfield (Steward Observatory; Arizona State University) and collaborators used JWST to collect secondary eclipse observations of Gl 486b, watching as the planet passed behind its host star. Secondary eclipse observations show the thermal glow of the planet and starlight reflected from its surface. Coupling the secondary eclipse observations with previously collected JWST and TESS data, the team found that the planet’s dayside temperature is a scorching 865K. This high temperature suggests that Gl 486b has either a thin atmosphere or no atmosphere at all, since a thick atmosphere would redistribute the heat and lower the dayside temperature.

plots showing well-fitting bare surface and atmosphere models as well as poorly fitting models

Best-fitting bare surface (top) and atmosphere (middle) models. The bottom panel shows poorly fitting models. Click to enlarge. [Mansfield et al. 2024]

Atmosphere Likely Lacking

With these constraints in hand, Mansfield’s team used forward modeling to explore the types of atmospheres that could be present. They found that a planet with no atmosphere provided the best fit to the data, but a thin atmosphere was also acceptable, as long as it contained only a small amount of water or carbon dioxide.

Given the realities of 6.6 billion years spent around an active M-dwarf star, the team concluded that Gl 486b is unlikely to have an atmosphere. In addition to weighing in on the likelihood of Gl 486b having a water-rich atmosphere, the team also achieved the most precise measurement yet of a planet’s dayside temperature, constraining the temperature to within just 14K.

Location of Gl 486b relative to the approximate cosmic shoreline

Location of Gl 486b relative to the approximate cosmic shoreline. [Mansfield et al. 2024]

Overall, these findings help to define the location of the cosmic shoreline and suggest that in order to find M-dwarf exoplanets that have retained their atmospheres, exoplanet atmosphere hunters must probe more massive or less strongly irradiated worlds.

Citation

“No Thick Atmosphere on the Terrestrial Exoplanet Gl 486b,” Megan Weiner Mansfield et al 2024 ApJL 975 L22. doi:10.3847/2041-8213/ad8161

Artist's rendition of a brown dwarf.

Somewhere between planets and stars is a cool, cloudy class of objects called brown dwarfs. A new study suggests that swirling clouds at the poles of these intriguing objects may provide clarity on their observed characteristics.

Trendy Brown Dwarfs

Not quite big enough to spark hydrogen fusion, but not quite small enough to lump in with gas giant planets, brown dwarfs sit in a perplexing in-between space and are not yet fully understood. As space-based observations of brown dwarfs have evolved, recent studies have revealed long-term variations in the brightness of brown dwarfs as well as unexplained trends in their color and spectral characteristics depending on from which angle we happen to view them. 

Previous studies have theorized that these variations are driven by changes in atmospheric properties of brown dwarfs. Changes in the rotation of clouds in a brown dwarf’s atmosphere can explain the observed short-term variability, but the observed long-term variations are not well explained by this same property. Could there be a single contributor driving both the observed short- and long-term variations in brown dwarfs? 

Brown dwarf atmosphere components

Three modeled atmosphere components, which include the evolving polar vortex that changes with time, the ambient atmosphere that is stable, and the bands of active clouds that change and evolve on short timescales. The top panel shows a 30 degree inclination angle where the polar vortex is clearly visible and will impact the observed photometric and spectroscopic properties of the brown dwarf. The bottom panel shows an 80 degree inclination angle where the equatorial bands cover most of the brown dwarf’s disk. Click to enlarge. [Fuda & Apai 2024]

Polar Vortex Potential

Two astronomers at the University of Arizona, Nguyen Fuda and Dániel Apai, posit that the color– and spectral–inclination trends observed in brown dwarfs could be driven by polar vortices — an expanse of swirling air sitting atop the pole of a planet. To test this hypothesis, the authors explore three model atmospheres (no vortex, evolving vortex, and stationary vortex) and compare the simulated observations of each to understand how the presence or absence of a polar vortex may impact brown dwarf observations over time.

Depending on the viewing angle, we may see primarily equatorial cloud bands or we may see a significant portion or all of the brown dwarf’s pole. By making simulated observations across a series of inclinations, the authors find that both the evolving and stationary vortex models produce color–inclination variations that align with previous observations of brown dwarfs — polar vortices tend to have bluer infrared colors compared to the redder equatorial regions.

Brown dwarf trends with no vortex, evolving vortex, and stationary vortex.

Resulting variability–inclination (top) and color–inclination (bottom) trends across various inclinations for short-term (left) and long-term (right) simulated observations of each modeled scenario. Click to enlarge. [Fuda & Apai 2024]

All three scenarios, over short-term monitoring, exhibit variability–inclination trends. As more and more active equatorial bands become visible, short-term variations in brightness increase. On the other hand, over long-term monitoring, the evolving vortex scenario produces a variability-inclination trend opposite to that seen in the stationary and no vortex scenarios.

Solar System Similarities

Brown dwarfs share similar characteristics to gas giants, like those in our own solar system. Many observations of Jupiter, Saturn, Uranus, and Neptune reveal clear vortex-dominated poles that produce similar color and brightness variability. These planets, though smaller than brown dwarfs, show variations from pole to equator that grant validity to the idea of polar vortices being present in brown dwarf atmospheres.

Jupiter's South Pole

Jupiter’s bright blue south pole covered in vortices, swirling around like hurricanes. [NASA/JPL-Caltech/SwRI/MSSS/Betsy Asher Hall/Gervasio Robles]

What does this mean moving forward? The authors suggest that, based on their results, brown dwarf atmospheres are more complex than simpler, non-changing brown dwarf atmosphere models that have traditionally been used. As more long-term space-based observations of brown dwarfs become available, the polar vortex hypothesis can be tested further, allowing astronomers to unwind some of the mysteries of brown dwarfs. 

Citation

“The Polar Vortex Hypothesis: Evolving, Spectrally Distinct Polar Regions Explain Short- and Long-term Light-curve Evolution and Color–Inclination Trends in Brown Dwarfs and Giant Exoplanets,” Nguyen Fuda and Dániel Apai 2024 ApJL 975 L32. doi: 10.3847/2041-8213/ad87e9

illustration of a neutron star merger

With the Neil Gehrels Swift Observatory nearing the 20th anniversary of its launch, astronomers have demonstrated a new way to use the seasoned telescope. The technique, which involves rapidly slewing to the location of a gravitational wave signal, could open a window onto the critical first minutes after a neutron-star merger.

When Gravitational Waves and Radiation Meet

gravitational wave signal from a neutron-star merger

The “chirp” signal of the neutron-star merger GW170817, as seen by the Laser Interferometer Gravitational-wave Observatory in Livingston, Louisiana. [Caltech/MIT/LIGO Laboratory]

In 2017, researchers detected gravitational waves and electromagnetic radiation from the collision of two neutron stars for the first time. Just 1.7 seconds after the gravitational wave signal was detected, two all-sky gamma-ray observatories — the Fermi Gamma-ray Space Telescope and the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) — happened to detect a burst of gamma rays from the collision.

As fortuitous as this detection was, Fermi and INTEGRAL lacked the localization ability needed to pinpoint the burst’s position. The delay in localization cost astronomers precious time, and it wasn’t until another 12 hours had passed that optical and ultraviolet emission from the collision was identified — wavelengths that are critical to collect as soon as possible to understand the physics of the merger and the cascade of element formation that follows. Is there a way to find a neutron-star merger faster?

Swift Follow-Up

In a recent article, Aaron Tohuvavohu (University of Toronto; California Institute of Technology) and collaborators showed how we can rapidly locate neutron-star mergers on the sky with an existing tool: the 20-year-old Swift Observatory. Gamma rays, which are produced just seconds after a neutron-star collision, are ideal for quickly spotting the radiation from a merger — and Swift has the most sensitive gamma-ray detector currently in operation. Once a gamma-ray source falls within Swift’s field of view, which spans 17% of the sky, the telescope has the ability to localize the source to within 1–3 arcminutes, giving follow-up telescopes sensitive to other wavelengths a small area to search. But unlike all-sky gamma-ray telescopes, Swift can’t look everywhere at once, raising the question of how to get the telescope to point toward the right place at the right time.

maps of on-sky localizations of gravitational wave signals

Demonstration of how the localization of a gravitational wave signal evolves as the time to merger decreases. Click to enlarge. [Tohuvavohu et al. 2024]

Tohuvavohu and collaborators outlined a way to use Swift to rapidly follow up on early-warning events — gravitational waves detected from inspiraling neutron stars before they merge. Essentially, after receiving notice of an impending neutron-star merger and a rough map of the merger’s location on the sky, Swift reorients to the likely location of the event, aiming to catch the gamma rays from the collision and pinpoint its location. But when is the best time for Swift to start turning toward the source? Current gravitational wave instruments can give up to 70 seconds of lead time before a neutron-star merger, but at that early time, they can only localize the event to an area spanning thousands of square degrees. As the merger draws nearer, the map of potential on-sky locations gets more precise, but waiting for better location information leaves Swift less time to move — and once the observatory begins slewing, it can’t be redirected until it’s finished moving.

Doubling the Odds

After being alerted to an imminent neutron-star merger, should Swift immediately race to the likeliest location, or should it wait to get a better idea of where the event is happening? Given the time required to wend toward the signal’s origin, curving to avoid forbidden zones too close to Earth or the Sun, Tohuvavohu’s team finds that it’s best for Swift to act on the earliest possible knowledge of where the merger is likely to happen, even though later maps are more accurate.

plot showing the fraction of gravitational wave events falling within the Burst Alert Telescope field of view as a function of time until merger

Fraction of gravitational wave events encompassed by the Swift Burst Alert Telescope field of view as a function of the notice given in advance of the merger. [Adapted from Tohuvavohu et al. 2024]

This method isn’t foolproof — Swift will sometimes rush to a location, only for it to become clear that the merger happened elsewhere — but the team estimated that it could more than double the rate at which neutron-star mergers are precisely localized. Quick localization, in turn, enables the rapid follow-up at optical and ultraviolet wavelengths that is key to distinguishing between competing merger models.

Future, more sensitive gravitational wave observatories will be able to detect the faint hum of spiraling neutron stars even longer before impact, helping Swift get into position more frequently. In the meantime, Tohuvavohu and coauthors recommend working to improve the speed at which notice of an impending neutron-star merger can be transmitted to Swift.

Citation

“Swiftly Chasing Gravitational Waves Across the Sky in Real Time,” Aaron Tohuvavohu et al 2024 ApJL 975 L19. doi:10.3847/2041-8213/ad87ce

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