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photograph of a Centaur second-stage rocket

Upcoming surveys will find dozens of near-Earth objects each night, but some of those objects will be space junk rather than asteroids. How will we tell the difference?

Masquerading as Asteroids

When humankind sent its first rocket toward the Moon 65 years ago, it marked the beginning of a long era of lunar exploration. Now, the cast-off rocket parts from decades of lunar missions are cropping up in an unexpected place: searches for near-Earth asteroids.

Phase angle and light curve of an artificial object mistaken for an asteroid

Phase angle (top) and light curve (bottom) of an artificial object mistaken for an asteroid. [Battle et al. 2024]

Several artificial objects have already been mistakenly cataloged as asteroids, and that number is likely to grow. More than 100 lunar missions are planned for the next decade, creating opportunities for more space debris to masquerade as natural near-Earth objects. Future surveys, like Rubin Observatory’s Legacy Survey of Space and Time that will be underway in 2025, will discover dozens of near-Earth objects every night. To probe ways to differentiate between asteroids and space debris, researchers have turned their astronomical tools toward a near-Earth object that is likely a rocket body discarded in 1966.

One Person’s Space Junk Is Another’s Research Opportunity

In 2020, a planetary defense survey discovered a candidate near-Earth object named 2020 SO. Later, dynamical studies suggested that rather than being an asteroid, 2020 SO is instead a discarded Centaur-D rocket body from NASA’s Surveyor 2 mission to the Moon. 2020 SO’s orbit is unusual for a natural object, and while it’s large, roughly 10 meters by 3 meters, it’s lightweight enough that just the gentle pressure of the Sun’s radiation can push it off course.

reflectance spectra of 2020 SO and two classes of asteroids

Reflectance spectrum of 2020 SO (black) compared to those of two types of asteroids (green and blue). The spectra are normalized at 0.7 μm. [Battle et al. 2024]

Adam Battle (University of Arizona) and collaborators observed 2020 SO to search for photometric or spectral signatures that could differentiate between artificial objects and asteroids. Using visible-light observations from the Large Binocular Telescope, Battle’s team showed that 2020 SO is redder than is typical for most asteroids.

Conversely, it’s close in color to three known Centaur-D rocket bodies in Earth orbit, although the colors don’t match perfectly. This suggests that if 2020 SO is also a Centaur-D rocket body, its surface has changed after decades of space aging from radiation and particle impacts.

Spectroscopy Has the Last Word

While it’s possible for artificial objects to be similar in color and reflectance to asteroids, spectroscopy lays their differences bare by revealing what they’re made of. A Centaur-D rocket body is largely covered in stainless steel and a polymer called polyvinyl fluoride, and its spectrum likely contains features from both materials. Using data from the Infrared Telescope Facility, Battle and coauthors compared the spectrum of 2020 SO to the spectrum of a known Centaur-D rocket as well as laboratory spectra of stainless steel and polyvinyl fluoride.

Infrared spectra of 2020 SO and a Centaur-D rocket body

Infrared spectra of 2020 SO (black) and a Centaur-D rocket body (green). [Battle et al. 2024]

The spectrum of 2020 SO is similar in shape to the spectrum of stainless steel, and 2020 SO sports a distinct absorption feature at 2.3 microns (1 micron = 10-6 meter), just like the spectrum of polyvinyl fluoride. Taken together, 2020 SO’s area-to-mass ratio, color, and spectrum all point to the object being a rocket body rather than an asteroid.

This study shows that photometry and infrared spectroscopy can reveal the colors and spectral features necessary to identify near-Earth objects. To take the identification of artificial objects a step further, future work may require a database of artificial-object spectra and laboratory studies of common spacecraft materials.

Citation

“Challenges in Identifying Artificial Objects in the Near-Earth Object Population: Spectral Characterization of 2020 SO,” Adam Battle et al 2024 Planet. Sci. J. 5 96. doi:10.3847/PSJ/ad3078

How do cosmic rays spread through the galaxy? Historically, that’s been a hard question to answer from within the Sun’s protective bubble that shields us from some fraction of these high-energy particles. However, now that Voyager 1 has completed its hard-fought journey into interstellar space, astronomers can study cosmic rays beyond the reach of the Sun to build a fuller picture of the lives of these lightning-fast bits of matter.

Tiny Bullets

A schematic of Voyager 1. The cosmic ray experiments are in the upper right. [NASA’s Goddard Space Flight Center]

Cosmic rays, or tiny chunks of atomic nuclei that whiz around at nearly the speed of light, permeate the entire Milky Way and the spaces between the galaxies. For nearly a century, astronomers have observed them flying through our own backyard, first via balloon experiments that caught them colliding with particles in Earth’s upper atmosphere, then later via satellites perched in orbit. Throughout the entire span of these observations, we’ve known that the cosmic rays we’ve recorded might not be representative of the broader population, since we only see those that managed to barrel into the inner solar system without being dissuaded by the protective influence of the Sun. This has made it challenging to model how cosmic rays flow through space in general, and consequently harder to constrain where they all come from.

For about a decade, however, astronomers have been able to call upon a hardy scout that has flown beyond the reaches of our Sun and into the region where cosmic rays roam free. Voyager 1, launched an astounding 46 years ago and still limping along despite recent communication challenges, was the first human-made object to reach interstellar space back in 2012. Even as the spacecraft shut down other instruments to ration its limited power source, it kept its cosmic ray experiments powered up and collecting data. Now, by combining measurements from Voyager 1 with data collected by instruments on the International Space Station, astronomers can revisit their models of how cosmic rays move through the galaxy and within our solar system.

The Alpha Magnetic Spectrometer after its installation aboard the International Space Station. [NASA]

Many Models Mostly Working

Ethan Silver, University of California Berkeley, and Elena Orlando, University of Trieste, recently attempted this comparison. Using about a decade’s worth of data from Voyager 1 and from the Alpha Magnetic Spectrometer aboard the International Space Station, they were able to fit six different types of models of how cosmic rays move through the galaxy, each with many parameters that needed to be carefully tuned. Some of these models assumed that one can capture the behavior of cosmic rays using only a diffusion framework, while others were much more complex and allowed for numerous discontinuities in the input spectra of energies for each type of particle.

Silver and Orlando found that in general, all of the models they tested could explain the observed data with reasonable success. Though unsurprisingly the more complex models with more free parameters agreed with the measurements the best, the lighter-weight models held their own and couldn’t be confidently ruled out. Interestingly, all of the models tested failed on the same subset of observations: none of the schemes they tested, including the most complex diffusion-convection-reacceleration models, accurately predicted the flux of antiprotons with energies around ∼10 giga-electron volts. This missed prediction suggests that even with the help of probes far beyond our confines here on Earth, there is still much we don’t know about the production and transportation of cosmic rays. However, the general success of the models indicates that we’re on the right track, a validation enabled in large part by a distant and aging but well-loved spacecraft.

Citation

“Testing Cosmic-Ray Propagation Scenarios with AMS-02 and Voyager Data,” Ethan Silver and Elena Orlando 2024 ApJ 963 111. doi:10.3847/1538-4357/ad1ce8

photograph of the rings of Saturn and five of Saturn's moons

The giant planets in our solar system — Jupiter, Saturn, Uranus, and Neptune — collectively host nearly 300 moons. What can craters tell us about how old these moons are and how they formed?

How Are Moons Made?

submillimeter-wavelength image of the PDS 70 planetary system, which contains a circumplanetary disk

A submillimeter image of the PDS 70 planetary system. Between the outer ring of gas and the central star sits the Jupiter-like exoplanet PDS 70c, which is surrounded by a possible moon-forming circumplanetary disk. [ALMA (ESO/NAOJ/NRAO)/Benisty et al.; CC BY 4.0]

Theories of how the solar system’s many satellite worlds came to be are nearly as varied as the satellites themselves. The leading theory is that disks of gas, dust, and pebbles surrounding newborn planets provided the material for moons to form, much like how planets form out of gas and dust surrounding a newborn star. Other theories suggest that massive ring systems around giant planets are the sites of moon formation, or that some satellites coalesce from the debris of previous satellite collisions.

Crater counting provides a way to estimate the ages of satellite surfaces — placing limits on the ages of the objects themselves — and helps test these theories of satellite formation. To understand how the craters on a moon’s surface relate to the age of the surface, researchers had to imagine conditions in the solar system nearly 4.5 billion years ago, when things were far more chaotic than they are today.

plot of cumulative number of impactors versus impactor size

Modeled cumulative number and size of impactors striking giant planet satellites since the beginning of the bombardment period. Satellites with similar collision probabilities are grouped together for clarity. Click to enlarge. [Bottke et al. 2024]

Shuffling the Solar System

A few tens of millions of years after the gaseous nebula that enveloped the young Sun and its budding planets dissipated, Neptune shook up the solar system. As Neptune migrated outward from the Sun, it crossed into a primordial disk of planetesimals, kicking 99.9% of these icy bodies into the solar system. (The remaining 0.1% of planetesimals remained behind and formed what is today known as the Kuiper Belt.) Thus began a long period of bombardment that is recorded on the surfaces of the many moons of the outer solar system.

Recently, William Bottke (Southwest Research Institute) and collaborators assigned ages to the surfaces of 26 satellites of Jupiter, Saturn, Uranus, and Neptune by modeling crater formation during this bombardment period.

To measure surface ages from the craters observed on these satellites today, the team modeled the sizes and numbers of craters forming due to bombardment on each satellite as a function of time. This model accounts for the relationship between the size of the impacting body and the size of the crater, changes in the sizes of the impacting bodies over time due to collisions between impactors, and other factors.

Missing Early History

modeled and observed crater size and frequency values

Modeled (black lines) and observed (colored circles) crater size and frequency values for several small moons. The derived surface ages are listed in each panel. Bombardment begins at T = 20 million years after the gas disk disperses. Click to enlarge. [Bottke et al. 2024]

All of the satellites in the study were missing evidence of the large impacts that should have occurred early on in the bombardment period. This suggests that the satellites were resurfaced through surface melting, shattering, or total disruption of the object, hiding these large early craters. The oldest surfaces, belonging to Iapetus, Hyperion, Phoebe, and Oberon, go back to just a few million years after the bombardment began, while the youngest surfaces, belonging to the largest moons, could be billions of years younger.

With early history erased by large impacts, it’s not possible to definitively state how these worlds formed — though Bottke’s team notes that for midsize and large moons, their model results are consistent with what’s expected for satellites forming in a disk around their host planets. Significant roadblocks exist for other theories. If future spacecraft can make sensitive gravity measurements of these worlds, revealing bombardment-induced changes to their crusts and interiors, it could reveal this hidden era of solar system history.

Citation

“The Bombardment History of the Giant Planet Satellites,” William F. Bottke et al 2024 Planet. Sci. J. 5 88. doi:10.3847/PSJ/ad29f4

phases of a total solar eclipse

Today’s the day! The Moon’s shadow will sweep across Mexico, the United States, and eastern Canada, plunging millions of eclipse viewers into a few minutes of mid-afternoon twilight. I’ll be enjoying a partial eclipse in partly cloudy Colorado, having bailed on my original plans for totality in Texas due to thunderstorms. See you all in Spain in August 2026, perhaps?

Whether your plans have been sidelined by weather or you’re enjoying the eclipse under sunny skies, take a moment to delve into three solar physics research articles with us today to learn how researchers are studying our home star.

Launching the Solar Wind with Tiny Jets

The solar wind is a hot, tenuous plasma that constantly streams out from the Sun, whipping past Earth at about 400 kilometers per second. How the solar wind is launched from the Sun’s atmosphere is a hot topic, with recent research suggesting that thousands of small jets powered by the release of pent-up magnetic energy could provide the boost the solar wind needs to get going.

plot of number of events versus size of events for erupting filament-like features on the Sun

Number and size distribution of erupting filaments on the Sun. From left to right, the black circles represent microfilament eruptions, jetlets, minifilament eruptions, and solar flares and coronal mass ejections. [Sterling et al. 2024]

Recently, Alphonse Sterling (NASA’s Marshall Space Flight Center,) and collaborators examined the physics behind this hypothesis. Coronal jets and their smaller counterparts, jetlets, seem to be associated with the eruption of loops of coronal plasma called filaments. Sterling’s team cataloged a wide range of solar phenomena, from tiny minifilaments that span just a few thousand kilometers (“tiny” is relative!) to huge coronal mass ejections that shrug off the Sun’s gravity and explode into space. The team found that the sizes and occurrence rates of these coronal structures fall along a power law, suggesting that they may all be triggered by the same mechanism: the eruption of a twisted tube of magnetic fields called a magnetic flux rope.

Looking more closely at the connection between jets and the solar wind, Sterling’s team proposed that when a flux rope erupts, the material ensnared within the rope is launched outward, and the intrinsic twist of the magnetic field in a flux rope could be transferred to field lines that extend out into the solar system. The movement of this magnetic field twist could produce magnetic zigzags called switchbacks, which have been observed in detail by the Parker Solar Probe and Solar Orbiter, connecting the beginnings of the solar wind to phenomena seen farther downwind.

A Coronal Mass Ejection Seen from Mercury, Earth, and Other Locations in Space

With spacecraft orbiting Earth, zooming around other planets, and even skimming the Sun’s atmosphere, we can study the Sun from more angles than ever before. In February 2022, three spacecraft made ultraviolet observations of a loop of solar plasma suspended within the Sun’s magnetic field. The loop of plasma, called a prominence, eventually erupted as a coronal mass ejection. The BepiColombo spacecraft and the Parker Solar Probe weathered the coronal mass ejection from their positions near Mercury’s orbit, just 0.35 au from the Sun.

three extreme-ultraviolet images of the Sun, showing different perspectives on an erupting filament

Three extreme-ultraviolet viewpoints of the same event on 15 February 2022. From left to right: the STEREO spacecraft, at Earth’s orbital distance; Solar Orbiter, around 0.75 au; and the GOES spacecraft, in orbit around Earth. [Palmerio et al. 2024]

As the fleet of spacecraft exploring our solar system grows, it has become more common for coronal mass ejections to interact with multiple spacecraft (although it’s still quite rare!). This event gave researchers an exceptional opportunity to study coronal mass ejections, as BepiColombo and the Parker Solar Probe were separated by only a few million kilometers and were only a small angular distance apart.

This allowed Erika Palmerio (Predictive Science Inc.) and collaborators to study how the structure of a coronal mass ejection varies on these scales. Despite the small separation of the spacecraft, many of the properties they measured were considerably different. For example, they measured different directions of the shock as it passed over them, and the time it took for the coronal mass ejection to pass over them was measured to be different by more than 9 hours. This reveals that coronal mass ejections have considerable variations over the scales examined in this study, and more work is needed to understand how and why these variations exist.

A Potential Solution to a Persistent Problem

Magnetic field lines that emerge from the Sun’s surface and loop back to re-enter the surface are called closed field lines, while those that extend out into the solar system are called open field lines. For decades, researchers modeling the solar corona have encountered a persistent issue: models predict far less open magnetic flux than spacecraft measure. The discrepancy can be a factor of two or more, and the mismatch tends to be greatest when modeling the Sun at its most active.

comparison of open magnetic flux observations and derived measurements

Comparison of the observed open magnetic flux (red line) with the results of various methods of tallying the flux. The blue line shows the results of the method explored in this study. Click to enlarge. [Arge et al. 2024]

A research team led by C. Nick Arge (NASA’s Goddard Space Flight Center) has proposed that the solution to this problem lies at the boundaries between regions of open and closed field lines. Arge’s team showed that the traditional method of summing up regions of open magnetic flux at the outer boundary of the model — around 2.5 solar radii — undercounts the amount of open flux, while a method that relies on tracing the magnetic field from the outer boundary down to the solar surface and from the solar surface to the outer boundary nearly matches the observed values.

The mismatch between the two methods is greatest when the grid cells of the model fall upon an active region — an area of especially strong and complicated magnetic fields — that lies on the edge of a coronal hole, where a concentration of open magnetic field lines carries solar plasma into space. This fact hints at why previous methods were least successful in matching the observed open flux when the Sun was active; high solar activity means more active regions, and more opportunities to miss open flux.

While the new method hasn’t yet been perfected, tending to overestimate the open magnetic flux in some cases, the authors hope that it will finally bring the open flux mystery to a close.

Citation

“How Small-Scale Jetlike Solar Events from Miniature Flux Rope Eruptions Might Produce the Solar Wind,” Alphonse C. Sterling et al 2024 ApJ 963 4. doi:10.3847/1538-4357/ad1d5f

“On the Mesoscale Structure of Coronal Mass Ejections at Mercury’s Orbit: BepiColombo and Parker Solar Probe Observations,” Erika Palmerio et al 2024 ApJ 963 108. doi:10.3847/1538-4357/ad1ab4

“Proposed Resolution to the Solar Open Magnetic Flux Problem,” C. Nick Arge et al 2024 ApJ 964 115. doi:10.3847/1538-4357/ad20e2

Illustration of a dusty accretion disk surrounding the supermassive black hole at the center of a galaxy

Do models of turbulent, magnetized gas in accretion disks around black holes “remember” the conditions they started with, or is that information washed away as the model evolves?

Magnetic Fields and Accretion Disks

If you’re an astronomer, you may already know this joke: an astronomer is speaking with a therapist, discussing a recent bout of sleeplessness. The therapist asks, “What do you think it is that’s keeping you up at night?” Staring wide-eyed at the ceiling, the astronomer whispers, “Magnetic fields…”

Today’s article presents one example of why astronomers might lose sleep over magnetic fields. Magnetic fields are a key component of accretion disks, which feed material to growing stars, black holes, and other objects. Researchers have had great success in modeling magnetized accretion disks, but the huge computational cost of increasingly complex models has prompted the use of time-saving shortcuts with the potential for unintended side effects.

Testing a Model’s Memory

One way to speed up simulations is to start out with the model’s magnetic fields close to the desired end state. Recently, Payton Rodman (University of Cambridge) and Christopher Reynolds (University of Cambridge and University of Maryland) tackled the question of whether it matters what kind of magnetic fields a model starts with when simulating accretion disks around supermassive black holes.

illustration of poloidal and toroidal directions

Illustration of the poloidal (red arrow) and toroidal (blue arrow) directions. [Wikipedia user DaveBurke; CC BY 2.5]

Our best observations of supermassive black holes suggest that the magnetic fields close to the black hole are strong and poloidal, meaning that the fields are vertical close to the black hole and arc up over the disk. These strong poloidal fields are thought to be responsible for launching powerful black hole jets.

To save time, many models start out with a strong poloidal magnetic field. However, real accretion disks might instead start out with weak toroidal magnetic fields, which circle the black hole horizontally. If all magnetic fields, regardless of initial strength of configuration, “forget” their initial state and eventually evolve into the strong poloidal fields that researchers observe, that means that modelers can choose any reasonable starting parameters. If the magnetic fields retain a memory of their initial state, modelers will need to carefully examine their starting conditions and how the fields evolve.

A Clear Imprint

Rodman and Reynolds compared the evolution of weak and strong toroidal magnetic field models to test the “memory” of their accretion disk model. They found that the weak and strong magnetic field simulations diverged quickly and didn’t reach the same end point — the final outcome retained an imprint of the initial field strength.

plot of magnetic field streamlines from the simulation results

Magnetic field streamlines for simulations with a weak initial magnetic field (left) and a strong initial magnetic field (right). Click to enlarge. [Rodman & Reynolds 2024]

Ultimately, the weak and strong toroidal magnetic fields both produced poloidal fields, but how strong the fields were and how widely they were distributed depended on the initial field strength. Critically, neither simulation generated poloidal fields strong and widespread enough to reach the magnetically arrested disk regime, which is thought to govern how supermassive black holes at the centers of galaxies accrete matter and produce jets.

The results of this study make it clear that initial magnetic field conditions must be chosen carefully, as accretion models appear to have long memories. Future work should delve into this issue further, hopefully helping astronomers everywhere to rest easier.

Citation

“Evolution of the Magnetic Field in High- and Low-β Disks with Initially Toroidal Fields,” Payton E. Rodman and Christopher S. Reynolds 2024 ApJ 960 97. doi:10.3847/1538-4357/ad0384

Illustration of a blue supergiant star

New research shows that the properties of some blue supergiant stars can be explained by the merger of a massive star with a smaller companion. This suggests that many of the brightest and hottest stars in our galaxy are not born, but made.

One Star or Two?

JWST image of SN 1987A

JWST image of SN 1987A. Some research suggests that the intricate ejecta pattern could not be possible without the supernova progenitor star undergoing a merger. [NASA, ESA, CSA, Mikako Matsuura (Cardiff University), Richard Arendt (NASA-GSFC, UMBC), Claes Fransson (Stockholm University), Josefin Larsson (KTH)]

The blue supergiant stage is a brief phase in the lives of hot, massive stars. Though the phase is short-lived, there seem to be a lot of blue supergiants, and despite the fact that massive stars tend to live in pairs or trios, there seem to be a lot of supergiants without a stellar partner. These curious findings could be explained by a scenario in which some blue supergiants form when a massive star swallows its companion.

Researchers have applied this line of thinking to several famous massive stars like Eta Carinae and the progenitor of the supernova SN 1987A, showing that the merger model explains these stars’ unusual properties. Now, a research team led by Athira Menon (Institute of Astrophysics of the Canary Islands and University of La Laguna) has taken the investigation a step further, using this model to explain the varied properties of a population of blue supergiant stars in a neighboring galaxy.

evolutionary track of a stellar merger product

Evolutionary track for the merger product of a 31.6-solar-mass star and a 3.2-solar-mass star. [Menon et al. 2024]

A Star Is Made

Menon’s team used the Modules for Experiments in Stellar Astrophysics (MESA) to model the evolution of post-merger stars and to compare the properties of blue supergiants coming from single stars to those coming from stellar mergers.

In the stellar merger scenario, blue supergiants result from the collision of a massive post-main-sequence star and its main-sequence binary companion. As the massive star expands, it donates some of its mass to its companion, which eventually becomes entangled in the extensive outer atmosphere of the larger star. Doomed by friction and tidal forces, the smaller star “dissolves” within the larger star, setting the resulting star on a new evolutionary path.

Clues from Chemical Abundances

The team found that blue supergiants formed through mergers have different surface abundances of elements like carbon and oxygen compared to supergiants arising from single stars. To compare these results to the properties of actual blue supergiants, the team amassed a sample of 59 blue supergiant stars in the Large Magellanic Cloud and divided them into three groups based on the ratios of their carbon, nitrogen, and oxygen abundances.

chemical abundances of modeled and observed blue supergiants

Chemical abundance ratios for modeled stars (star icons) and observed blue supergiants (BSGs; triangles). Click to enlarge. [Menon et al. 2024]

The first group, which had relatively little nitrogen compared to carbon and oxygen, matched the outcomes of the single-star models — these stars are likely “true” blue supergiants that evolved from single stars. The second group, with moderate N/C and N/O ratios, could be explained by either single stars or stellar mergers. The final group, which made up about 40% of the sample, had larger abundance ratios that the single-star model couldn’t reproduce.

Taking into account other factors, such as systematic offsets, Menon’s team concluded that more than half of the 59 stars in their sample came from stellar mergers. This suggests that many blue supergiant stars owe their status to a stellar merger, and the merger model is a valuable tool to understand blue supergiant populations throughout the universe.

Citation

“Evidence for Evolved Stellar Binary Mergers in Observed B-type Blue Supergiants,” Athira Menon et al 2024 ApJL 963 L42. doi:10.3847/2041-8213/ad2074

Pulsars are the universe’s natural lighthouses, and we can learn much about them by modeling their periodic flashes. But, fitting these models is a tricky task that often requires manual decision-making. A new algorithm, however, promises to offload that work to the computers.

Listening to the Sky

A photograph of a 100m radio dish in an otherwise empty field.

The Green Bank Telescope, a workhorse among pulsar timing observatories. [NRAO/AUI/NSF, CC BY 3.0]

Pulsars, or neutron stars formed from the collapse of a massive star that can rotate hundreds of times each second, make excellent cosmic metronomes. As they spin, they slash Earth with beams of radiation that swing around once per revolution, and for decades now astronomers have used radio telescopes to record the arrival times of these periodic pulses. After collecting data over a few years, observers can try to fit them with models of the pulsar’s spin and a handful of other relevant properties.

At first glance, carrying out these fits might seem like a straightforward task since pulsar system can be described by only a handful of parameters including the spin frequency and its drift rate over time. So, naively one would think that if you’re just looking for the best-fitting parameters, you could code up a model, then fiddle around with its inputs until the fit looks correct. Unfortunately, radio astronomers are not so lucky.

Counting (Pulsar Rotations) Can Be Hard

A residual vs. time plot where the residuals are nearly uniformly distributed in phase from -0.5 to 0.5, indicating a poor fit to the data.

The residuals to the initial fit for one example pulsar. Since the model’s spin frequency is slightly incorrect, the scatter in the residuals is far larger than expected given the tiny error bars. [Taylor et al. 2024]

To illustrate why this simple task is actually quite finicky, consider just the variable describing the pulsar’s period and two measurements separated by a year. If we tweak the period by only a billionth of a second, that would imply that the pulsar has completed tens of thousands of more or fewer rotations in that time. It’s unlikely that the model’s predicted times precisely line up with the data for both observations, and once you also add the rate that the period changes, it suddenly becomes difficult to find the uniquely best combination of parameters that describe the data.

So, astronomers typically proceed cautiously. Instead of rushing in and immediately trying to fit all of the data with all of the parameters at once, they instead fit subsets of the data with simpler models, then iteratively add complications until they’re left with the full, final model. Historically, this was a manual process that involved a lot of choices about what to include and reject at each step, and scientists would often spend hours on each pulsar to get it right.

New Automation

A tree diagram with about 100 branches terminating around level 10, while one continues down to level 30.

An illustration of how the new algorithm tries different paths towards the final model fit. [Taylor et al. 2024]

In the past few years, however, the community has begun to develop more automated and efficient codes to offload more of the manual work to computers. The latest and most complex of these was just released by Jackson Taylor of West Virginia University. Called “Algorithmic Pulsar Timer for Binaries,” this open-source, Python-based script uses a tree-like structure to decide which of the model components to add or remove at each iteration, and it can automatically search hundreds of different decision paths without requiring expert intervention.

Taylor and collaborators designed their algorithm to handle not just isolated pulsars, but also more complicated systems where the pulsar has a nearby companion that affects its motion. To test their code, they let their new algorithm loose on two newly discovered pulsars that would have been very challenging to fit “by hand” due to the limited number of observations. Happily, they found that their procedure succeeded where a human would struggle: after exploring many different decision trees, it landed on a satisfactory solution in both cases.

Though it still requires several hours to arrive at the global best fit, being able to check so many paths in such a uniform way is an enormous benefit to previously bogged-down astronomers. As we continue to discover more and more pulsars, tools like this will likely see increased use over the next several years.

Citation

“Algorithmic Pulsar Timer for Binaries,” Jackson Taylor et al 2024 ApJ 964 128. doi:10.3847/1538-4357/ad1ce9

the Milky Way's central black hole in polarized light

In 2022, the Event Horizon Telescope Collaboration released the first image of the supermassive black hole at the center of the Milky Way. Now, the team has taken a closer look at our hometown black hole to understand the nature of its magnetic fields, the spin of its accretion disk, and more.

A First Look at the Milky Way’s Supermassive Black Hole

Event Horizon Telescope image of the Milky Way's central supermassive black hole

The first EHT image of Sgr A*. [EHT Collaboration]

Nearly seven years ago, a global network of radio dishes named the Event Horizon Telescope (EHT) turned toward the center of our galaxy. These telescopes peered through 27,000 light-years of gas, dust, and stars to study the massive object that is nestled there: Sagittarius A*, or Sgr A* (pronounced “sadge-ay-star”) for short, is a compact object four million times more massive than the Sun.

These observations revealed a bright ring of emission from the superheated accretion disk surrounding Sgr A* and confirmed that it is indeed a supermassive black hole with an event horizon — a surface beyond which nothing, including light, can escape the black hole’s immense gravitational pull.

polarization of Sagittarius A*

Top: Average polarization fraction for Sgr A*. Bottom: Polarization “field lines” showing the spiral pattern of the polarization angles. [EHT Collaboration et al. 2024]

From Brightness to Polarization

But the EHT did more than just measure the brightness of the glowing gas surrounding Sgr A* — it also measured the light’s polarization. As light waves wiggle through space, they can be oriented in any direction. If the light is oriented every which way, that light is unpolarized. If instead the waves are oriented along a particular plane, the light is polarized.

Hot, magnetized plasma emits linearly polarized light because the magnetic field influences the direction in which light waves oscillate. If this polarized light travels through magnetized material, the direction of polarization might rotate or become random. By analyzing the polarization of an object’s emitted light, researchers can study the strength and structure of the object’s magnetic fields and determine whether or not there is magnetized material between the source and our telescopes.

The EHT data show that the light from the hot, gaseous accretion disk surrounding Sgr A* is strongly polarized, with certain parts of the glowing disk being up to 40% linearly polarized. Some of the light — 5–10% — is circularly polarized, which means that the orientation of the waves rotates as the light travels. The polarization direction follows a distinct pattern, forming a spiral that curls around the black hole.

Measurements, Modeling, and Magnetic Fields

The amount of polarized light and the direction of polarization can help researchers disentangle the structure of the magnetic field lines close to the black hole and determine the properties of its accretion disk. The EHT collaboration used simple analytic models and rigorous numerical general relativistic magnetohydrodynamics models to study these properties.

simulated images of polarized emission from Sgr A*

The best-fit model of Sgr A* showing simulated images that are unblurred (left column) and blurred to the approximate resolution of the EHT (right column). The top row shows the total intensity, strength, and direction of the linearly polarized light. The bottom row shows the circularly polarized light. Click to enlarge. [Adapted from EHT Collaboration et al. 2024]

One significant challenge in modeling Sgr A* is its large rotation measure, or the change in the angle of polarization with wavelength. This rotation can come from magnetized material within or outside of the region emitting the polarized light, and it’s not yet clear which is the case for Sgr A*.

If the material is inside the emitting region, this suggests that the black hole’s accretion disk rotates in the opposite direction from what researchers have inferred from previous observations. Additionally, none of the models that place the material inside the emitting region can meet all of the observational constraints. If the material is instead outside this region, this would mean that the disk rotates in the expected direction, and the team finds one model that can meet all of the observational constraints. In this model, Sgr A* is surrounded by strong magnetic fields and the black hole rotates in the same direction as the material swirling around it.

hubble space telescope image of the galaxy M87 and its particle jet

This visible-light image from the Hubble Space Telescope shows the massive elliptical galaxy Messier 87. The Milky Way’s central black hole is much less massive than Messier 87’s, and Messier 87’s larger black hole powers a jet of electrons and other particles, seen here. [NASA and The Hubble Heritage Team (STScI/AURA)]

How Does Sgr A* Stack Up?

In addition to Sgr A*, the EHT has observed M87*, the supermassive black hole at the center of the massive elliptical galaxy Messier 87. M87* is about 1,500 times more massive than the Milky Way’s black hole, and it’s far more active, too — a jet of particles powered by the black hole stretches 5,000 light-years beyond the galaxy, whereas Sgr A* appears to have no jet at all.

Compared to Sgr A*, M87*’s light is much less strongly polarized, with the polarization fraction topping out around 15%. This means that hot, magnetized plasma close to the black hole is scrambling the polarization of the emitted light.

Despite these differences, both M87* and Sgr A* favor magnetically arrested disk models, in which magnetic fields pile up at the inner edge of the accretion disk and moderate the rate at which gas from the disk falls onto the black hole. Because magnetic fields are thought to play a critical role in the creation of supermassive black hole jets, the similarities in the magnetic fields of M87* and Sgr A* could mean that our galaxy’s black hole has a jet that’s yet to be discovered.

Looking Ahead

The EHT Collaboration’s results show that the closest supermassive black hole isn’t necessarily the easiest one to study; Sgr A* varies rapidly, changing noticeably even over the course of a single day’s observations, making the challenging feat of extracting information from the new polarized images even more complicated. The articles published today represent just the beginning of the investigation into Sgr A*’s polarized light, and future advances could come from new observations as well as improvements in modeling.

Models that have different starting conditions, include electrons with different energies, or fuel the black hole with helium gas instead of hydrogen gas, may provide a path forward. Future observations by the EHT will probe Sgr A* at a higher frequency, potentially allowing researchers to discern whether the material that is rotating the polarized light from the disk lies within the emitting region or outside of it.

While you wait for new measurements and modeling breakthroughs, be sure to check out all the Sgr A* research from the EHT team in the Focus Issue on First Sgr A* Results from the Event Horizon Telescope.

Citation

“First Sagittarius A* Event Horizon Telescope Results. VII. Polarization of the Ring,” EHT Collaboration et al 2024 ApJL 964 L25. doi:10.3847/2041-8213/ad2df0

“First Sagittarius A* Event Horizon Telescope Results. VIII. Physical Interpretation of the Polarized Ring,” EHT Collaboration et al 2024 ApJL 964 L26. doi:10.3847/2041-8213/ad2df1

photograph of a white dwarf

Most stars in the Milky Way will evolve into white dwarfs: ultra-hot, crystallized stellar cores, some of which have magnetic fields millions of times stronger than Earth’s. Could the crystallization of white dwarf interiors explain why some of these stars have such strong magnetic fields?

Magnetic Mystery

Hubble image of the Ring Nebula

When a super-hot white dwarf illuminates the diffuse shells of gas that surround it, we see a glowing planetary nebula. The central white dwarf is visible in this image of the Ring Nebula. [NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration]

Roughly 5–6 billion years from now, the Sun will cease all nuclear fusion in its core and cast off the outer layers of its atmosphere. Left behind will be a blazingly hot, Earth-sized core of carbon and oxygen wreathed in a colorful and ephemeral planetary nebula. This carbon–oxygen core — a white dwarf — will slowly cool over trillions of years and fade from view. Such is the fate of more than 95% of the stars in our galaxy.

Some white dwarfs have extremely strong magnetic fields, and the origin of these fields isn’t yet clear. Though the magnetic fields in question are a million times stronger than Earth’s, they might form in similar ways, as new research from José Rafael Fuentes (University of Colorado Boulder) and collaborators shows.

Creating Crystal Interiors

Many magnetic fields in the universe, including Earth’s, form in liquids that have three properties: they’re electrically conductive, they rotate, and they convect — rising and falling like the globs of wax in a lava lamp. As white dwarfs begin to cool, a process begins by which their liquid interiors may achieve all three criteria necessary to generate a magnetic field.

plot of composition flux as a function of time

The composition flux, τ, as a function of time for a 0.9-solar-mass white dwarf. Convection of the white dwarf’s liquid layer is only efficient while the composition flux is large. [Fuentes et al. 2024]

When first formed, white dwarfs are filled with a hot quantum liquid of carbon and oxygen. As they cool, their centers crystallize into a solid, with a layer of quantum liquid surrounding the crystal core. Crystallization changes the composition of the interior, as oxygen tends to be pulled into the crystal core and carbon tends to remain in the liquid. The difference in chemical makeup causes the electrically conductive, rotating fluid to convect — setting the stage for magnetic-field creation.

To probe whether crystallization could help create the million-Gauss magnetic fields seen in some white dwarfs, Fuentes and collaborators modeled the interiors of white dwarfs as they crystallize. The team used the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution model to show that during a brief, 10-million-year period, strong convection could generate magnetic fields of 1–100 million Gauss.

Plot of modeled and observed magnetic fields strengths of white dwarfs

Comparison of the magnetic field strengths obtained though modeling (blue line) with the observed magnetic fields of white dwarfs (symbols). The filled symbols show white dwarfs that are expected to be crystallizing, given their ages, while the open symbols show white dwarfs that are likely not yet crystallizing. Click to enlarge. [Adapted from Fuentes et al. 2024]

Short Phase, Lasting Consequences

While the period of strong convection that creates magnetic fields is short lived, the magnetic field is likely to be long lasting; it takes a long time for magnetic fields to dissipate in a white dwarf, especially once it crystallizes completely.

The models used by Fuentes and coauthors reproduce some observed properties of white-dwarf magnetic fields, such as the lack of a dependence of the field strength on the rotation rate. However, the models also predict that magnetic fields should be stronger for more massive white dwarfs, which observations don’t support. Extending the modeling forward in time may reveal how the magnetic fields evolve and diffuse as the star cools, helping to make sense of these magnetic crystalline stars.

Citation

“A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs,” J. R. Fuentes et al 2024 ApJL 964 L15. doi:10.3847/2041-8213/ad3100

active galaxy Centaurus A

A new modeling method allows black holes and the gas that surrounds them to “talk” back and forth, painting a more realistic picture of how black holes collect material and churn out energy.

A Problem of Scale

Centaurus A

Composite image of Centaurus A, a galaxy whose appearance is dominated by the large-scale jets powered by the supermassive black hole at its center. [ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray); CC BY 4.0]

Black holes at the centers of galaxies across the universe consume gas, dust, and even stars from their surroundings. In exchange for this feast, accreting black holes emit powerful jets and radiation that disrupt and heat nearby gas. This process, known as feedback, cements the link between a black hole and its home galaxy.

Supermassive black holes, though enormous, are tiny compared to their host galaxies — the Milky Way’s central black hole’s event horizon stretches roughly 15 million miles, just a minuscule fraction of our galaxy’s half-quintillion-mile diameter. Despite this size mismatch, supermassive black holes are so powerful that they can influence entire galaxies, leaving researchers with the enormous challenge of modeling the complex processes of accretion and feedback across a wide range of spatial scales.

Building a Two-Way Radio

Typically, models of hungry black holes handle the spatial scale issue by nesting simulations spanning different scales within each other and running them in sequence, starting far from the black hole and spiraling in toward it. This strategy helps the model communicate to the black hole what’s going on around it — how much gas there is to snack on, for example — but it needs to let the black hole talk back, too. That’s where a new technique from a team led by Hyerin Cho (조혜린), Center for Astrophysics | Harvard & Smithsonian and the Black Hole Initiative, comes in.

This new technique uses general relativistic magnetohydrodynamics to model black hole accretion and feedback across seven orders of magnitude in spatial scale. The key advance is that the model spirals from large scales down to small scales — and back — hundreds of times, allowing the black hole to chat freely with its surroundings.

Focusing on Feedback

To demonstrate the new method’s capabilities, Cho and collaborators first showed that they could reproduce the standard analytical solution for a black hole accreting unmagnetized gas. Then, they moved on to a more realistic system that includes magnetic fields. Unlike the unmagnetized case, where gas swirls toward the black hole in a smooth and orderly way, the magnetized case is chaotic: random, turbulent movements as the gas is pulled toward the black hole make the accretion rate vary wildly.

simulation results showing the plasma beta and the plasma density across eight orders of magnitude of spatial scale

Maps of plasma beta (β; the ratio of thermal pressure to magnetic pressure within a plasma) and plasma density (ρ) across eight orders of magnitude in spatial scale. Click to enlarge. [Adapted from Cho et al. 2023]

Where does the turbulence come from? Cho’s team found that magnetic field lines close to the black hole are constantly rearranging, relaxing into new configurations that convert pent-up magnetic energy into kinetic energy. In other words, the reconfiguring of the magnetic field heats and accelerates the surrounding gas, prompting large-scale motions that transport energy away from the black hole — and this outward transport of energy signals that black hole feedback is actually taking place!

Importantly, Cho’s team’s results mesh with what researchers have seen for the black holes they’ve observed closely, especially the central supermassive black holes of the Milky Way and the massive elliptical galaxy Messier 87. While this two-way communication represents a huge advance in the modeling of black hole accretion and feedback, there’s more work to be done; future investigations will tackle spinning black holes surrounded by rotating gas.

Citation

“Bridging Scales in Black Hole Accretion and Feedback: Magnetized Bondi Accretion in 3D GRMHD,” Hyerin Cho et al 2023 ApJL 959 L22. doi:10.3847/2041-8213/ad1048

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