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a photograph of a total solar eclipse with the solar corona showing

A recent research article describes a new way to measure the magnetic field of the Sun’s tenuous upper atmosphere, or corona, from images taken during total solar eclipses.

Illuminating the Solar Corona

An illustration of the regions of the Sun's atmosphere and interior.

An illustration of the regions of the Sun’s atmosphere and interior. Click to enlarge. [NASA/Goddard]

The outermost layer of the Sun’s atmosphere is normally hidden from view, but a total solar eclipse reveals ghostly tendrils flowing out from the Sun: the corona. The solar corona is exceptionally hot, sparse, and dynamic, and it’s the source of a variety of exciting space weather phenomena. It’s this last point that makes understanding the properties of the corona important, especially the strength and direction of its magnetic field.

Because direct measurements of the coronal magnetic field are hard to come by — the only spacecraft yet designed to travel into the harsh coronal environment, the Parker Solar Probe, has barely dipped a toe into the region — researchers must use complex models to reproduce measurements made from a distance to understand this important region of the Sun.

comparison of output from the two methods

A projection of the magnetic field strength derived using the total solar eclipse (TSE) method and the magnetohydrodynamic (MHD) modeling method. Click to enlarge. [Bemporad 2023]

A Simpler Solution?

Alessandro Bemporad (National Institute for Astrophysics, Italy; Purple Mountain Observatory) explored a simpler way. Bemporad’s analysis method starts with polarized-light images of the corona from the 21 August 2017 total solar eclipse. The author first used an established technique to calibrate the images and convert the brightness of each pixel to an important physical quantity: how densely packed or rarefied the plasma is within the corona.

To convert from plasma density to magnetic field strength, Bemporad made a bit of a leap. In many physical systems, energy is distributed fairly equally among different types — like thermal, kinetic, magnetic, or potential energy. For a system like the solar corona, the dominant forms of energy are magnetic and gravitational potential. By making the assumption that these forms of energy are balanced, Bemporad estimated the magnetic field strength without needing complex modeling.

Looking Forward, Looking Back

comparison of output from the new method and the more complex fluid dynamics modeling

Left: Relative difference between the magnetic field strength derived using the total solar eclipse method and the fluid modeling. Right: Cumulative distribution of relative differences between pixel values. Click to enlarge. [Bemporad 2023]

To test the validity of the new method, Bemporad compared his results against those from fluid dynamics models. In general, the output from the two methods differed by less than 50%. Given that the results from the more complicated fluid models aren’t ground truth, Bemporad considers these discrepancies small.

Bemporad notes that the new method can be applied to any white-light images of the solar corona, not just those taken during total solar eclipses. This means that any ground- or space-based images made using a coronagraph (an instrument that blocks the light from the Sun’s bright disk so that the fainter corona can be photographed) are ripe for further exploration, including images taken decades ago — opening a new window into the magnetic conditions of past solar cycles.

Citation

“Coronal Magnetic Fields Derived with Images Acquired during the 2017 August 21 Total Solar Eclipse,” A. Bemporad 2023 ApJ 946 14. doi:10.3847/1538-4357/acb8b8

A rendering of large star on the left, and a very nearby gaseous planet on the right.

JWST’s data are revolutionary, and exoplanet astronomers are learning to squeeze it for all it’s worth. Danger lies at the bleeding edges, though, and a recent study highlights how subtle assumptions can strongly influence final conclusions about a planet’s atmosphere.

Sniffing an atmosphere

As unprecedented as JWST data are, it is crucial to remember that after sniffing an exoplanet’s atmosphere, the telescope does not beam back a full inventory of the molecules it found. Instead, it sends back raw data from its distant perch at L2 that must be interpreted and analyzed by astronomers on the ground before any conclusions are drawn. Behind every discovery claim of a certain molecule in an exoplanet atmosphere is a laborious two-step process: “reduction” and “retrieval.”

A corner plot illustrating the results of an atmosphere retrieval, here computing different abundances and atmospheric profiles fit to a combination of the Tiberius output and Hubble data. [Constantinou et al. 2023]

The first of these, reduction, refers to the process of extracting the signal of interest from the raw data. For most exoplanet studies this means the transmission spectrum of the planet’s atmosphere, which is unfortunately convolved with other nuisance signals originating from quirks of the detector. To automate this extraction and cleaning, astronomers write software “pipelines” that turn the raw data downlinks into a clean final spectrum. Different pipelines often produce similar but slightly different outputs depending on the design choices underpinning each.

Once raw data are pushed through a pipeline and the spectrum is extracted, astronomers move on to the second step: retrieval. Here, the goal is to explain the spectrum by using complex models that simulate a planet’s atmosphere to match what was observed.

Products Depend on Pipelines

Two reduction pipelines popular in the JWST literature are named Tiberius and Eureka. When fed the exact same raw data, they return spectra that look qualitatively similar but differ slightly at each wavelength. Recently, a collaboration led by Savvas Constantinou (University of Cambridge) decided to investigate the implications of these subtle differences: they passed each pipeline an identical copy of observations of the “hot Saturn” exoplanet named WASP-39 b, then ran retrievals on each output spectrum to check how the final atmospheric profiles depended on the choice of pipeline.

Differences in retrieved parameters when using Tiberius and Eureka. Appending Hubble data to each lessened but did not fully resolve the discrepancies between them. [Constantinou et al. 2023]

They found that while the analyses agreed on the largest signals (both successfully recovered the much-lauded detections of CO2 and SO2), they differed in statistically meaningful ways when estimating more subtle parameters. For example, the inferred mixing ratio of most species differed by more than 1σ, and in some cases, one pipeline led to a molecule detection while the other concluded it wasn’t present. Although adding in older data from the Hubble Space Telescope helped reconcile the two somewhat, some serious differences remained even then.

The authors concluded that while there is no cause for concern when it comes to interpreting the most obvious whopping signals like the >15σ CO2 absorption, astronomers are going to need to take care when interpreting weaker signals. With many more JWST transit observations to come, the community will have to double check that they don’t miss any molecules, and that they ones they claim float in exo-airs are really there and aren’t just figments of our reductions.

Citation

“Early Insights for Atmospheric Retrievals of Exoplanets Using JWST Transit Spectroscopy,” Savvas Constantinou et al 2023 ApJL 943 L10. doi:10.3847/2041-8213/acaead

An X-ray image of GRB 221009A's emission scattering off of dust

Gamma-ray bursts are the most luminous explosions in the universe, and we’ve learned much about these superlative outbursts since their discovery in 1967. A new Focus Issue of the Astrophysical Journal Letters released yesterday showcases results related to the gamma-ray burst GRB 221009A: the brightest of all time. Today’s post briefly introduces the Focus Issue articles that have already been published or are in press — be sure to check out the full articles linked below, and keep an eye out for future articles in this issue!

A Superlative Burst

an image of the GRB 221009A afterglow in a field of stars

An infrared image of the afterglow of GRB 221009A (circled), taken 1–2 months after the onset of the burst. The burst’s host galaxy is the faint, extended source behind the burst. Click to enlarge. [NASA, ESA, CSA, STScI, A. Levan (Radboud University); Image Processing: Gladys Kober]

Gamma-ray bursts are brief flashes of high-energy emission, lasting anywhere from a fraction of a second to several hours. The shortest gamma-ray bursts likely mark the collision of two compact stellar remnants called neutron stars, and the longest bursts are thought to arise when a massive, rapidly spinning star collapses to form a black hole. After the initial burst of gamma-rays, dubbed the “prompt” emission, a sustained afterglow shines for days or weeks at radio to X-ray wavelengths.

On 9 October 2022, a remarkable new gamma-ray burst named GRB 221009A was picked up by 25 satellites, most of which weren’t designed to detect gamma-ray bursts — like Voyager 1 and a pair of Mars orbiters. The burst even made an impact on Earth’s atmosphere, creating a disturbance as large as a solar flare would. To put that into context, this means that an explosion roughly 2 billion light-years away had as large an effect on our atmosphere as a solar flare more than 100 trillion times closer!

GRB 221009A Across the Electromagnetic Spectrum

An article led by Maia Williams (Pennsylvania State University) describes the initial discovery and the properties of this extremely bright, long-lived burst. GRB 221009A’s X-ray afterglow is more than an order of magnitude brighter than other gamma-ray bursts observed by the Swift Observatory, and analysis of its emission suggests that the jet it produced is extremely narrow. However, the authors found that GRB 221009A’s afterglow emission isn’t fit well by some of our standard models for how the emission is produced — namely, a jet of energetic particles interacting with material surrounding the object — suggesting that further work is needed to understand the event.

Plot of the gamma-ray burst's flux density as a function of frequency for multiple time periods

Radio, optical, and X-ray observations of GRB 221009A at different times, with the observation day indicated by the symbol color. The solid lines indicate the results of a shock emission model. The model fails to capture the observed radio flux. Click to enlarge. [Laskar et al. 2023]

After its initial discovery at high energies, astronomers monitored GRB 221009A at wavelengths across the electromagnetic spectrum, aiming to understand the nature of the event and pin down its unusual properties. Tanmoy Laskar (University of Utah) and collaborators inspected GRB 221009A’s behavior from radio to gamma rays, finding evidence for a shock forming where the gamma-ray burst’s jet collides with surrounding material. The team noted that the radio emission should linger for years more, providing further opportunities to study this event.

In addition to space-based gamma-ray telescopes, researchers used a ground-based gamma-ray observatory to study the event. Luckily for life on Earth, most gamma rays fail to reach our planet’s surface, but we can detect the faint light emitted when gamma rays interact with particles in the atmosphere. Researchers used the High Energy Stereoscopic System (HESS) — a set of five telescopes located in Namibia — to search for emission linked to GRB 221009A, but found none. Despite this, the upper limits derived from the observations allowed the HESS team to constrain the possible emission mechanism for the burst.

David Kann (Goethe University Frankfurt) and collaborators presented optical and X-ray data spanning from the prompt phase of the burst out to 60 days post-burst. The team used these observations to study the dust along the path between the burst and Earth, finding that the burst’s host galaxy is probably moderately dusty. Modeling of the burst’s jet presented some curiosities: the simplest jet model failed to reproduce the observations, and adding structure and other features to the jet didn’t improve the fit.

Supernova or No?

In addition to the prompt and afterglow emission, gamma-ray bursts are often accompanied by a supernova, powered by nuclear reactions within the material expelled as the star collapses. As the emission from the burst fades, emission from the supernova brightens, eventually peeking out from beneath the fading afterglow as a bump in the light curve days or weeks after the onset of the burst. One of the many intriguing features of GRB 221009A’s evolution is that it might not show any supernova emission.

Optical and near-infrared light curves for GRB 221009A

Optical and near-infrared light curves for GRB 221009A. [Shrestha et al. 2023]

Manisha Shrestha (University of Arizona) and collaborators examined GRB 221009A’s light curves and spectra for signs of a supernova, but didn’t find a convincing supernova signal in either. The team’s modeling suggested that a supernova could be hidden beneath the bright afterglow, though, depending on how much the host galaxy’s dust stifles the light from the supernova. Michael Fulton (Queen’s University Belfast) and coauthors monitored the burst at optical wavelengths as it faded over the course of nearly two months, finding a potential but inconclusive supernova bump around 20 days after the burst.

Andrew Levan (Radboud University) and collaborators turned two exceptional telescopes toward the burst — JWST and the Hubble Space Telescope — and obtained the first mid-infrared spectrum ever taken of a gamma-ray burst. These observations suggest that if there is a supernova accompanying the burst, it’s faint or its spectrum peaks at wavelengths bluer than those covered by JWST and Hubble. Future work will help to disentangle the emission from the gamma-ray burst afterglow, the possible accompanying supernova, and the gamma-ray burst host galaxy that is visible in the Hubble images. If this burst occurred sans supernova, it might mean that the newly formed black hole swallowed the debris of the exploded star, or it could mean — though the team deems this unlikely — that GRB 221009A was instead caused by the merger of neutron stars.

Polarization, Particles, and Dusty Pathways

A team led by Michela Negro (University of Maryland, Baltimore County) observed the burst with the Imaging X-ray Polarimetry Explorer, obtaining the first measurement of polarized X-rays from a gamma-ray burst afterglow. The observations revealed a bright central core of emission surrounded by rings created by photons scattering off of dust grains. In other words, the team was able to study the prompt emission (in echo form) and the afterglow simultaneously! The team placed an upper limit on the polarization fraction of 13.8% for the afterglow emission and somewhere between 55% and 82% for the prompt emission.

plot of the electron flux during the gamma-ray burst

Electron flux measured by one of the three detectors making up the High-Energy Particle Package (black) and the scaled photon count from the High Energy Burst Searcher (blue). Click to enlarge. [Battison et al. 2023]

High-energy radiation from a gamma-ray burst can produce a significant increase in the ionization of Earth’s upper atmosphere. Some of these newly formed ions can be detected by spacecraft, yielding another way to study gamma-ray bursts as they happen. Robert Battison (University of Trento) and collaborators presented data from the High-Energy Particle Package on the China Seismo-Electromagnetic Satellite, which shows a sudden increase in the flux of charged particles at the time of the gamma-ray burst.

The IceCube Collaboration also searched for particles associated with the burst, but neutrinos — chargeless, nearly massless particles produced in a variety of high-energy astronomical phenomena — proved elusive. The link between neutrinos and gamma-ray bursts is theorized but yet to be proven, and the team detected no neutrinos from the event. Planned upgrades to the IceCube Neutrino Observatory might allow us to detect neutrinos, if present, from future gamma-ray bursts.

Images of the expanding rings

Images of the expanding rings at 0.7–4 kiloelectronvolts. Click to enlarge. [Tiengo et al. 2023]

To reach us on Earth, emission from GRB 221009A had to carve a path through the Milky Way. As the narrow beam of emission pierced our galaxy, it illuminated dust clouds along its path like the beam of a flashlight. Because of this, researchers observed shifting rings of X-ray emission for weeks after the event, showing where X-rays had scattered off of dust clouds within the Milky Way. Andrea Tiengo (IUSS – School for Advanced Studies and National Institute of Astrophysics, Italy) and collaborators reported the detection of 20 such rings, corresponding to echoes off dust clouds located 1,000–61,000 light-years from Earth.

Brightest of All Time… So Far

Since its discovery, GRB 221009A has been referred to as the BOAT — the Brightest Of All Time. In the final Focus Issue article published yesterday, Eric Burns (Louisiana State University) and collaborators explored whether that moniker is truly deserved. By comparing against decades of gamma-ray burst observations, the team found that GRB 221009A indeed had by far the highest peak flux of any gamma-ray burst measured to date, and it topped the list for two out of the remaining three measures of brightness. Looking more closely at the properties of events observed over the past 50 years, the team estimated that GRB 221009A is a once-in-ten-millennia event. So while GRB 221009A is certainly not the brightest gamma-ray burst in the history and future of the universe, it’s probably the brightest burst in the history of human civilization — and its rarity means we’re lucky it happened while we have working gamma-ray telescopes!

Over the past few months, GRB 221009A has been hidden from view, blocked by the Sun’s disk. Soon, though, its reemergence will give researchers the chance to study it further and hopefully solve some of the lingering mysteries about its evolution.

Bonus

In addition to the Focus Issue articles published yesterday and those still to come, you can watch a recording of the GRB 221009A press conference presented yesterday at the 20th meeting of the High Energy Astrophysics Division.

Citation

Articles belonging to the Focus Issue will be collected here: Focus on the Ultra-luminous GRB 221009A

Hubble image of a star cluster surrounded by some wispy gas clouds

Researchers have explored the best places to search for ultra-high-energy cosmic rays: charged particles that can travel at nearly the speed of light. New cosmic ray “treasure maps” bring us one step closer to tracking down the origins of these rare particles.

Cosmic Particle Accelerators

diagram of the particle shower created when a cosmic ray enters Earth's atmosphere

A diagram illustrating the shower of particles created when a cosmic ray enters Earth’s atmosphere. Click to enlarge. [CERN]

Across the universe, cosmic rays are accelerated to extraordinary velocities. These speedy particles are mostly protons or the nuclei of helium atoms, with electrons and the nuclei of atoms heavier than helium rounding out the population. When these particles reach Earth, they can make quite a splash — the highest-energy cosmic ray ever detected had an energy of 320 exaelectronvolts (that’s 3.2×1020 electronvolts!) and earned the moniker the “Oh-My-God particle.”

Where in the universe cosmic rays reach their extreme speeds is still up for debate, though supernovae, accreting supermassive black holes, highly magnetized stellar remnants called magnetars, and gamma-ray bursts are all possible sites of cosmic-ray acceleration. Complicating the hunt for these sites is the fact that after cosmic rays are shot into space, they’re buffeted and misdirected by a tangled web of magnetic fields. As a result, where we see a cosmic ray come from might not be where it actually came from.

plot showing simulation results for the detectability of iron nuclei and nitrogen nuclei for sources of varying distance.

The loss-of-number density, aGZK, as a function of the distance of the cosmic-ray source, shown for nitrogen nuclei (dot-dashed green line) and iron nuclei (dashed orange line) with energies greater than 150 exaelectronvolts. The source distance at which 95% of particles fail to reach Earth is marked for each species with an arrow. In this simulation, the detector is sensitive to particles with masses greater than 12 atomic mass units. [Adapted from Globus et al. 2023]

Looking for Paired Particles

In a recent research article, a team led by Noémie Globus (University of California, Santa Cruz, and the Institute of Physical and Chemical Research, Japan) suggested that we may be able to identify the sites of cosmic-ray acceleration by detecting two cosmic rays from the same source arriving from the same direction at the same time. Magnetic fields between the cosmic-ray source and Earth make this kind of coordination unlikely. Globus and collaborators proposed that these paired particles might arrive more often from some parts of the sky than others.

The team focused on cosmic rays with energies in excess of 150 exaelectronvolts, about two of which are caught by our cosmic ray detectors each year. In addition to considering how magnetic fields affect the passage of these particles, the team also considered which cosmic rays are most likely to survive the journey to Earth. As cosmic rays zip through space, they interact with photons from the cosmic microwave background that suffuses the universe. This interaction strips protons and neutrons away from the cosmic ray, reducing its mass and energy. This means that the distance a cosmic ray can travel before being disassembled — and therefore, the distance of the cosmic-ray sources we can detect — depends on its initial mass and energy.

Treasure Maps for Rare Cosmic Rays

An example treasure map for a nitrogen nucleus with an energy of 150 exaelectronvolts. The projection is centered on the galactic anticenter (GA), while the galactic center (GC) appears at the right and left sides. [Adapted from Globus et al. 2023]

By modeling how cosmic rays of different masses, energies, and source locations navigate the magnetic fields within the Milky Way and beyond it, Globus and coauthors determined the likeliest places on the sky to search for paired cosmic rays. These “treasure maps” show that the detection probability depends on the location of the cosmic-ray observatory, the location on the sky, and the mass of the cosmic-ray particle.

In addition to suggesting where to look, the team notes that their work can be used to guide how to look. Because a cosmic ray’s mass determines how far it can travel before being destroyed, building detectors that can measure the mass of cosmic rays reaching Earth can help us pinpoint where the particles originated.

Citation

“Treasure Maps for Detections of Extreme Energy Cosmic Rays,” Noémie Globus et al 2023 ApJ 945 12. doi:10.3847/1538-4357/acaf5f

artist's impression of a magnetar

Astronomers have observed polarised X-rays emitted by a magnetar, opening the door to understanding these extreme objects in more detail.

Magnetic Remnants

an illustration of a neutron star placed next to a map of New York to show the object's size

A magnetar is a sub-type of a stellar remnant called a neutron star. Magnetars and neutron stars are made solely of neutrons, and they are about 10–15 miles across. This illustration places a neutron star next to Manhattan for comparison. [NASA/Goddard Space Flight Center]

A magnetar is the remnant of the death a medium-sized star. It is a type of rapidly spinning neutron star with a typical rotation period of between one and twelve seconds. However, its magnetic field is one thousand times stronger than an ordinary neutron star. In fact, if you were to replace the Moon with a magnetar it would wipe the credit card details of everyone on Earth.

Their strong magnetic fields make magnetars prone to dramatic outbursts, emitting radiation across the electromagnetic spectrum. A magnetar’s X-ray bursts are particularly spectacular, with their X-ray flux suddenly leaping by a factor of up to a thousand.

artist's rendition of the Imaging X-ray Polarimetry Explorer (IXPE) spacecraft with Earth in the background

An artist’s impression of the Imaging X-ray Polarimetry Explorer (IXPE) spacecraft. [NASA]

Focusing on Flares

It hasn’t always been clear, however, where these flares are coming from. They could spring from the crust of the magnetar itself, or they could launch from material trapped in an atmosphere around it. A team led by Silvia Zane (University College London) recently went looking for answers using the Imaging X-ray Polarimetry Explorer (IXPE). Launched at the end of 2021, it is a joint mission between NASA and the Italian Space Agency.

As its name suggests, IXPE observes the polarisation of X-rays. Light is polarised if its waves all vibrate in the same plane. Zane’s team used IXPE to observe the polarisation of the magnetar known as 1RXS J170849.0-40091 to see if they could work out what was corralling the X-rays.

Belts, Caps, and More

plot of the polarization percentage and polarization angle of the source in different energy bands

Polarisation of the magnetar in different energy bands. The colors of the circles represent different detector units (DUs), and the orange stars and green crosses show the predictions for the “belt plus cap” and “cap plus cap” models, respectively. [Adapted from Zane et al. 2023]

They found that about 20% of the low-energy X-rays produced by 1RXS J170849.0-40091 are polarised, but this rises to around 80% for higher-energy X-rays.

The team ran simulations of various magnetar configurations to see if they could explain this result, settling on two possibilities. The first, which they call “belt plus cap,” has a warm region around the magnetar’s equator and a hotter atmosphere confined to a circular polar cap. The second, called “cap plus cap,” has the X-ray emission emanating from two circular spots. One is on the surface of the magnetar, and there’s a hotter atmospheric spot located in the opposite hemisphere.

Further work to untangle these two possibilities could have wider implications. Astronomers have previously suggested that magnetars are the main source of fast radio bursts, for example. Understanding why magnetars flare could help cement this link and assist astronomers in explaining these mysterious events.

Citation

“A Strong X-Ray Polarization Signal from the Magnetar 1RXS J170849.0-400910,” Silvia Zane et al 2023 ApJL 944 L27. doi:10.3847/2041-8213/acb703

A photograph of a galaxy which appears in the foreground as a diagonal line because of the edge-on viewing geometry.

In the early universe, galaxies were misshapen blobs that could only dream of transforming into something as structured and stately as our present Milky Way. Unfortunately, pinning down exactly when in history some early galaxies flattened into disks is a challenging task; now, though, the formidable combination of JWST and deep learning offers steps towards an answer.

Challenging Finds

When do disky galaxies form? In theory, answering this question is straightforward: astronomers just need to look back in time, which they (somewhat remarkably) can do by looking at higher and higher redshifts, and see when disks first appear. Unfortunately but unsurprisingly, what’s simple in description is difficult in practice. High-redshift galaxies appear extremely faint in visible light and instead are brightest in the infrared, which historically was much harder to detect with high-spatial-resolution detectors. Although the Hubble Space Telescope was a transformative telescope in so many ways, it struggled to find or resolve many high-redshift galaxies.

Enter JWST. It was designed in part to find these evasive galaxies: its detectors all are sensitive to longer wavelengths of light, and especially its NIRCam instrument is expected to be a workhorse of galaxy discovery and classification. This work has already begun in earnest, and several of the observations associated with the Cosmic Evolution Early Release Science Survey (CEERS) have already been taken.

Finding Disks

Images taken by JWST of a CEERS field. Along the top and bottom are rows of inset frames which magnify specific galaxies in the image. The magnified images show generally extended disk-like shapes, but are heavily blurred/pixelated given the small angular size of these targets.

False-color JWST images and a selection of galaxies that Morpheus classified as “disky.” The photometric redshift of each galaxy is noted in the upper corner of each inset. [Robertson et al. 2023]

Though the CEERS team is still hard at work on their analysis, their data were made public immediately to allow other researchers to take a look. This allowed a collaboration led by Brant Robertson (University of California, Santa Cruz) to run their own analysis tools on a series of images taken of a region called the Extended Groth Strip.

The tool they deployed was a deep learning model named Morpheus, which back in 2020 was trained to ingest images taken by Hubble, spot high-redshift galaxies, and then classify them according to their morphology. In that earlier study, the authors compared how many galaxies were “disks” vs. “spheroids” (and other categories) at different redshifts to estimate when structure first emerged. With JWST images in hand of the exact same patch of sky, they played the same game on the same galaxies with the new data and compared the results.

A 3 row, 4 column panel figure. From left to right, the columns are labeled "Morpheus", "JWST/F150W", "Model", and "Residual". Each row corresponds to a different example galaxy, and when read across left to right demonstrates Morpheus's ability to build a model which accurately matches the data.

An example of Morpheus’s classification “under the hood.” The network assigns a probability to each pixel that it belongs to a certain galaxy classification, then assigns a classification for any object that has >50% probability of being in a certain class. [Robertson et al. 2023]

In this first-ever application of AI/ML tools to JWST data, the authors cross-matched >7,000 galaxies between the Hubble and JWST images, and interestingly, noticed that Morpheus changed its mind about how to classify some of the faintest and highest-redshift targets. Of the 160 faintest (H>24.5AB!) galaxies Morpheus claimed were disk galaxies after seeing the JWST images, it had previously only thought 5% were flat in the Hubble data. Most of the others were too faint to resolve at those shorter wavelengths, so they had previously been mislabeled as “compact,” point-like sources, when in fact there was some structure Hubble had simply missed. Excitingly, some of the newly-classified disk galaxies have photometric redshifts of 4-5, a surprisingly high value which will require further analysis to fully contextualize.

While it’s still too early for any precise estimates of the disk formation timescale, these results confirm that JWST is a transformative tool for the job. Now more than a year since launch, we can be sure to expect more galaxies, more accurate classifications, and more rewrites to our current understanding of galaxy formation in short order.

Citation

“Morpheus Reveals Distant Disk Galaxy Morphologies with JWST: The First AI/ML Analysis of JWST Images,” Brant E. Robertson et al 2023 ApJL 942 L42. doi:10.3847/2041-8213/aca086

JWST image of the galaxy cluster SMACS 0723

In the months since the release of the first JWST image of the galaxy cluster SMACS J0723.3–7327 (SMACS 0723), astronomers have studied the image from every possible angle. Today’s post takes a look at five research articles that have advanced our understanding of galaxies within the cluster and beyond.

How to Weigh a Cluster

SMACS 0723 is a galaxy cluster located four billion light-years away. It gets its name from the Southern MAssive Cluster Survey (SMACS), in which the Hubble Space Telescope observed 124 galaxy clusters that are bright at X-ray wavelengths. In addition to the galaxies bound to the cluster, observations of SMACS 0723 contain images of galaxies far more distant. This is because SMACS 0723 is so massive that it warps spacetime, causing photons from galaxies billions of light-years away to curve around the foreground galaxies and into our field of view. As a result, we see stretched-out, wiggly, or multiple images of distant galaxies.

two mass density distribution maps for galaxy cluster SMACS 0723

Top: Mass distribution derived by Mahler and collaborators. The magenta lines are mass contours and the white lines are X-ray surface brightness contours. Bottom: Mass distribution derived by Pascale and collaborators. The black lines show the surface mass density distribution. Note that there is a difference in scale and orientation between the two maps. [Adapted from Mahler et al. 2023; Pascale et al. 2022]

Researchers can use these wiggly galaxies to reconstruct a map of SMACS 0723’s mass distribution. Understanding how a cluster’s mass is distributed is important because it can tell us about the underlying distribution of dark matter within the cluster and how the cluster has evolved over time. As is often the case when exciting new data are released, several teams worked in parallel to map SMACS 0723’s mass distribution using different techniques. Two articles published in AAS journals detail efforts by teams led by Massimo Pascale (University of California, Berkeley) and Guillaume Mahler (Durham University) to use JWST’s superior capabilities to improve upon the existing mass map based on Hubble data.

In previous observations of the cluster, astronomers picked out five lensed background galaxies, but JWST’s impressive capabilities allowed the teams to pick out many more; at 6.5 meters in diameter, JWST is already a massive space telescope, but as Pascale and coauthors noted, pointing it toward a galaxy cluster that bends the light from more distant sources effectively creates a telescope with a diameter of 20–30 meters!

Pascale’s and Mahler’s teams used different computer algorithms to analyze the lensed galaxies newly identified in the JWST images, and both teams achieved a large increase in the precision of their mass maps compared to those based on Hubble data. Both teams’ maps show evidence of a past disturbance in the galaxy cluster, such as gravitational interactions or mergers between galaxies.

A Trio of Distant Galaxies

Karla Arellano-Córdova (The University of Texas at Austin) and collaborators took advantage of SMACS 0723’s lensing ability to study three galaxies with redshift, z, between 7.6 and 8.5, corresponding to when the universe was about 600–700 million years old. Using JWST’s Near-Infrared Spectrograph (NIRSpec), the team identified emission lines in the spectra of these distant galaxies. By measuring the strength of several prominent emission lines and the ratios of their strengths relative to one another, Arellano-Córdova’s team determined the properties of the gas in each galaxy, including the abundance of elements like carbon, oxygen, neon, and potentially iron. Notably, the team measured the neon-to-oxygen abundance ratio for a galaxy at z > 7 for the first time, and they obtained the most distant carbon-to-oxygen ratio measurement ever.

metallicity versus stellar mass for star-forming galaxies

Metallicity versus stellar mass for galaxies in the local universe (light blue circles) and at high redshift. Values from this work are shown with stars, while other researchers’ results are shown with triangles, diamonds, pentagons, and squares. [Arellano-Córdova et al. 2022]

By comparing the chemical abundances for the three high-redshift galaxies to those of nearby galaxies, the team gained a sense of how each element’s abundance has changed over time, which can inform us about how galactic stellar populations evolved early in the universe. While some of the abundances followed expected trends, others showed behavior that will require further inspection; a tentative detection of emission lines from iron atoms suggests that these galaxies might be rich in iron compared to oxygen, which the authors propose could be due to gaseous outflows. The authors emphasized that this is just the beginning of what JWST will accomplish in terms of investigating the chemical makeup of galaxies in the first billion years of the universe — we’ve cracked open a window into that time period, and future studies should open it wider still.

Investigating Point Sources in SMACS 0723

The final two articles focus on goings on within the cluster itself, using JWST’s exceptional precision to investigate individual star clusters four billion light-years away.

several panels illustrating the locations of the star clusters within the SMACS 0723 field

Location of the star clusters studied in this work within the larger SMACS 0723 field. Click to enlarge. [Faisst et al. 2022]

A team led by Andreas L. Faisst (California Institute of Technology — Infrared Processing and Analysis Center) searched the SMACS 0723 images for signs of star clusters. The team manually selected 178 promising sources from the images, finding that the human eye was better at picking out the faint clusters against the background galactic light than a search algorithm. Because the individual star clusters were so faint, the team combined the measurements for all 178 clusters to determine the average properties of the sample.

Overall, the clusters appear to be about 1.5 billion years old, although the team could not rule out the possibility that the clusters are up to 9 billion years old (roughly the age of the universe at SMACS 0723’s redshift). Given the clusters’ sizes (no more than 160 light-years across), masses (about 2.4 million solar masses, on average), and metallicity (about 20–30% the metal abundance of the Sun), the team concluded that the star clusters are most likely to be globular clusters: roughly spherical collections of hundreds of thousands of stars found on the outskirts of nearly all galaxies. However, the authors acknowledged another possibility: as dwarf galaxies surf the gravitational swells of the massive galaxy cluster where they were born, they may lose their outermost stars, resulting in dense star clusters similar to those seen in SMACS 0723.

Galaxy Cluster Globular Clusters

Myung Gyoon Lee (Seoul National University) and collaborators also went globular cluster–hunting in the SMACS 0723 images, using an algorithm to identify and characterize point sources throughout the galaxy cluster. By grouping point sources based on their color and brightness, the team identified those that are most likely to be globular clusters associated with the SMACS 0723 galaxy cluster.

illustration of the locations of the sources studied in this work

Illustration of the location of some of the sources studied in this work. Click to enlarge. [Lee et al. 2022]

Many of the sources appear to be linked to the brightest galaxy in the cluster, which lies near the cluster center. These star clusters are most concentrated near the bright central galaxy and become more scattered farther out, and similar concentrations are seen around other bright galaxies within the cluster.

In addition to tracing the distribution of galaxies in SMACS 0723, the candidate globular clusters also follow some of the intracluster structures, such as the distribution of diffuse intracluster light. This faint emission comes from stars that have been ejected from their home galaxy due to gravitational interactions between galaxies in the cluster. The team also compared the star cluster distribution to the dark matter distribution determined in other research articles, finding considerable similarities between the two distributions. These findings suggest that star clusters can be used to trace dark matter within galaxy clusters.

Citation

“Unscrambling the Lensed Galaxies in JWST Images behind SMACS 0723,” Massimo Pascale et al 2022 ApJL 938 L6. doi:10.3847/2041-8213/ac9316

“Precision Modeling of JWST’s First Cluster Lens SMACS J0723.3–7327,” Guillaume Mahler et al 2023 ApJ 945 49. doi:10.3847/1538-4357/acaea9

“A First Look at the Abundance Pattern—O/H, C/O, and Ne/O—in z > 7 Galaxies with JWST/NIRSpec,” Karla Z. Arellano-Córdova et al 2022 ApJL 940 L23. doi:10.3847/2041-8213/ac9ab2

“What Are Those Tiny Things? A First Study of Compact Star Clusters in the SMACS0723 Field with JWST,” Andreas L. Faisst et al 2022 ApJL 941 L11. doi:10.3847/2041-8213/aca1bf

“Detection of Intracluster Globular Clusters in the First JWST Images of the Gravitational Lens Cluster SMACS J0723.3–7327 at z = 0.39,” Myung Gyoon Lee et al 2022 ApJL 940 L19. doi:10.3847/2041-8213/ac990b

a representation of the Sun's spectrum

Solar physicists have identified the best way to measure magnetic fields in the solar atmosphere, opening the door to deciphering the mysteries of a poorly understood part of the Sun.

An illustration of the regions of the Sun's atmosphere and interior.

An illustration of the regions of the Sun’s atmosphere and interior. Click to enlarge. [NASA/Goddard]

Our Multilayered Star

The Sun’s atmosphere has three layers: the photosphere, the chromosphere and the corona. The photosphere is the Sun’s visible surface and the corona is the wispy, tenuous outermost layer visible during solar eclipses. The chromosphere, a name that means sphere of colour, is sandwiched between the two and is incredibly thin. It takes up just one per cent of the Sun’s radius and yet understanding it is important work.

Strong magnetic fields corral hot plasma in the chromosphere into a network of super-sized granulation cells that can stretch to 20,000 miles across — some 2.5 Earth diameters — and 1,000 miles deep. On the edges of the cells sit spicules: spikes of magnetically confined plasma that shoot up into the corona above. The release of magnetic energy is also linked to events such as solar flares and coronal mass ejections, which can cause issues for our electrical infrastructure on Earth. So solar physicists want to know how magnetic fields emerge from, heat, and accelerate chromospheric plasma.

Magnetic Field Measurement

One way to measure the magnetic fields in the chromosphere is to take advantage of the Zeeman effect. Strong magnetic fields split the lines observed in the Sun’s spectrum. Recently a team led by Philip Judge (National Center for Atmospheric Research, Colorado) did something no one had ever done before: assess the comparative merits of using all spectral lines from X-ray to infrared wavelengths to measure magnetic fields as high as possible in the solar atmosphere.

an example solar spectrum

A sample solar spectrum with prominent spectral lines labeled. Click to enlarge. [Judge et al. 2022]

The team concluded that the best lines for the job are the magnesium h and k lines near 2800 Ångstroms (Å). However, they also found that combining them with the numerous chromospheric lines of iron between 2585 Å and the h line at 2803 Å would provide a far more discriminating probe of magnetic structure in the chromosphere. All of these wavelengths sit in the ultraviolet part of the spectrum.

Future Prospects

The recent Chromosphere Lyman-Alpha Spectro-Polarimeter (CLASP-2) instrument launched aboard a sounding rocket demonstrated that measurements at these wavelengths can be made successfully. A modest space-borne telescope focussing on the same part of the spectrum could provide a way to untangle some of the biggest riddles about the Sun’s chromosphere and the violent eruptions associated with it.

Citation

“Optimal Spectral Lines for Measuring Chromospheric Magnetic Fields,” P. Judge et al 2022 ApJ 941 159. doi:10.3847/1538-4357/aca2a5

artist's impression of a kilonova explosion

Astronomers are still debating the origin of the blue-tinted emission from colliding neutron stars detected in 2017. Stymied by missing data from the crucial first few hours of that event, researchers have determined just how quickly we’ll need to catch the next one.

A Groundbreaking Event

light curves for the electromagnetic counterpart to a gravitational wave signal detected in August 2017

Observed (symbols) and modeled (lines) light curves for AT2017gfo, the electromagnetic counterpart to the gravitational wave signal GW170817. [Adapted from Cowperthwaite et al. 2017]

In August 2017, a pair of gravitational wave detectors identified a signal from two neutron stars — the ultra-dense remnants of massive stars — colliding 130 million light-years away. In the hours that followed, a network of telescopes raced to search for the electromagnetic counterpart to the event (dubbed AT2017gf), with telescopes locking on to the source 11 hours after the merger was detected.

These observations revealed bright but rapidly fading emission at blue wavelengths in the first day after the collision was detected. Though scientists have proposed many causes for this early blue emission, two theories have risen to the top: radioactive decay of newly fused elements and heating from a shock wave. Researchers suspect that in order to distinguish between these theories, we need ultraviolet data from the first few hours after the merger is detected. We missed those critical data in 2017, with ultraviolet observations beginning 15 hours after detection — how can we ensure we capture them in the future?

When Theories Collide

Bas Dorsman (University of Amsterdam) and collaborators proposed that an ultraviolet satellite with a wide field of view could capture the necessary data next time around. To test this possibility, the team modeled the emission from an AT2017gfo–like collision of two neutron stars, finding that both theories produce ultraviolet emission that matches observations at 15 hours after the event was detected but diverges at earlier times.

plot of simulated light curves for the two competing theories

Simulated ultraviolet light curves for an AT2017gfo–like neutron star merger. The orange and green lines show the results for two versions of the radioactive decay model, the blue line shows the output from the shock heating model, and the gray line shows the time at which the ultraviolet observations of AT2017gfo began. [Dorsman et al. 2023]

To estimate just how quickly we’d need to begin collecting ultraviolet data to draw firm conclusions, Dorsman and coauthors considered the capabilities of a proposed ultraviolet satellite called Dorado. Using their model results, the team simulated what a Dorado-like spacecraft would detect during the early hours of an AT2017gfo–like event. Ultimately, they found that if the ultraviolet data collection starts 1.2 hours after the event is detected — the best-case scenario based on the satellite’s capabilities — we’d be able to distinguish between the two theories for events within about 522 million light-years of Earth. As the time of first observation grows later, the events need to be closer for us to tell what drives the early blue emission: out to 424 million light-years for data starting at 3.2 hours and out to 196 million light-years for data starting at 5.2 hours.

Awaiting Spacecraft

While the team’s results show the promise of collecting early ultraviolet data, we’ll need to wait at least a few years to put the plan into practice. The Dorado spacecraft described in this work didn’t proceed beyond a concept study, but there are two similar spacecraft farther along in the selection process: the Ultraviolet Transient Astronomy Satellite (ULTRASAT; launch planned for 2026) and the Ultraviolet Explorer (UVEX; launch in 2027 or later, if selected).

These missions should overlap with the fifth observing run for the LIGO, Virgo, and KAGRA gravitational wave observatories, which is slated to begin in 2027. Hopefully — a decade after the detection of AT2017gfo — we’ll be able to get some answers!

Citation

“Prospects of Gravitational-wave Follow-up through a Wide-field Ultraviolet Satellite: A Dorado Case Study,” Bas Dorsman et al 2023 ApJ 944 126. doi:10.3847/1538-4357/acaa9e

A photograph of a building and other infrastructure atop an ice sheet at night. Several stars are visible overhead.

The universe is crackling with gravitational waves and sparkling with neutrino emissions, but detecting both signals from the same source is an enormous challenge. A new publication summarizes the state of this effort and reports on non-detections of high-energy neutrinos associated with the merger events caught by LIGO/Virgo.

Multiple Messengers

Historically, astronomers studied our universe by analyzing the electromagnetic radiation (light) that lands on Earth after a journey through the depths of the cosmos. In the last decade, however, researchers have expanded upon this paradigm and now routinely scrutinize other “messengers” that collide with our home planet and leave their imprints on our scientific instruments. Two of these messengers include high-energy neutrinos, first observed in 2013, and gravitational waves, first spotted in 2015.

Certain astrophysical processes, including the mergers of compact objects like black holes and neutron stars, are thought to produce both of these messengers. Therefore, to wring as much information as possible from these distant events, astronomers are incentivized to detect coincident signals of different messengers from the same source. Unfortunately, pulling this off requires overcoming technical and instrumental hurdles across two different subfields of physics, and to date the positional uncertainties of gravitational wave detections especially have prevented any confident claims.

Vigilance and Patience

A histogram displaying the distribution of delays between initial detection of a gravitational wave and the release of IceCube's analysis. The median is marked with an orange vertical line marking 0.94 hours.

The distribution of delay times between a gravitational wave event and release of preliminary IceCube analysis. Most often the team turned around results in less than one hour. [Abbasi et al. 2023]

Between April 2019 and March 2020, a pair of gravitational wave detectors named LIGO and Virgo listened intently for the “bangs” of compact objects merging somewhere in the distant universe. Each time the detectors heard something, their teams quickly released a rough estimate of its source location on the sky. This in turn kickstarted a global scramble to try and detect the same merger with a different messenger, whether optical light, gamma rays, or high-energy neutrinos.

The IceCube team, a collaboration that searches for flashes of light created when high-energy neutrinos interact with Antarctic ice, watched carefully for these real-time estimates. Whenever one was released, they immediately checked their logs for high-energy neutrinos that came around the same time from around the same place as the gravitational wave, then released their own reports of any encouraging signals. Thanks to their preparation leading up to the run, this entire process happened usually within the hour following the initial LIGO/Virgo observations. Once the runs were complete and the gravitational wave data were collated into a more confident catalog, the team also ran an after-the-fact analysis to check if anything was missed in real time.

Close Calls

A heatmap with right ascension on the horizontal axis and declination on the vertical. Darker colors, which represent regions with a higher probability of hosting the gravitational wave event, form a vertical stripe down the middle. Overplotted on the heatmap is a circular contour marking the high-likelihood region for the neutrino's source region. The circle largely overlaps with the darkest region.

A sky map showing the overlap between high-likelihood regions of a gravitational wave event and of the most likely direction of a high-energy neutrino that arrived 360 seconds earlier. Though there is much overlap, it was not classified as statistically significant coincident emission. [Abbasi et al. 2023]

While the IceCube team did not confidently detect any high-energy neutrinos associated with a gravitational wave event, there were four close calls that prompted further scrutiny. Just as importantly, their non-detections enabled the calculation of upper bounds on neutrino emission for each gravitational wave event. As LIGO and Virgo gear up for another observing run and IceCube considers a next generation detector, hopefully the tools developed here will reveal the first confirmed neutrino emission from a merger event soon.

Citation

“IceCube Search for Neutrinos Coincident with Gravitational Wave Events from LIGO/Virgo Run O3,” R. Abbasi et al 2023 ApJ 944 80. doi:10.3847/1538-4357/aca5fc

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