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Artist’s impression of a fast radio burst traveling through space and reaching Earth

The repeating fast radio burst FRB 20190520B traveled through an unusually large amount of matter on its journey to Earth. Could unidentified galaxy clusters in the billions of light-years that separate us from the burst’s source explain why?

An Astrophysical Mystery

Fast radio bursts are among the most mysterious events in the universe. Most of these powerful, milliseconds-long radio blips occur just once, each burst an astronomical flash in the pan that leaves researchers puzzling over its origin. In rare cases, fast radio bursts repeat, giving us a clue that at least some sources of these mysterious bursts survive the event.

signal from the first fast radio burst ever detected

The signal from the first fast radio burst ever detected. The highest frequencies arrive first, and the lower frequencies follow. [Wikipedia user Psr1909; CC BY-SA 4.0]

Fast radio bursts illuminate gas and dust as they travel across millions to billions of light-years, providing a way to study matter along their paths. This is reflected in what researchers call the dispersion measure, which is related to the amount of matter the burst travels through from its origin to an observer on Earth. Researchers determine the dispersion measure of a fast radio burst by measuring the delay between when its highest and lowest frequencies arrive.

The matter that delays the arrival of the lowest-frequency radio waves in a burst — free-roaming electrons are the best at holding up low-frequency waves — can be located anywhere along the burst’s path: in the immediate vicinity of the source, in the source’s host galaxy, in intergalactic space, or in our own galaxy. To disentangle the contributions from these different regions, researchers must take a wide view of the situation.

Surveying a Superlative Burst

The dispersion measure of the repeating fast radio burst FRB 20190520B is more than twice as large as expected given its distance. This unusually high value caught the attention of a team led by Khee-Gan Lee (Kavli Institute for the Physics and Mathematics of the Universe), which is carrying out the Fast Radio Burst (FRB) Line-of-sight Ionization Measurement From Lightcone AAOmega Mapping survey, or FLIMFLAM. This survey aims to map the distribution of luminous matter in the universe by searching for galaxy groups that are revealed by fast radio bursts.

Plot of newly identified galaxy clusters and other galaxies in FRB 20190520B's field

Snapshot of an interactive figure showing the locations of the newly identified galaxy clusters relative to FRB 20190520B’s location. Click to enlarge. You can interact with this figure here. [Lee et al. 2023]

The team spectroscopically determined the distances to galaxies in the field of view surrounding FRB 20190520B’s location and used a group-finding algorithm to identify galaxy groups and clusters. They found multiple galaxy groups in the field of view, including two galaxy clusters that lie directly between us and FRB 20190520B. By using models to estimate the masses of these galaxies and their halos, Lee’s team determined how much these intervening galaxy clusters contributed to the burst’s dispersion measure.

A Revised Estimate

Based on FRB 20190520B’s extremely high dispersion measure, previous research estimated its host galaxy’s dispersion to be the highest of any known fast radio burst, a fact that has been difficult to reconcile with other observations of the galaxy. Now, with the new estimate of the foreground galaxies’ contribution, FRB 20190520B’s host galaxy has been assigned a more moderate value that aligns with its observational properties. This study demonstrates that even when focusing closely on a single fast radio burst, it’s still important to zoom out and consider the big picture!

Citation

“The FRB 20190520B Sight Line Intersects Foreground Galaxy Clusters,” Khee-Gan Lee et al 2023 ApJL 954 L7. doi:10.3847/2041-8213/acefb5

Hubble image of a star-forming region in the Small Magellanic Cloud

Earlier this year, researchers using JWST discovered a galaxy that stopped forming stars just 700 million years after the Big Bang. Cosmological simulations provide a way to study sudden star-formation shutdowns in early galaxies like this one.

Hubble and JWST image of side-by-side elliptical and spiral galaxies

This image of the VV 191 galaxy pair combines data from the Hubble Space Telescope and JWST. Elliptical galaxies, like the left-hand galaxy in this image, usually have little or no star formation, while spiral galaxies, like the one on the right, are alight with new stars. [NASA, ESA, CSA, Rogier Windhorst (ASU), William Keel (University of Alabama), Stuart Wyithe (University of Melbourne), JWST PEARLS Team, Alyssa Pagan (STScI); CC BY 4.0]

Young Galaxies in the Early Universe

In the first few million years after the Big Bang, stars came to light and galaxies assembled, steadily brightening the infant universe. But star formation cannot continue forever, as evidenced by the many galaxies in the universe today whose star formation has drawn to a close.

For galaxies to exhaust their star-forming gas after 13 billion years may not be surprising, but JWST has revealed a galaxy that stopped its star formation far sooner, less than one billion years after the Big Bang. The galaxy, JADES-GS-z7-01-QU, provides an opportunity to study processes that halt star formation — either temporarily or permanently — in the early universe.

Stop-and-Go Star Formation

To understand why a galaxy might stop forming stars so early in the universe’s history, a team led by Viola Gelli (University of Florence and Italian National Institute for Astrophysics/Arcetri Observatory) turned to cosmological simulations. Using the Sᴇʀʀᴀ simulations, Gelli and collaborators gathered a sample of 130 galaxies with redshifts and masses similar to JADES-GS-z7-01-QU. The team found that about 30% of these galaxies had no star formation, and less massive galaxies in the sample were less likely to be star forming than more massive galaxies.

simulation snapshot showing active and quiescent galaxies

Snapshots of the Sᴇʀʀᴀ simulations at a redshift of 6 showing the stars (left) and gas (right) of typical active and quiescent galaxies. Click to enlarge. [Gelli et al. 2023]

A quick peek through the galaxies’ star-formation histories showed that all galaxies in that era with masses less than one billion solar masses (about a thousand times less massive than the Milky Way) had experienced periods when star formation flourished and periods when it stalled. These galaxies undergo bursts of star formation that gradually diminish, consistent with incremental heating of the interstellar gas by supernova explosions, which prevents new stars from forming. Once the gas has cooled, star formation begins again.

Abrupt Transition Needed

Plot showing the spectral energy distributions of simulated galaxies compared to a galaxy observed by JWST

Spectral energy distributions of simulated galaxies compared to JADES-GS-z7-01-QU. The simulated galaxy singled out for further study is named Lilium. Click to enlarge. [Adapted from Gelli et al. 2023]

But when Gelli’s team focused on a single simulated galaxy with almost exactly the same mass and redshift as JADES-GS-z7-01-QU, it became clear that this picture of stop-and-go star formation doesn’t fully explain JADES-GS-z7-01-QU’s behavior. Despite the similarities between the two galaxies, their spectral energy distributions — a measure of how energy output is distributed across the electromagnetic spectrum — didn’t agree. The team brought them into alignment by adding a sharp cutoff in the simulated galaxy’s star formation just 5 million years after it started.

What could be the cause of such a sharp shutoff in star formation? Although heating of star-forming gas by supernovae appears to be ubiquitous in low-mass galaxies in the early universe, the lag between when massive stars form and when they explode as supernovae means a shutdown due to supernovae can’t happen less than 30 million years after star formation starts. In the case of JADES-GS-z7-01-QU, something else must be at play — perhaps the powerful winds of young stars or an accreting supermassive black hole at the galaxy’s center could be responsible for rapidly shutting off star formation in JADES-GS-z7-01-QU. As JWST observes more quiescent galaxies in the early universe, we’ll be able to investigate the causes of stalled star formation further.

Citation

“Quiescent Low-Mass Galaxies Observed by JWST in the Epoch of Reionization,” Viola Gelli et al 2023 ApJL 954 L11. doi:10.3847/2041-8213/acee80

elliptical galaxy NGC 4150

Discovery of a bright, rapidly evolving source led researchers to propose a new class of transients called luminous fast coolers. The physical origin of these rare events, which arise in quiescent elliptical galaxies rather than spiral galaxies like many transients, is still unknown.

Plot of flux versus rest days from explosion showing the large, sudden increase in flux that marks the discovery of a new transient

ATLAS data of AT 2022aedm. [Adapted from Nicholl et al. 2023]

Something New in the Universe

In 2022, the Asteroid Terrestrial impact Last Alert Survey (ATLAS) detected a rapidly brightening source in a quiet elliptical galaxy. The event, labeled AT 2022aedm, was tentatively tagged as a supernova at first, but its fast evolution soon revealed it to be something other than an ordinary exploding star. In a recent research article, Matt Nicholl (Queens University Belfast) and collaborators described the object’s dynamic light curve, investigated its spectrum, and pondered its origins.

Spectra of AT 2022aedm and its host galaxy. The spectra are largely featureless and the shift of the peak wavelength to longer wavelengths shows the rapid cooling of the source.

Spectra of AT 2022aedm at multiple points in time after discovery, plus a spectrum of its host galaxy (gray). Click to enlarge. [Adapted from Nicholl et al. 2023]

Assembling the Puzzle Pieces

In the weeks after AT 2022aedm’s discovery, Nicholl’s team collected data to characterize the event’s swiftly changing light curve. Optical data show that it reached its peak brightness just nine days after its onset, rocketing up to a spectacular peak magnitude of −21.5, before fading nearly as quickly. To put that into context, this means that this single event was about 2.5 times brighter than our entire galaxy! This incredible brightness placed AT 2022aedm in the realm of superluminous supernovae, which make up just 0.1% of supernovae. But other pieces of the puzzle didn’t fit this explanation; AT 2022aedm’s brightness increased and decreased too quickly and its spectrum never gained the tell-tale broad spectral lines of a supernova.

Even stranger than AT 2022aedm‘s light curve and spectrum is where it was found: in a massive elliptical galaxy with very little active star formation. Most core-collapse supernovae happen in lower-mass spiral galaxies, where ongoing star formation is constantly creating new massive stars to fuel these explosions, and superluminous supernovae especially seem to be confined to star-forming spirals.

A Class of Three

Location of AT 2022aedm within its host galaxy.

The location of AT 2022aedm within its host galaxy is marked with magenta crosshairs. [Adapted from Nicholl et al. 2023]

Based on the key properties of the event — extremely high peak brightness and rapid fading — the team identified two more likely members of the same class among previous observations and coined the term “luminous fast coolers” to describe them. The other two events, named Dougie and AT 2020bot, also occurred in the outer parts of elliptical galaxies, further hinting at a common origin.

Despite the similarities between the light curves and spectra of the three events, Nicholl and collaborators were unable to find a physical explanation that fit all three events. Among the options considered were tidal disruption events (when a star is shredded by a massive black hole), exploding white dwarfs, supernovae giving birth to extremely dense, highly magnetized stellar remnants called magnetars, and colliding neutron stars. Each of these scenarios encountered serious roadblocks during the team’s analysis, but the team found promising signs that with further modeling, either a star interacting with a stellar-mass black hole or a shock colliding with a gas cloud could someday explain the origins of these newly described events.

Citation

“AT 2022aedm and a New Class of Luminous, Fast-cooling Transients in Elliptical Galaxies,” M. Nicholl et al 2023 ApJL 954 L28. doi:10.3847/2041-8213/acf0ba

JWST image of the Chamaeleon I molecular cloud

Astronomers have made the first detection of interstellar glycolamide, a molecule closely related to the simplest of the amino acids necessary for life on Earth. Glycolamide is the latest interstellar molecule detected in the G+0.693–0.027 molecular cloud near the center of the Milky Way.

An Elusive Acid

model of the structure of a glycine atom

A model of a glycine molecule. Carbon atoms are shown as dark gray, hydrogen atoms are light gray, oxygen atoms are red, and nitrogen is blue. [Ben Mills; Public Domain]

Amino acids are among the most important molecules for life as we know it, providing the means for all life on Earth to build proteins. Beyond Earth, researchers have discovered amino acids in meteorites, comets, and the hot gas surrounding protostars. If amino acids can form in the sparse gas of the interstellar medium, as observations suggest, these critical molecules can be inherited by protoplanetary disks and, eventually, planets themselves.

The simplest of the amino acids necessary for life on Earth is glycine, which contains just 10 atoms. Glycine has been detected in various places in our solar system, but we’ve yet to detect it definitively in the interstellar medium. In a recent article, a research team led by Víctor Rivilla (National Institute of Aerospace Technology–Spanish National Research Council) took the search for interstellar glycine in a new direction by widening the investigation to include its chemical cousins.

Isolating an Isomer

A glycine molecule contains two carbon atoms, two oxygen atoms, five hydrogen atoms, and one nitrogen atom. But there’s more than one way to arrange these atoms into a molecule, and glycine has many isomers: molecules with the same atoms but arranged in a different way. To search for glycine and its isomers, Rivilla and collaborators turned radio telescopes toward G+0.693–0.027, an interstellar molecular cloud near the center of the Milky Way that is already known to host a number of complex organic molecules.

Structure of a glycolamide molecule and one of the emission lines detected

Left: Example of an emission line from glycolamide detected in this work. Right: The structure of the glycolamide molecule. The molecule is similar to glycine except the carbon atom that is doubly bonded to an oxygen atom is adjacent to the nitrogen atom. [Adapted from Rivilla et al. 2023]

Even in this molecule-rich cloud, glycine remained elusive. However, Rivilla’s team was able to detect one of its isomers for the first time, a molecule called glycolamide, and place upper limits on the abundance of glycine and several of its other isomers.

Tracing Chemical Pathways

Rivilla and collaborators used the measured abundances to explore the likeliest ways glycolamide is created in the interstellar medium. These molecules likely form on the surface of dust grains, where atoms and molecules can gather and link up in the sparse environment of a molecular cloud. For the case of G+0.693–0.027 specifically, ultraviolet photons might create an abundance of highly reactive molecules called radicals, which could interact in this environment to form glycolamide.

graphic showing the proposed way to create glycolamide in the interstellar medium

Pathway proposed in this work to form glycolamide (NH2C(O)CH2OH) in the interstellar medium. Click to enlarge. [Rivilla et al. 2023]

And as for glycine? Comparing the abundances of similar molecules suggests that only small amounts of this critical amino acid exist in G+0.693–0.027 — maybe 3–8 times less than the upper limit measured in this study. Given the low abundance, detecting glycine in the interstellar medium will remain difficult with our current instruments.

Citation

“First Glycine Isomer Detected in the Interstellar Medium: Glycolamide (NH2C(O)CH2OH),” Víctor M. Rivilla et al 2023 ApJL 953 L20. doi:10.3847/2041-8213/ace977

An image of the milky way oriented horizontally, with overlaid blue blobs along the plane corresponding to areas of large neutrino emission.

We live in a universe teeming with neutrinos, tiny particles that zip around near the speed of light and pass right through most solid objects. Where are they all coming from, and what could be responsible for so many bizarre little bits of matter? Astronomers still aren’t sure, but researchers at the IceCube Neutrino Observatory are beginning to sketch out an answer.

Lawless Speeders

Since 2013, astronomers have known that Earth, its telescopes, and all of its inhabitants are constantly weathering a continuous but nearly undetectable barrage of speeding neutrinos. What they are still less sure about, however, are where all of these ultralight and intrusive particles are coming from. Five years ago they discovered and interrogated a possible suspect, a faraway active galactic nucleus with the catchy name TXS 0506+056; however, although they found that this temperamental blazar was indeed responsible for at least some of the high-energy neutrinos zipping through our solar system, it was a small-time player that couldn’t account for the vast majority caught darting through.

An equatorial projection of the sky. Each of the detections is shown as a set of nested contours, each of which denotes a level of confidence.

The on-sky location and uncertainty of each of the detected high-energy neutrino events. [R. Abbasi et al. 2023]

To catch the real culprits, in principle astronomers try to follow their literal tracks. When a high-energy neutrino careens through the earth, sometimes it bumps into the nucleus of an intervening atom. Charged particles created in the aftermath of these rare collisions create a streaking flashes of light pointing along the direction of the incoming neutrino. If scientists can measure the orientation of this streak before it fades, they can trace it backwards and draw a line into the sky pointing towards its origin. This is one of the many goals of the IceCube Neutrino Observatory, a kilometer-sized instrument at the South Pole that looks for tell-tale neutrino flashes through the Antarctic ice.

In reality, measuring these tracks is much easier said than done. While IceCube has detected a few dozen bright streaks since construction finished in 2010, it hasn’t been able to measure any of their orientations accurately enough to guide other telescopes confidently to their sources. Luckily, however, IceCube detects more than just the highest-energy neutrinos, so astronomers have other evidence to consider. It also records millions of smaller, tamer events; with some careful filtering to ignore the neutrinos that are created within the atmosphere by local processes, astronomers can extract a much larger catalog of astrophysical neutrinos, each of which is tagged with a rough estimate of its source location.

Coincidence or Coincident?

The IceCube Collaboration, a vast network of hundreds of scientists, recently dove into these lower-energy events to answer an interesting question: did any of these neutrinos come from the same direction as their brighter counterparts? If they did, that might suggest that whatever is creating all of these racing particles does so at a steady rate, even if it only sends out the heavy-hitters on rarer occasions. Alternatively, if every neutrino came from its own random direction, that would imply that most are created during short outbursts from otherwise quiet sources.

A photograph of a building sitting on a large ice sheet in front of a pink sky.

The IceCube Neutrino Observatory, located at the geographic South Pole [Sven Lidstrom, IceCube/NSF]

After an intensive statistical analysis, the Collaboration found no significant correlation in the direction of neutrino tracks across a wide range of timescales. Even within minutes of a high-energy event, most of the detected low-energy tracks came from random directions, implying that whatever causes a large burst likely doesn’t produce a shower of lower-energy particles at the same time. In other words, neutrinos that arrived at the same time did so by accident: they probably traveled alone from different sources.

Though astronomers still haven’t pinned down the object or process responsible for the background flux of neutrinos, they are making progress building the profile of what they must be like. As studies like this accumulate and the picture improves, eventually the community will settle on an explanation. For, now though, the neutrinos will keep coming, and IceCube will keep watching for flashes.

Citation

“Constraints on Populations of Neutrino Sources from Searches in the Directions of IceCube Neutrino Alerts,” R. Abbasi et al 2023 ApJ 951 45. doi:10.3847/1538-4357/acd2ca

image of the "dragon scale" texture on the surface of Mars

New laboratory experiments suggest that salty water mixed with Martian surface material can remain a liquid under colder and drier conditions than water alone. This means that liquid water might be found over a larger area of Mars’s surface than previously thought, as well as throughout more of the Martian year, with important implications for habitability and exploration.

The Search for Water on Mars

dark streaks on sandy slopes on Mars

Warm temperatures on Mars are associated with the appearance of dark streaks on sloping terrain. On Earth, these streaks are caused by water, but on Mars they may be caused by shifting sand grains instead. [NASA/JPL-Caltech/UA/USGS]

Mars’s sinuous riverbeds and dry lake basins tell a tale of a planet once awash with water, but what about today? Proving the presence of liquid water on Mars’s surface has been tricky, and claims of evidence for modern-day liquid water often find themselves rebutted; for example, the dark streaks thought to indicate subsurface water seeping through the sand were reinterpreted as sand sliding down steep slopes (say that five times fast!).

But the search continues, with evidence mounting that liquid water might exist in the form of brine: a concentrated mixture of water and salt. Martian brine can form in several ways including by water vapor collecting on the surface of salt crystals. In the lab, researchers have tested the conditions under which brine remains a liquid, rather than freezing or evaporating in Mars’s cold, dry climate. But brine on Mars doesn’t exist in isolation. Instead, it’s muddled together with regolith: the loose mixture of rocks, sand, and dust that coats the planet’s surface. Could the mashup of these two materials help water remain a liquid on Mars’s surface?

photograph of Martian soil

An image of Martian soil scooped up by the Phoenix Mars Lander. For this study, the team used simulated Martian soil made from volcanic rocks in the Mojave Desert. [NASA/JPL-Caltech/University of Arizona/Max Planck Institute]

Throwing Regolith into the Mix

To explore this question, Andrew Shumway (University of Washington) and collaborators measured the properties of regolith–brine mixtures in a lab. Since we don’t yet have actual Martian regolith to experiment on, Shumway’s team used a simulated regolith that was originally developed to help NASA scientists test the navigation and sample-collecting skills of the Mars rovers. For their Martian brine, the team swirled together water and a salt called magnesium perchlorate (magnesium and perchlorate are common components of Mars’s surface material).

The team measured two key factors for each of their regolith–brine samples: 1) the freezing point, which partly determines where on the planet’s surface the mixture can remain a liquid, and 2) the amount of water that’s available to participate in chemical reactions and other processes important for life.

Briny Findings

Plot showing the melting points of samples with various concentrations

Melting temperature of frozen regolith–brine samples. Samples with a lower melting temperature also freeze at lower temperatures, making them remain liquid under colder conditions. Click to enlarge. [Shumway et al. 2023]

Shumway’s team found that mixtures of brine and regolith have more water available and freeze at a lower temperature than brine alone, and water can persist when the ambient air is drier, as well. This means that liquid water might be found across more of the Martian surface and during more of the Martian year than previously thought. While this is exciting news for the prospect of finding life on Mars, it also means that we’ll need to be even more careful not to spread earthly microbes to the Martian surface, as water helps to support Earth life as well!

Citation

“Regolith Inhibits Salt and Ice Crystallization in Mg(ClO4)2 Brine, Implying More Persistent and Potentially Habitable Brines on Mars,” Andrew O. Shumway et al 2023 Planet. Sci. J. 4 143. doi:10.3847/PSJ/ace891

A graphic of a white star surrounded by a pink haze of radiation. Streams of white particles flow away from the surface in lines, presumably following the magnetic field lines.

They’re powerful, they’re fast, and we aren’t sure about what causes them, but astronomers are closer than ever to understanding the source of mysterious fast radio bursts.

Flashes Without a Cause

Fast radio bursts: one of the most recent mysteries to appear in the sky and one of the most active fields of astronomical research. Since the discovery of the first of these powerful <1-second eruptions eruptions of radio waves back in 2007, astronomers have recorded hundreds of similar events. We know that they must originate from beyond our Milky Way galaxy, but, beyond that, astronomers have still yet to settle on a consensus about what might cause such brief, energetic flashes. This mystery of the origins of these bursts has driven many astronomers into an exciting, frustrating, and increasingly productive quest to understand whatever immense forces power them.

It is not easy to study a flash; by their very nature, they appear for only a fraction of a second, then vanish to almost never return. While a precious few do eventually repeat, they do so at largely irregular intervals, meaning astronomers can never really be sure when or where a fast radio burst might happen. The ones astronomers do manage to spot are almost always flagged by survey telescopes that scan huge swaths of the sky at once. A wide field of view comes with a tradeoff, though: although these telescopes can monitor enough sky that they have a good chance of catching a burst, their view of the sky is fairly blurry. So, although astronomers have recorded a few hundred bursts by now, they usually can’t say exactly where each one came from.

For the 16 years since the discovery of the first fast radio burst, astronomers have been trying to piece together their secrets without even knowing the location of each flash. Recently, however, they have made progress both in narrowing in on their quarry and on understanding their source. Below are three recent studies published in AAS Journals detailing this progress.

A pixelated image of a galaxy that appears as a roughly circular blob at center. The white circle marking the uncertainty in the position of the FRB is much smaller than the galaxy itself.Zooming In

First up is a study published in May of this year by a team led by Shivani Bhandari, Netherlands Institute for Radio Astronomy. Bhandari and collaborators describe their discovery of a fast radio burst using the Australian Square Kilometre Array Pathfinder, a relatively new and phenomenally capable radio telescope that they used the pin down the location of the flash to within one arcsecond (about 0.03% of a degree!). This extreme precision allowed the team to identify which galaxy the flash came from, and what they found was somewhat surprising: the host was a small, somewhat boring dwarf galaxy with almost no ongoing star formation. This is in contrast to the handful of known hosts of repeating fast radio burst, which were all more lively, active galaxies. Considering both the host and the properties of the burst itself, the team concluded that their burst could have been caused by an “accretion jet from a hyperaccreting black hole.”

Magnetar Earthquakes?

A month later in early June, a team led by Fayin Wang, Nanjing University, published their own analysis of archival data to suggest an alternative source. By digging through all of the observations of two known repeating fast radio burst collected by the Five-hundred-meter Aperture Spherical radio Telescope (FAST), Wang and colleagues realized that the gaps between bursts were not quite as random as previously thought. Instead, whatever was causing the bursts seemed to have “memory,” meaning the triggers must be correlated in time. Building from this, they advocate for a different explanation, positing that the bursts occur whenever a highly magnetized neutron star undergoes “crustal fractures” — in other words, earthquakes. After a shift, the magnetic stresses will build up again and cause the process to repeat, which could give rise to the recurring bursts.

More Than One

A photograph of two nearby spiral galaxies, with cyan ellipses overplotted to show the uncertainty in the location of the FRB.

The location of one of the thirteen repeating fast radio burst, which lines up perfectly with a pair of merging spiral galaxies. [Michilli et al. 2023]

Finally, in late June, another study led by Daniele Michilli, Massachusetts Institute of Technology, offered a bridge between the two previous ones. This publication describes a re-analysis of data collected by the Canadian Hydrogen Intensity Mapping Experiment (CHIME), which resulted in newer, much more precise estimates of previous fast radio burst locations. The team focused on 13 repeating bursts and pinned down each of their locations to within about 10 arcseconds. While that isn’t precise enough to nail the host galaxy for all 13, they did mange to conclusively identify the host for two of them. Intriguingly, these two galaxies were nothing alike: one is a peaceful, quiescent galaxy, and the other is one in a pair of merging spiral galaxies that are actively forming many stars. This suggests that fast radio burst can come from a range of environments, or even that there could be multiple causes that each produce a similar looking signal.

While the final, well-supported model to describe all fast radio burst is still out of reach, astronomers are actively getting closer to this final goal. As new telescopes and processing techniques come online, it is only a matter of time until enough data is collected and analyzed that a clearer picture emerges. Soon, what now appear as mysterious flashes will be the subjects of well-documented chapters in the next textbooks, and this knowledge will be based on studies happening today, like these three.

Citation

“A Nonrepeating Fast Radio Burst in a Dwarf Host Galaxy,” Shivani Bhandari et al 2023 ApJ 948 67. doi:10.3847/1538-4357/acc178

“Repeating Fast Radio Bursts Reveal Memory from Minutes to an Hour,” F. Y. Wang et al 2023 ApJL 949 L33. doi:10.3847/2041-8213/acd5d2

“Subarcminute Localization of 13 Repeating Fast Radio Bursts Detected by CHIME/FRB,” Daniele Michilli et al 2023 ApJ 950 134. doi:10.3847/1538-4357/accf89

illustration of a quasar

Using JWST, researchers have validated a new tool for studying quasars — the extremely luminous nuclei of galaxies in the early universe, powered by the accretion of gas onto a growing supermassive black hole.

Studying Superlative Sources

Hubble image of the quasar 3C 273

This Hubble image shows the quasar 3C 273. [ESA/Hubble & NASA; CC BY 4.0]

Quasars are among the most luminous objects in the universe. Not only are they extremely bright, some quasars also create immense outflows that inject even more energy into their surroundings. The energy generated by quasars is enough to turn the tides of star formation in entire galaxies, transforming busy, star-forming galaxies into quiescent ones.

Astronomers have identified the best optical wavelengths for studying quasar outflows, but with a powerful new infrared telescope in our toolkit, we’ll need to develop new diagnostics for studying these features in the infrared and measure how they stack up against our existing tools.

Visible vs. Infrared

To expand our toolkit into the infrared, David Rupke (Rhodes College) and collaborators compared infrared observations from JWST and visible-light observations from the Gemini North telescope. The team focused on the quasar F2M110648.35+480712.3, or F2M1106 for short, which at roughly 5 billion light-years away is the nearest of the quasars in the JWST Q3D Early Release Science program sample.

Rupke‘s team analyzed observations at two main wavelengths: visible light at 500 nanometers, which comes from oxygen atoms that have lost two of their electrons, and infrared light at 10.5 microns (1 micron = 10-6 meter), which comes from sulfur atoms that have lost three of their electrons. The 500-nanometer oxygen line is a proven tool for studying quasar outflows since it traces oxygen atoms that have lost their electrons to the harsh ultraviolet radiation from the quasar. Coincidentally, it takes nearly the same amount of energy to remove three electrons from a sulfur atom as it does to steal two electrons from an oxygen atom. This hints that the infrared sulfur line might trace the same structures that are traced by the oxygen line — namely, the warm, ionized gas that makes up a quasar’s powerful outflows.

Future Quasar Investigations

comparison of emission from oxygen and sulfur atoms in the outflow region of a quasar

Comparison of emission from doubly ionized oxygen (OIII) at 500 nm/5007 Å to emission from triply ionized sulfur (SIV) at 10.51 microns. Click to enlarge. [Rupke et al. 2023]

Imaged at 500 nanometers, the quasar F2M1106 clearly has two lobes extending roughly 33,000 light-years out from the quasar in opposite directions. At 10.5 microns, the scene is similar, with two oppositely directed lobes with their brightest points located at the same position at both wavelengths.

While the two emission lines don’t paint exactly the same picture — the infrared sulfur line is inherently weaker than the optical oxygen line, so it fades out of view in certain regions — it’s clear that 10.5-micron emission is an effective way to study quasar outflows with JWST. And for dust-reddened quasars like F2M1106, it has a particular advantage, as its longer wavelength is better at piercing dusty clouds. This study demonstrates JWST’s potential as a new tool for studying quasar outflows, helping us to understand the impact of these structures on galaxy evolution.

Citation

“First Results from the JWST Early Release Science Program Q3D: Benchmark Comparison of Optical and Mid-infrared Tracers of a Dusty, Ionized Red Quasar Wind at z = 0.435,” David S. N. Rupke et al 2023 ApJL 953 L26. doi:10.3847/2041-8213/aced85

ultraviolet image of Mars

As the Sun’s magnetic activity cycle ramps up, solar storms are brewing. A recent research article describes what two Mars-orbiting spacecraft saw when a solar storm struck the red planet.

image of the Sun releasing two coronal mass ejections

This image from the Solar and Heliospheric Observatory shows two coronal mass ejections during an event in November 2000. [ESA/NASA/SOHO]

Storms from the Sun

The Sun’s atmosphere is suffused with pent-up magnetic energy that periodically escapes, powering coronal mass ejections (CMEs): immense eruptions of plasma and magnetic fields. CMEs are common — the frequency varies from about 50 a year when the Sun is at its calmest to more than 1,000 a year at the peak of the Sun’s activity cycle — but only a small fraction of these storms strike the planets in our solar system.

When a CME strikes Earth, it disturbs and compresses our planet’s protective magnetic shield and creates brilliant auroras. While Earth has a strong global magnetic field to shield our atmosphere from these storms, other planets are not as fortunate. Mars has feeble magnetic fields buried in some of its crust, perhaps a remnant of a stronger and larger magnetic field from long ago, but its atmosphere is largely unprotected. How solar activity like CMEs affects the atmosphere of Mars is important for our understanding of how the red planet’s atmosphere and habitability has evolved over billions of years.

Day and Night

On 4 December 2021, a CME burst from the Sun. Two days later, it swept past the BepiColombo spacecraft, which was surfing through the solar wind after passing close to Mercury for the first time, and charged toward Mars. There, two spacecraft stood at the ready: Tianwen-1 and MAVEN. When the CME reached Mars on 10 December 2021, Tianwen-1 witnessed the impact head on from its vantage point on the sunlit side of the planet. MAVEN took notes from the night side, spared the direct impact of the CME but positioned to witness how its effect propagated around the planet.

illustration of the orbits of Tianwen-1 and MAVEN during the CME

An illustration of the orbits of the Tianwen-1 and MAVEN spacecraft during the CME’s passage. Tianwen-1 sampled the day side atmosphere while MAVEN measured the night side atmosphere. BepiColombo witnessed the event from much closer to the Sun. Click to enlarge. [Yu et al. 2023]

A team led by Bingkun Yu (Institute of Deep Space Sciences) described the impact of the CME on Mars from the perspective of these two spacecraft. These spacecraft monitor Mars’s ionosphere, a weakly ionized layer of the atmosphere created by high-energy solar photons. As the CME reached Tianwen-1 on Mars’s day side, it compressed the ionosphere, pushing the top of this layer progressively lower over the course of several days.

Under normal conditions, some ionospheric plasma travels around the planet to its night side. What MAVEN witnessed as the CME raged on was notably different: a major decrease in the number of ions present in the shadow of the planet. This decrease suggests that rather than swinging around to the night side, the ions were escaping the atmosphere altogether, swept downstream by the CME.

Escape of Martian Plasma

plot summarizing the effects of the CME on the Martian ionosphere

Summary of the effects of the interplanetary CME (ICME) on Mars’s ionosphere. The density of the ionospheric plasma, shown by the solid lines, dipped, and the ionopause — the altitude above with the plasma density drops off sharply — was pushed lower day by day. Click to enlarge. [Adapted from Yu et al. 2023]

Essentially, Yu and collaborators reported that Tianwen-1 and MAVEN witnessed the removal of ions from Mars’s atmosphere by the CME. MAVEN saw a loss of 34%, 61% and 73% of the electrons, O+ ions, and O2+ ions, respectively, from altitudes above 180 km.

Since only a small fraction of Mars’s atmosphere is ionized, this means the CME stole just a tiny amount of the atmosphere — but the compounded effects of CMEs over billions of years could be huge; the theft of ions likely shaped the evolution of Mars’s atmosphere, playing a role in transforming our neighboring planet from one that was warm and hospitable to the unforgiving, arid world it is today.

Citation

“Tianwen-1 and MAVEN Observations of the Response of Mars to an Interplanetary Coronal Mass Ejection,” Bingkun Yu et al 2023 ApJ 953 105. doi:10.3847/1538-4357/acdcf8

A circular red gas bubble, more opaque on the edges than in the center, sits in a field of stars.

In astronomy, not detecting something can tell us something useful. A recent article details a radio search for six supernovae that resulted in no detections — but still gives us hints about the companions of these exploding stars.

How Stars Explode

Illustration of a white dwarf accreting gas from a red giant companion star

Accretion from a companion star onto a white dwarf is one way to trigger a supernova. [Still image from an animation by NASA’s Goddard Space Flight Center Conceptual Image Lab]

Supernovae happen in two main ways. In the first, a massive star loses its battle with gravity, and its outer layers rebound off its collapsed core in a massive explosion. In the second, an evolved low- to intermediate-mass star experiences pulsations that cause its outer layers to gently waft off into space, leaving behind its core as a white dwarf. If the white dwarf has a binary companion, the companion can donate material to the white dwarf through winds, accretion, or a collision. If the white dwarf gains too much mass, its core ignites and the star explodes.

Researchers suspect that nearly any type of star, from compact white dwarfs to puffed-up supergiants, can donate mass to a white dwarf and trigger a supernova. Which stars actually do is an active area of research, especially since these explosions are useful cosmic distance markers. But how can we tell from the aftermath of the explosion what kind of star was involved?

A Rapid Radio Search

When a star donates mass to its white dwarf companion, some of that gas remains in the space between the two stars, and the distribution of the gas can tell us about the type of companion star; for example, the billowing stellar winds of a supergiant should create an extended region of low-density gas. When a supernova’s shock wave collides with and compresses this gas, it generates synchrotron radiation from electrons traveling in helical paths around magnetic field lines. Synchrotron radiation is produced even when the supernova collides with very low-density material that would be impossible to see through other means.

Chelsea Harris (Michigan State University) and collaborators performed a search for synchrotron radiation from six nearby supernovae that had been detected at optical wavelengths. Judging by the evolution of their optical light curves, these supernovae all resulted from exploding white dwarfs with mass-donating stellar companions. However, the team found no radio emission to accompany the rapidly fading optical light of any of the supernovae in their sample.

Plot showing the types of winds ruled out by the radio non-detections of the supernovae in the authors' sample

Non-detections of the six supernovae rule out winds with the parameters spanned by the green bars. Click to enlarge. [Harris et al. 2023]

Winds, Shocks, and Shells

By modeling how much radio emission we’d expect to see from various distributions of circumstellar gas, the team was able to rule out the presence of low-density stellar winds from a supergiant companion. The non-detections also ruled out most winds due to accretion of material onto the white dwarf.

As part of their investigation, Harris and collaborators also modeled the expected synchrotron emission from a shock wave colliding with a dense shell of circumstellar gas. This scenario might arise when a pre-supernova white dwarf undergoes one or more novae before the ultimate explosion. Unexpectedly, the team found that these dense shells probably don’t produce a detectable amount of synchrotron emission. While radio observations are a powerful tool to study circumstellar gas, these shells might make themselves known on the other side of the electromagnetic spectrum, with fleeting bursts of X-rays or gamma rays.

Citation

“Radio Observations of Six Young Type Ia Supernovae,” C. E. Harris et al 2023 ApJ 952 24. doi:10.3847/1538-4357/acd84f

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