astrophysics

how-kepler’s-400-year-old-sunspot-sketches-helped-solve-a-modern-mystery

How Kepler’s 400-year-old sunspot sketches helped solve a modern mystery

astronomy, astrophysics, Johannes Kepler, Physics, Science, solar physics, sun spots / /
A naked-eye sunspot group on 11 May 2024

Enlarge / A naked-eye sunspot group on May 11, 2024. There are typically 40,000 to 50,000 sunspots observed in ~11-year solar cycles.

E. T. H. Teague

A team of Japanese and Belgian astronomers has re-examined the sunspot drawings made by 17th century astronomer Johannes Kepler with modern analytical techniques. By doing so, they resolved a long-standing mystery about solar cycles during that period, according to a recent paper published in The Astrophysical Journal Letters.

Precisely who first observed sunspots was a matter of heated debate in the early 17th century. We now know that ancient Chinese astronomers between 364 and 28 BCE observed these features and included them in their official records. A Benedictine monk in 807 thought he’d observed Mercury passing in front of the Sun when, in reality, he had witnessed a sunspot; similar mistaken interpretations were also common in the 12th century. (An English monk made the first known drawings of sunspots in December 1128.)

English astronomer Thomas Harriot made the first telescope observations of sunspots in late 1610 and recorded them in his notebooks, as did Galileo around the same time, although the latter did not publish a scientific paper on sunspots (accompanied by sketches) until 1613. Galileo also argued that the spots were not, as some believed, solar satellites but more like clouds in the atmosphere or the surface of the Sun. But he was not the first to suggest this; that credit belongs to Dutch astronomer Johannes Fabricus, who published his scientific treatise on sunspots in 1611.

Kepler read that particular treatise and admired it, having made his sunspot observations using a camera obscura in 1607 (published in a 1609 treatise), which he initially thought was a transit of Mercury. He retracted that report in 1618, concluding that he had actually seen a group of sunspots. Kepler made his solar drawings based on observations conducted both in his own house and in the workshop of court mechanic Justus Burgi in Prague.  In the first case, he reported “a small spot in the size of a small fly”; in the second, “a small spot of deep darkness toward the center… in size and appearance like a thin flea.”

The earliest datable sunspot drawings based on Kepler's solar observations with camera obscura in May 1607.

Enlarge / The earliest datable sunspot drawings based on Kepler’s solar observations with camera obscura in May 1607.

Public domain

The long-standing debate that is the subject of this latest paper concerns the period from around 1645 to 1715, during which there were very few recorded observations of sunspots despite the best efforts of astronomers. This was a unique event in astronomical history. Despite only observing some 59 sunspots during this time—compared to between 40,000 to 50,000 sunspots over a similar time span in our current age—astronomers were nonetheless able to determine that sunspots seemed to occur in 11-year cycles.

German astronomer Gustav Spörer noted the steep decline in 1887 and 1889 papers, and his British colleagues, Edward and Annie Maunder, expanded on that work to study how the latitudes of sunspots changed over time. That period became known as the “Maunder Minimum.” Spörer also came up with “Spörer’s law,” which holds that spots at the start of a cycle appear at higher latitudes in the Sun’s northern hemisphere, moving to successively lower latitudes in the southern hemisphere as the cycle runs its course until a new cycle of sunspots begins in the higher latitudes.

But precisely how the solar cycle transitioned to the Maunder Minimum has been far from clear. Reconstructions based on tree rings have produced conflicting data. For instance, one such reconstruction concluded that the gradual transition was preceded either by an extremely short solar cycle of about five years or an extremely long solar cycle of about 16 years. Another tree ring reconstruction concluded the solar cycle would have been of normal 11-year duration.

Independent observational records can help resolve the discrepancy. That’s why Hisashi Hayakawa of Nagoya University in Japan and co-authors turned to Kepler’s drawings of sunspots for additional insight, which predate existing telescopic observations by several years.

How Kepler’s 400-year-old sunspot sketches helped solve a modern mystery Read More »

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Astronomers find first emission spectra in brightest GRB of all time

astronomy, astrophysics, B.O.A.T., Fermi space telescope, Gamma ray bursts, Physics, Science / /

shine on, you beautiful BOAT —

Chance that first detected emission line is a noise fluctuation is one in half a billion.

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A jet of particles moving at nearly light speed emerges from a massive star in this artist’s concept.

Enlarge / A jet of particles moving at nearly light-speed emerges from a massive star in this artist’s concept of the BOAT.

NASA’s Goddard Space Flight Center Conceptual Image Lab

Scientists have been all aflutter since several space-based detectors picked up a powerful gamma-ray burst (GRB) in October 2022—a burst so energetic that astronomers nicknamed it the BOAT (Brightest Of All Time). Now an international team of astronomers has analyzed an unusual energy peak detected by NASA’s Fermi Gamma-ray Space Telescope and concluded that it was an emission spectra, according to a new paper published in the journal Science. Per the authors, it’s the first high-confidence emission line ever seen in 50 years of studying GRBs.

As reported previously, gamma-ray bursts are extremely high-energy explosions in distant galaxies lasting between mere milliseconds to several hours. There are two classes of gamma-ray bursts. Most (70 percent) are long bursts lasting more than two seconds, often with a bright afterglow. These are usually linked to galaxies with rapid star formation. Astronomers think that long bursts are tied to the deaths of massive stars collapsing to form a neutron star or black hole (or, alternatively, a newly formed magnetar). The baby black hole would produce jets of highly energetic particles moving near the speed of light, powerful enough to pierce through the remains of the progenitor star, emitting X-rays and gamma rays.

Those gamma-ray bursts lasting less than two seconds (about 30 percent) are deemed short bursts, usually emitting from regions with very little star formation. Astronomers think these gamma-ray bursts are the result of mergers between two neutron stars, or a neutron star merging with a black hole, comprising a “kilonova.” That hypothesis was confirmed in 2017 when the LIGO collaboration picked up the gravitational wave signal of two neutron stars merging, accompanied by the powerful gamma-ray bursts associated with a kilonova.

Several papers were published last year reporting on the analytical results of all the observational data. Those findings confirmed that GRB 221009A was indeed the BOAT, appearing especially bright because its narrow jet was pointing directly at Earth. But the various analyses also yielded several surprising results that puzzled astronomers. Most notably, a supernova should have occurred a few weeks after the initial burst, but astronomers didn’t detect one, perhaps because it was very faint, and thick dust clouds in that part of the sky were dimming any incoming light.

Earlier this year, astronomers confirmed that the BOAT came from a supernova, thanks to the telltale signatures of key elements like calcium and oxygen that one would expect to find with a supernova. However, they did not find evidence of the expected heavy elements like platinum and gold, which bears on the longstanding question of the origin of such elements in the universe. The BOAT might just be special in that regard; further data will tell us more.

“It gave me goosebumps”

A few minutes after the BOAT erupted, Fermi’s Gamma-ray Burst Monitor recorded an unusual energy peak. Scientists now say this feature is the first high-confidence emission line ever seen in 50 years of studying GRBs.

The newly detected spectral emission line was likely caused by the collision of matter and anti-matter, according to the authors, producing a pair of gamma rays that are blue-shifted toward higher energies because we are looking into the jet. Having a spectral emission associated with a GRB is important because it can shed light on the specific chemicals involved in the interactions. There have been prior studies reporting possible evidence for absorption or emission lines in other GRBs, but they have usually turned out likely to be statistical noise.

That’s not the case with this latest detection, according to co-author Om Sharan Salafia at INAF-Brera Observatory in Milan, Italy, who added that the odds of this turning out to be a statistical fluctuation “are less than one chance in half a billion.” His INAF colleague and co-author, Maria Edvige Ravasio, said that when she first saw the signal, “it gave me goosebumps.”

Why did astronomers take so long to detect it? When the BOAT first erupted in 2022, it saturated most of the space-based gamma-ray detectors, including the Fermi Space Telescope, making them unable to measure the most intense part of that blast. The emission line didn’t appear until a good five minutes after the burst when it had sufficiently dimmed for Fermi to make a measurement. The spectral emission lasted for about 40 seconds and reached a peak energy of about 12 MeV, compared to 2 or 3 MeB for visible light, per the authors.

Science, 2024. DOI: 10.1126/science.adj3638  (About DOIs).

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Nearby star cluster houses unusually large black hole

astronomy, astrophysics, black hole, globular cluster, Science / /

Big, but not that big —

Fast-moving stars imply that there’s an intermediate-mass black hole there.

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Three panel image, with zoom increasing from left to right. Left most panel is a wide view of the globular cluster; right is a zoom in to the area where its central black hole must reside.

Enlarge / From left to right, zooming in from the globular cluster to the site of its black hole.

ESA/Hubble & NASA, M. Häberle

Supermassive black holes appear to reside at the center of every galaxy and to have done so since galaxies formed early in the history of the Universe. Currently, however, we can’t entirely explain their existence, since it’s difficult to understand how they could grow quickly enough to reach the cutoff for supermassive as quickly as they did.

A possible bit of evidence was recently found by using about 20 years of data from the Hubble Space Telescope. The data comes from a globular cluster of stars that’s thought to be the remains of a dwarf galaxy and shows that a group of stars near the cluster’s core are moving so fast that they should have been ejected from it entirely. That implies that something massive is keeping them there, which the researchers argue is a rare intermediate-mass black hole, weighing in at over 8,000 times the mass of the Sun.

Moving fast

The fast-moving stars reside in Omega Centauri, the largest globular cluster in the Milky Way. With an estimated 10 million stars, it’s a crowded environment, but observations are aided by its relative proximity, at “only” 17,000 light-years away. Those observations have been hinting that there might be a central black hole within the globular cluster, but the evidence has not been decisive.

The new work, done by a large international team, used over 500 images of Omega Centauri, taken by the Hubble Space Telescope over the course of 20 years. This allowed them to track the motion of stars within the cluster, allowing an estimate of their speed relative to the cluster’s center of mass. While this has been done previously, the most recent data allowed an update that reduced the uncertainty in the stars’ velocity.

Within the update data, a number of stars near the cluster’s center stood out for their extreme velocities: seven of them were moving fast enough that the gravitational pull of the cluster isn’t enough to keep them there. All seven should have been lost from the cluster within 1,000 years, although the uncertainties remained large for two of them. Based on the size of the cluster, there shouldn’t even be a single foreground star between the Hubble and the Omega Cluster, so these really seem to be within the cluster despite their velocity.

The simplest explanation for that is that there’s an additional mass holding them in place. That could potentially be several massive objects, but the close proximity of all these stars to the center of the cluster favor a single, compact object. Which means a black hole.

Based on the velocities, the researchers estimate that the object has a mass of at least 8,200 times that of the Sun. A couple of stars appear to be accelerating; if that holds up based on further observations, it would indicate that the black hole is over 20,000 solar masses. That places it firmly within black hole territory, though smaller than supermassive black holes, which are viewed as those with roughly a million solar masses or more. And it’s considerably larger than you’d expect from black holes formed through the death of a star, which aren’t expected to be much larger than 100 times the Sun’s mass.

This places it in the category of intermediate-mass black holes, of which there are only a handful of potential sightings, none of them universally accepted. So, this is a significant finding if for no other reason than it may be the least controversial spotting of an intermediate-mass black hole yet.

What’s this telling us?

For now, there are still considerable uncertainties in some of the details here—but prospects for improving the situation exist. Observations with the Webb Space Telescope could potentially pick up the faint emissions from gas that’s falling into the black hole. And it can track the seven stars identified here. Its spectrographs could also potentially pick up the red and blue shifts in light caused by the star’s motion. Its location at a considerable distance from Hubble could also provide a more detailed three-dimensional picture of Omega Centauri’s central structure.

Figuring this out could potentially tell us more about how black holes grow to supermassive scales. Earlier potential sightings of intermediate-mass black holes have also come in globular clusters, which may suggest that they’re a general feature of large gatherings of stars.

But Omega Centauri differs from many other globular clusters, which often contain large populations of stars that all formed at roughly the same time, suggesting the clusters formed from a single giant cloud of materials. Omega Centauri has stars with a broad range of ages, which is one of the reasons why people think it’s the remains of a dwarf galaxy that was sucked into the Milky Way.

If that’s the case, then its central black hole is an analog of the supermassive black holes found in actual dwarf galaxies—which raises the question of why it’s only intermediate-mass. Did something about its interactions with the Milky Way interfere with the black hole’s growth?

And, in the end, none of this sheds light on how any black hole grows to be so much more massive than any star it could conceivably have formed from. Getting a better sense of this black hole’s history could provide more perspective on some questions that are currently vexing astronomers.

Nature, 2024. DOI: 10.1038/s41586-024-07511-z  (About DOIs).

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Swarm of dusty young stars found around our galaxy’s central black hole

astronomy, astrophysics, black holes, milky way, Science, Star formation / /

Hot young stars —

Stars shouldn’t form that close to the black hole, so these would need explaining.

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Image with a black background, large purple streaks, and a handful of bright blue objects.

Enlarge / The Milky Way’s central black hole is in a very crowded neighborhood.

Supermassive black holes are ravenous. Clumps of dust and gas are prone to being disrupted by the turbulence and radiation when they are pulled too close. So why are some of them orbiting on the edge of the Milky Way’s own supermassive monster, Sgr A*? Maybe these mystery blobs are hiding something.

After analyzing observations of the dusty objects, an international team of researchers, led by astrophysicist Florian Peißker of the University of Cologne, have identified these clumps as potentially harboring young stellar objects (YSOs) shrouded by a haze of gas and dust. Even stranger is that these infant stars are younger than an unusually young and bright cluster of stars that are already known to orbit Sgr A*, known as the S-stars.

Finding both of these groups orbiting so close is unusual because stars that orbit supermassive black holes are expected to be dim and much more ancient. Peißker and his colleagues “discard the en vogue idea to classify [these] objects as coreless clouds in the high energetic radiation field of the supermassive black hole Sgr A*,” as they said in a study recently published in Astronomy & Astrophysics.

More than just space dust

To figure out what the objects near Sgr Amight be the, researchers needed to rule out things they weren’t. Embedded in envelopes of gas and dust, they maintain especially high temperatures, do not evaporate easily, and each orbits the supermassive black hole alone.

The researchers determined their chemical properties from the photons they emitted, and their mid- and near-infrared emissions were consistent with those of stars. They used one of them, object G2/DSO, as a case study to test their ideas about what the objects might be. The high brightness and especially strong emissions of this object make it the easiest to study. Its mass is also similar to the masses of known low-mass stars.

YSOs are low-mass stars that have outgrown the protostar phase but have not yet developed into main sequence stars, with cores that fuse hydrogen into helium. These objects like YSO candidates because they couldn’t possibly be clumps of gas and space dust. Gaseous clouds without any objects inside to hold them together via gravity could not survive so close to a supermassive black hole for long. Its intense heat causes the gas and dust to evaporate rather quickly, with heat-excited particles crashing into each other and flying off into space.

The team figured out that a cloud comparable in size to G2/DSO would evaporate in about seven years. A star orbiting at the same distance from the supermassive black hole would not be destroyed nearly as fast because of its much higher density and mass.

Another class of object that the dusty blobs could hypothetically be—but are not—is a compact planetary nebula or CPN. These nebulae are the expanding outer gas envelopes of small to medium stars in their final death throes. While CPNs have some features in common with stars, the strength of a supermassive black hole’s gravity would easily detach their gas envelopes and tear them apart.

It is also unlikely that the YSOs are binary stars, even though most stars form in binary systems. The scorching temperatures and turbulence of SGR Awould likely cause stars that were once part of binaries to migrate.

Seeing stars

Further observations determined that some of the dust-obscured objects are nascent stars, and while others are thought to be stars of some kind, but haven’t been definitively identified.

The properties that made G2/DSO an exceptional case study are also the reason it has been identified as a YSO. D2 is another high-luminosity object about as massive as a low-mass star, which is easy to observe in the near- and mid-infrared. D3 and D23 also have similar properties. These are the blobs near the black hole that the researchers think are most likely to be YSOs.

There are other candidates that need further analysis. These include additional objects that may or may not be YSOs, but still show stellar characteristics: D3.1 and D5, which are difficult to observe. The mid-infrared emissions of D9 are especially low when compared to the other candidates, but it is still thought to be some type of star, though possibly not a YSO. Objects X7 and X8 both exhibit bow shock—the shockwave that results from a star’s stellar wind pushing against other stellar winds. Whether either of these objects is actually a YSO remains unknown.

Where these dusty objects came from and how they formed is unknown for now. The researchers suggest that the objects formed together in molecular clouds that were falling toward the center of the galaxy. They also think that, no matter where they were born, they migrated towards Sgr A*, and any that were in binary systems were separated by the black hole’s immense gravity.

While it is unlikely that the YSOs and potential YSOs originated in the same cluster as the slightly older S-stars, they still might be related in some way. They might have experienced similar formation and migration journeys, and the younger stars might ultimately reach the same stage.

“Speculatively, the dusty sources will evolve into low-mass S stars,” Peißker’s team said in the same study.

Even black holes look better with a necklace of twinkling diamonds.

Astronomy and Astrophysics, 2024.  DOI: 10.1051/0004-6361/202449729

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40 years later, X Window System is far more relevant than anyone could guess

astrophysics, dumb terminal, Linux, Sun microsystems, Tech, UNIX, VAX, Wayland, X, x window system, x11, zaphod / /

Widely but improperly known as X-windows —

One astrophysics professor’s memories of writing X11 code in the 1980s.

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low angle view of Office Buildings in Hong Kong from below, with the sky visible through an X-like cross

Getty Images

Often times, when I am researching something about computers or coding that has been around a very long while, I will come across a document on a university website that tells me more about that thing than any Wikipedia page or archive ever could.

It’s usually a PDF, though sometimes a plaintext file, on a .edu subdirectory that starts with a username preceded by a tilde (~) character. This is typically a document that a professor, faced with the same questions semester after semester, has put together to save the most time possible and get back to their work. I recently found such a document inside Princeton University’s astrophysics department: “An Introduction to the X Window System,” written by Robert Lupton.

X Window System, which turned 40 years old earlier this week, was something you had to know how to use to work with space-facing instruments back in the early 1980s, when VT100s, VAX-11/750s, and Sun Microsystems boxes would share space at college computer labs. As the member of the AstroPhysical Sciences Department at Princeton who knew the most about computers back then, it fell to Lupton to fix things and take questions.

“I first wrote X10r4 server code, which eventually became X11,” Lupton said in a phone interview. “Anything that needed graphics code, where you’d want a button or some kind of display for something, that was X… People would probably bug me when I was trying to get work done down in the basement, so I probably wrote this for that reason.”

Getty Images

Where X came from (after W)

Robert W. Scheifler and Jim Gettys at MIT spent “the last couple weeks writing a window system for the VS100” back in 1984. As part of Project Athena‘s goals to create campus-wide computing with distributed resources and multiple hardware platforms, X fit the bill, being independent of platforms and vendors and able to call on remote resources. Scheifler “stole a fair amount of code from W,” made its interface asynchronous and thereby much faster, and “called it X” (back when that was still a cool thing to do).

That kind of cross-platform compatibility made X work for Princeton, and thereby Lupton. He notes in his guide that X provides “tools not rules,” which allows for “a very large number of confusing guises.” After explaining the three-part nature of X—the server, the clients, and the window manager—he goes on to provide some tips:

  • Modifier keys are key to X; “this sensitivity extends to things like mouse buttons that you might not normally think of as case-sensitive.”
  • “To start X, type xinit; do not type X unless you have defined an alias. X by itself starts the server but no clients, resulting in an empty screen.”
  • “All programmes running under X are equal, but one, the window manager, is more equal.”
  • Using the “--zaphod” flag prevents a mouse from going into a screen you can’t see; “Someone should be able to explain the etymology to you” (link mine).
  • “If you say kill 5 -9 12345 you will be sorry as the console will appear hopelessly confused. Return to your other terminal, say kbd mode -a, and make a note not to use -9 without due reason.”

I asked Lupton, whom I caught on the last day before he headed to Chile to help with a very big telescope, how he felt about X, 40 years later. Why had it survived?

“It worked, at least relative to the other options we had,” Lupton said. He noted that Princeton’s systems were not “heavily networked in those days,” such that the network traffic issues some had with X weren’t an issue then. “People weren’t expecting a lot of GUIs, either; they were expecting command lines, maybe a few buttons… it was the most portable version of a window system, running on both a VAX and the Suns at the time… it wasn’t bad.”

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Supermassive black hole roars to life as astronomers watch in real time

active galactic nuclei, astronomy, astrophysics, black holes, Physics, Science, Very Large Telescope / /

Sleeping Beauty —

A similar awakening may one day occur with the Milky Way’s supermassive black hole

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Artist’s animation of the black hole at the center of SDSS1335+0728 awakening in real time—a first for astronomers.

In December 2019, astronomers were surprised to observe a long-quiet galaxy, 300 million light-years away, suddenly come alive, emitting ultraviolet, optical, and infrared light into space. Far from quieting down again, by February of this year, the galaxy had begun emitting X-ray light; it is becoming more active. Astronomers think it is most likely an active galactic nucleus (AGN), which gets its energy from supermassive black holes at the galaxy’s center and/or from the black hole’s spin. That’s the conclusion of a new paper accepted for publication in the journal Astronomy and Astrophysics, although the authors acknowledge the possibility that it might also be some kind of rare tidal disruption event (TDE).

The brightening of SDSS1335_0728 in the constellation Virgo, after decades of quietude, was first detected by the Zwicky Transient Facility telescope. Its supermassive black hole is estimated to be about 1 million solar masses. To get a better understanding of what might be going on, the authors combed through archival data and combined that with data from new observations from various instruments, including the X-shooter, part of the Very Large Telescope (VLT) in Chile’s Atacama Desert.

There are many reasons why a normally quiet galaxy might suddenly brighten, including supernovae or a TDE, in which part of the shredded star’s original mass is ejected violently outward. This, in turn, can form an accretion disk around the black hole that emits powerful X-rays and visible light. But these events don’t last nearly five years—usually not more than a few hundred days.

So the authors concluded that the galaxy has awakened and now has an AGN. First discovered by Carl Seyfert in 1943, the glow is the result of the cold dust and gas surrounding the black hole, which can form orbiting accretion disks. Gravitational forces compress the matter in the disk and heat it to millions of degrees Kelvin, producing radiation across the electromagnetic spectrum.

Alternatively, the activity might be due to an especially long and faint TDE—the longest and faintest yet detected, if so. Or it could be an entirely new phenomenon altogether. So SDSS1335+0728 is a galaxy to watch. Astronomers are already preparing for follow-up observations with the VLT’s Multi Unit Spectroscopic Explorer (MUSE) and Extremely Large Telescope, among others, and perhaps even the Vera Rubin Observatory slated to come online next summer. Its Large Synoptic Survey Telescope (LSST) will be capable of imaging the entire southern sky continuously, potentially capturing even more galaxy awakenings.

“Regardless of the nature of the variations, [this galaxy] provides valuable information on how black holes grow and evolve,” said co-author Paula Sánchez Sáez, an astronomer at the European Southern Observatory in Germany. “We expect that instruments like [these] will be key in understanding [why the galaxy is brightening].”

There is also a supermassive black hole at the center of our Milky Way galaxy (Sgr A*), but there is not yet enough material that has accreted for astronomers to pick up any emitted radiation, even in the infrared. So, its galactic nucleus is deemed inactive. It may have been active in the past, and it’s possible that it will reawaken again in a few million (or even billion) years when the Milky Way merges with the Andromeda Galaxy and their respective supermassive black holes combine. Only much time will tell.

Astronomy and Astrophysics, 2024. DOI: 10.1051/0004-6361/202347957  (About DOIs).

Listing image by ESO/M. Kornmesser

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Polarized light yields fresh insight into mysterious fast radio bursts

astronomy, astrophysics, CHIME, fast radio bursts, Physics, Science / /

CHIME-ing in —

Scientists looked at how polarization changed direction to learn more about origins

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Artist’s rendition of how the angle of polarized light from an FRB changes as it journeys through space.

Enlarge / Artist’s rendition of how the angle of polarized light from a fast radio burst changes as it journeys through space.

CHIME/Dunlap Institute

Astronomers have been puzzling over the origins of mysterious fast radio bursts (FRBs) since the first one was spotted in 2007. Researchers now have their first look at non-repeating FRBs, i.e., those that have only produced a single burst of light to date. The authors of a new paper published in The Astrophysical Journal looked specifically at the properties of polarized light emitting from these FRBs, yielding further insight into the origins of the phenomenon. The analysis supports the hypothesis that there are different origins for repeating and non-repeating FRBs.

“This is a new way to analyze the data we have on FRBs. Instead of just looking at how bright something is, we’re also looking at the angle of the light’s vibrating electromagnetic waves,” said co-author Ayush Pandhi, a graduate student at the University of Toronto’s Dunlap Institute for Astronomy and Astrophysics. “It gives you additional information about how and where that light is produced and what it has passed through on its journey to us over many millions of light years.”

As we’ve reported previously, FRBs involve a sudden blast of radio-frequency radiation that lasts just a few microseconds. Astronomers have over a thousand of them to date; some come from sources that repeatedly emit FRBs, while others seem to burst once and go silent. You can produce this sort of sudden surge of energy by destroying something. But the existence of repeating sources suggests that at least some of them are produced by an object that survives the event. That has led to a focus on compact objects, like neutron stars and black holes—especially a class of neutron stars called magnetars—as likely sources.

There have also been many detected FRBs that don’t seem to repeat at all, suggesting that the conditions that produced them may destroy their source. That’s consistent with a blitzar—a bizarre astronomical event caused by the sudden collapse of an overly massive neutron star. The event is driven by an earlier merger of two neutron stars; this creates an unstable intermediate neutron star, which is kept from collapsing immediately by its rapid spin.

In a blitzar, the strong magnetic fields of the neutron star slow down its spin, causing it to collapse into a black hole several hours after the merger. That collapse suddenly deletes the dynamo powering the magnetic fields, releasing their energy in the form of a fast radio burst.

So the events we’ve been lumping together as FRBs could actually be the product of two different events. The repeating events occur in the environment around a magnetar. The one-shot events are triggered by the death of a highly magnetized neutron star within a few hours of its formation. Astronomers announced the detection of a possible blitzar potentially associated with an FRB last year.

Only about 3 percent of FRBs are of the repeating variety. Per Pandhi, this is the first analysis of the other 97 percent of non-repeating FRBs, using data from Canada’s CHIME instrument (Canadian Hydrogen Intensity Mapping Experiment). CHIME was built for other observations but is sensitive to many of the wavelengths that make up an FRB. Unlike most radio telescopes, which focus on small points in the sky, CHIME scans a huge area, allowing it to pick out FRBs even though they almost never happen in the same place twice.

Pandhi et al. decided to investigate how the direction of the light polarization from 128 non-repeating FRBs changes to learn more about the environments in which they originated. The team found that the polarized light from non-repeating FRBs changes both with time and with different colors of light. They concluded that this particular sample of non-repeating FRBs is either a separate population or more evolved versions of these kinds of FRBs that are part of a population that originated in less extreme environments with lower burst rates. That’s in keeping with the notion that non-repeating FRBs are quite different from their rarer repeating FRBs.

The Astrophysical Journal, 2024. DOI: 10.3847/1538-4357/ad40aa  (About DOIs).

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Radio telescope finds another mystery long-repeat source

astronomy, astrophysics, neutron stars, pulsars, Science, weird stuff / /

File under W for WTF —

Unlike earlier object, the new source’s pulses of radio waves are erratic.

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Image of a purple, glowing sphere with straight purple-white lines emerging from opposite sides, all against a black background.

Enlarge / A slowly rotating neutron star is still our best guess as to the source of the mystery signals.

Roughly a year ago, astronomers announced that they had observed an object that shouldn’t exist. Like a pulsar, it emitted regularly timed bursts of radio emissions. But unlike a pulsar, those bursts were separated by over 20 minutes. If the 22 minute gap between bursts represents the rotation period of the object, then it is rotating too slowly to produce radio emissions by any known mechanism.

Now, some of the same team (along with new collaborators) are back with the discovery of something that, if anything, is acting even more oddly. The new source of radio bursts, ASKAP J193505.1+214841.0, takes nearly an hour between bursts. And it appears to have three different settings, sometimes producing weaker bursts and sometimes skipping them entirely. While the researchers suspect that, like pulsars, this is also powered by a neutron star, it’s not even clear that it’s the same class of object as their earlier discovery.

How pulsars pulse

Contrary to the section heading, pulsars don’t actually pulse. Neutron stars can create the illusion by having magnetic poles that aren’t lined up with their rotational pole. The magnetic poles are a source of constant radio emissions but, as the neutron star rotates, the emissions from the magnetic pole sweep across space in a manner similar to the light from a rotating lighthouse. If Earth happens to be caught up in that sweep, then the neutron star will appear to blink on and off as it rotates.

The star’s rotation is also needed for the generation of radio emissions themselves. If the neutron star rotates too slowly, then its magnetic field won’t be strong enough to produce radio emissions. So, it’s thought that if a pulsar’s rotation slows down enough (causing its pulses to be separated by too much time), it will simply shut down, and we’ll stop observing any radio emissions from the object.

We don’t have a clear idea of how long the time between pulses can get before a pulsar will shut down. But we do know that it’s going to be far less than 22 minutes.

Which is why the 2023 discovery was so strange. The object, GPM J1839–10, not only took a long time between pulses, but archival images showed that it had been pulsing on and off since at least 35 years ago.

To figure out what is going on, we really have two options. One is more and better observations of the source we know about. The second is to find other examples of similar behavior. There’s a chance we now have a second object like this, although there are enough differences that it’s not entirely clear.

An enigmatic find

The object, ASKAPJ193505.1+214841.0, was discovered by accident when the Australian Square Kilometre Array Pathfinder telescope was used to perform observations in the area due to detections of a gamma ray burst. It picked up a bright radio burst in the same field of view, but unrelated to the gamma ray burst. Further radio bursts showed up in later observations, as did a few far weaker bursts. A search of the telescope’s archives also spotted a weaker burst from the same location.

Checking the timing of the radio bursts, the team found that they could be explained by an object that emitted bursts every 54 hours, with bursts lasting from 10 seconds to just under a minute. Checking additional observations, however, showed that there were often instances where a 54 minute period would not end with a radio burst, suggesting the source sometimes skipped radio emissions entirely.

Odder still, the photons in the strong and weak bursts appeared to have different polarizations. These differences arise from the magnetic fields present where the bursts originate, suggesting that the two types of bursts differ not only in total energy, but also that the object that’s making them has a different magnetic field.

So, the researchers suggest that the object has three modes: strong pulses, faint pulses, and an off mode, although they can’t rule out the off mode producing weak radio signals that are below the detection capabilities of the telescopes we’re using. Over about eight months of sporadic observations, there’s no apparent pattern to the bursts.

What is this thing?

Checks at other wavelengths indicate there’s a magnetar and a supernova remnant in the vicinity of the mystery object, but not at the same location. There’s also a nearby brown dwarf at that point in the sky, but they strongly suspect that’s just a chance overlap. So, none of that tells us more about what produces these erratic bursts.

As with the earlier find, there seem to be two possible explanations for the ASKAP source. One is a neutron star that’s still managing to emit radiofrequency radiation from its poles despite rotating extremely slowly. The second is a white dwarf that has a reasonable rotation period but an unreasonably strong magnetic field.

To get at this issue, the researchers estimate the strength of the magnetic field needed to produce the larger bursts and come up with a value that’s significantly higher than any previously observed to originate on a white dwarf. So they strongly argue for the source being a neutron star. Whether that argues for the earlier source being a neutron star will depend on whether you feel that the two objects represent a single phenomenon despite their somewhat different behaviors.

In any case, we now have two of these mystery slow-repeat objects to explain. It’s possible that we’ll be able to learn more about this newer one if we can get some information as to what’s involved in its mode switching. But then we’ll have to figure out if what we learn applies to the one we discovered earlier.

Nature Astronomy, 2024. DOI: 10.1038/s41550-024-02277-w  (About DOIs).

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Nova explosion visible to the naked eye expected any day now

astronomy, astrophysics, nova, Science, white dwarf / /
Image of a blue sphere, surrounded by blue filaments, and enclosed in a partial sphere of pink specks.s

Enlarge / Aftermath of a nova at the star GK Persei.

When you look at the northern sky, you can follow the arm of the Big Dipper as it arcs around toward the bright star called Arcturus. Roughly in the middle of that arc, you’ll find the Northern Crown constellation, which looks a bit like a smiley face. Sometime between now and September, if you look to the left-hand side of the Northern Crown, what will look like a new star will shine for five days or so.

This star system is called T. Coronae Borealis, also known as the Blaze Star, and most of the time, it is way too dim to be visible to the naked eye. But once roughly every 80 years, a violent thermonuclear explosion makes it over 10,000 times brighter. The last time it happened was in 1946, so now it’s our turn to see it.

Neighborhood litterbug

“The T. Coronae Borealis is a binary system. It is actually two stars,” said Gerard Van Belle, the director of science at Lowell Observatory in Flagstaff, Arizona. One of these stars is a white dwarf, an old star that has already been through its fusion-powered lifecycle. “It’s gone from being a main sequence star to being a giant star. And in the case of giant stars, what happens is their outer parts eventually get kind of pushed into outer space. What’s left behind is a leftover core of the star—that’s called a white dwarf,” Van Belle explained.

The white dwarf stage is normally a super peaceful retirement period for stars. The nuclear fusion reaction no longer takes place, which makes white dwarfs very dim. They are still pretty hot, though, and they’re super dense, with a mass comparable to our Sun squeezed into a volume resembling the Earth.

But the retirement of the white dwarf in T. Coronae Borealis is hardly peaceful, as it has a neighbor prone to littering. “Its companion star is in the red giant phase, where it is puffed up. Its outer parts are getting sloughed off and pushed into space. The material that is coming off the red giant is now falling onto the white dwarf,” Van Belle said.

Ticking time bomb

And it doesn’t take much littering to make the white dwarf explode. “The material from the red giant will accumulate on the white dwarf’s surface until it forms a layer that’s actually not that thick. Just a few meters—the depth of a deep swimming pool,” Van Belle explained. Most of the material coming off the red giant is hydrogen. And since the red dwarf is still hot, there will eventually be a spark that triggers a runaway nuclear fusion reaction. “That is what causes the explosion,” Van Belle said.

The explosion is a nova, which means it doesn’t kill either the white dwarf or the red giant as a supernova would. “Only about 5 percent of the hydrogen layer fuses into heavier elements like helium, and the rest just gets ejected into space. Then the process starts all over again because the explosion isn’t large enough to disrupt the red giant, the donor of all this hydrogen, so it just keeps doing its thing,” Van Belle told Ars. This is why we can predict this event with such precision.

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Monster galactic outflow powered by exploding stars

astronomy, astrophysics, galaxy evolution, Science / /

A big burp —

Star death and birth both contribute to driving material out of a galaxy.

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Image of a galaxy showing lots of complicated filaments of gas.

Enlarge / All galaxies have large amounts of gas that influence their star-formation rates.

Galaxies pass gas—in the case of galaxy NGC 4383, so much so that its gas outflow is 20,000 light-years across and more massive than 50 million Suns.

Yet even an outflow of this immensity was difficult to detect until now. Observing what these outflows are made of and how they are structured demands high-resolution instruments that can only see gas from galaxies that are relatively close, so information on them has been limited. Which is unfortunate, since gaseous outflows ejected from galaxies can tell us more about their star formation cycles.

The MAUVE (MUSE and ALMA Unveiling the Virgo Environment) program is now changing things. MAUVE’s mission is to understand how the outflows of galaxies in the Virgo cluster affect star formation. NGC 4383 stood out to astronomer Adam Watts, of the University of Australia and the International Centre for Radio Astronomy Research (ICRAR), and his team because its outflow is so enormous.

The elements it releases into space can reveal the galaxy’s potential to form (or stop forming) stars. “Understanding the physics of stellar feedback-driven outflows… is essential to completing our picture of galaxy evolution,” the researchers said in a study recently published in Monthly Notices of the Royal Astronomical Society.

Star potential

Stellar feedback, which is all the radiation, particle winds, and other materials that stars blast into the interstellar medium, is what forms outflows as huge as that in NGC 4383. Much of this material comes from either bursts of star formation or the insides of massive stars when they die and go supernova. It includes heavier elements that escape into space with the outflow and float there for an indefinite amount of time, sometimes ending up in other galaxies.

Star formation in a galaxy depends on several processes. There has to be the right balance of gas accretion (growth from added gas), consumption (the burning of hydrogen and helium by stars), and ejection (when interstellar gas is blown out of the galaxy) between the intergalactic medium and circumgalactic medium, the gas surrounding galaxies. Some of the gas and other materials, such as iron and other heavy elements, that form stars can be recycled from supernova explosions.

The supply of gas is key because large amounts of gas eventually collapse in on themselves because of their immense gravity, eventually forming stars. A deficit of gas can squelch the formation of potential stars.

Watts and his team think that one source of the stellar feedback pushing star-forming gas out of NGC 4383 is multiple supernovae that occurred relatively close together. Supernovae can form gargantuan bubbles of scorching gas that eventually break out of a galactic disk vertically, extending from the top and bottom of the galaxy.

Hot gas continues into cooler regions of the interstellar medium, with its gravity pulling in more gas on the way out of the galaxy and increasing the total mass of the outflow (known as mass loading). The loss of so much gas decreases the chances of star formation even further.

Lost in space

Outflows can be observed at many different wavelengths. Emissions of X-rays from elements such as hydrogen and compounds such as carbon monoxide can be detected. It is also possible to observe outflows using UV, optical, and infrared. Some of the region’s emissions had already been observed with other telescopes, which was combined with MAUVE imaging of the Virgo Cluster and NGC 4383 at different wavelengths.

The problem with observing outflows accurately is that the scattered materials are notoriously difficult to spatially resolve, which means figuring out the distance of the entire outflow based on pixels. MAUVE, NGC 4383, and the Virgo Cluster were observed at a spatial resolution of about 261 light-years, so each pixel represented a square in space that measured 261 light-years on every side. Clumps of ionized gas that showed up in these pixels told the research team there was a bipolar outflow leaving the galaxy from the top and bottom.

So, does NGC 4383 have reduced star formation because of its massive outflow of star stuff? It turns out that stars are actually forming at the galaxy’s edge. While no stars form in the stream escaping the galaxy, there are still areas where there is enough accreted gas to give birth to them.

These starbursts, or areas of rapid star formation, are also providing stellar feedback—it’s not just supernovae. “There is an extension of blue knots that are much brighter in the near-UV and are clear evidence of star formation occurring outside the main body of the galaxy,” the researchers said in the same study.

Something that remains unclear about NGC 4383 is whether the gas outflow was set off by stellar feedback alone or whether a gravitational interaction with another galaxy intensified existing outflows. There is possibly evidence for this on the eastern side, where a disturbance in the gas suggests that a nearby dwarf galaxy might have interacted with it. For now, the research team is confident that the outflow is primarily driven by starbursts and supernovae.

There is still more that the researchers want to find out about NGC 4383 and its outflow. As telescopes become more advanced and spatial resolution improves, maybe something else will be revealed inside those clouds of gas.

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We may have spotted the first magnetar flare outside our galaxy

astronomy, astrophysics, gamma rays, giant flare, magnetar, neutron star, Science / /

Magnetars: how do they work? —

Not all gamma-ray bursts come from supernovae.

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Image of a whitish smear running diagonally across the frame, with a complex, branching bit of red material in the foreground.

Enlarge / M82, the site of what’s likely to be a giant flare from a magnetar.

NASA, ESA and the Hubble Heritage Team

Gamma rays are a broad category of high-energy photons, including everything with more energy than an X-ray. While they are often created by processes like radioactive decay, few astronomical events produce them in sufficient quantities that they can be detected when the radiation originates in another galaxy.

That said, the list is larger than one, which means detecting gamma rays doesn’t mean we know what event produced them. At lower energies, they can be produced in the areas around black holes and by neutron stars. Supernovae can also produce a sudden burst of gamma rays, as can the merger of compact objects like neutron stars.

And then there are magnetars. These are neutron stars that, at least temporarily, have extreme magnetic fields—over 1012 times stronger than the Sun’s magnetic field. Magnetars can experience flares and even giant flares where they send out copious amounts of energy, including gamma rays. These can be difficult to distinguish from gamma-ray bursts generated by the merger of compact objects, so the only confirmed magnetar giant bursts have happened in our own galaxy or its satellites. Until now, apparently.

What was that?

The burst in question was spotted by the ESA’s Integral gamma-ray observatory, among others, in November 2023. GRB 231115A was short, lasting only about 50 milliseconds at some wavelengths. While longer gamma-ray bursts can be produced by the formation of black holes during supernovae, this short burst is similar to those expected to be seen when neutron stars merge.

The directional data from Integral placed GRB 231115A right on top of a nearby galaxy, M82, which is also known as the Cigar Galaxy. M82 is what is called a starburst galaxy, which means that it’s forming stars at a rapid clip, with the burst likely to have been triggered by interactions with its neighbors. Overall, the galaxy is forming stars at a rate more than 10 times that of the Milky Way. That means lots of supernovae, but it also means a large population of young neutron stars, some of which will form magnetars.

That doesn’t rule out the possibility that M82 happened to be sitting in front of a gamma-ray burst from a distant event. However, the researchers use two different methods to show that this is pretty improbable, which leaves something happening inside the galaxy as being the most likely source of the gamma rays.

It could still be a gamma-ray burst happening within M82, except the estimated total energy of the burst is much lower than we’d expect from those events. A supernova should also be detected at other wavelengths, but there was no sign of one (and they typically produce longer bursts anyway). An alternative source, the fusion of two compact objects such as neutron stars, would have been detectable using our gravitational wave observatories, but no signal was apparent at this time. These events also frequently leave behind X-ray sources, but no new sources are visible in M82.

So, it looks like a magnetar giant flare, and the potential explanations for a brief burst of gamma radiation don’t really work for GRB 231115A.

Looking for more

The exact mechanism by which magnetars produce gamma rays isn’t entirely worked out. It is thought to involve the rearrangement of the crust of the neutron star, forced by the intense forces generated by the staggeringly intense magnetic field. Giant flares are thought to require magnetic field strengths of at least 1015 gauss; Earth’s magnetic field is less than one gauss.

Assuming that the event sent radiation off in all directions rather than directing it toward Earth, the researchers estimate that the total energy released was 1045 ergs, which translates to roughly 1022 megatons of TNT. So, while it’s less energetic than neutron star mergers, it’s still an impressively energetic event.

To understand them better, however, we probably need more than the three instances in our immediate neighborhood that are obviously associated with magnetars. So, being able to consistently identify when these events happen in more distant galaxies would be a big win for astronomers. The results could help us develop a template for distinguishing when we’re looking at a giant flare instead of alternative sources of gamma rays.

The researchers also note that this is the second candidate giant flare associated with M82 and, as mentioned above, starburst galaxies would be expected to be relatively rich in magnetars. Focusing searches on it and similar galaxies might be what we need to boost the frequency of our observations.

Nature, 2024. DOI: 10.1038/s41586-024-07285-4  (About DOIs).

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Second-biggest black hole in the Milky Way found

astronomy, astrophysics, black holes, Gaia, Science, stars / /
A dark background with a bright point at the end of a curved path, and a small red circle.

Enlarge / The star’s orbit, shown here in light, is influenced by the far more massive black hole, indicated by the red orbit.

As far as black holes go, there are two categories: supermassive ones that live at the center of the galaxies (and we’re unsure about how they got there) and stellar mass ones that formed through the supernovae that end the lives of massive stars.

Prior to the advent of gravitational wave detectors, the heaviest stellar-mass black hole we knew about was only a bit more than a dozen times the mass of the Sun. And this makes sense, given that the violence of the supernova explosions that form these black holes ensures that only a fraction of the dying star’s mass gets transferred into its dark offspring. But then the gravitational wave data started flowing in, and we discovered there were lots of heavier black holes, with masses dozens of times that of the Sun. But we could only find them when they smacked into another black hole.

Now, thanks to the Gaia mission, we have observational evidence of the largest black hole in the Milky Way outside of the supermassive one, with a mass 33 times that of the Sun. And, in galactic terms, it’s right next door at about 2,000 light-years distant, meaning it will be relatively easy to learn more.

Mapping the stars

Although stellar-mass black holes are several times the mass of the Sun, they aren’t really all that heavy in the grand scheme of things. The sorts of stars that tend to leave black holes behind also tend to lead violent existences, spewing a lot of themselves into space before dying. And the supernova that forms the black hole obviously expels a lot of the star’s mass, rather than feeding it into the black hole. It had been thought that these processes set limits on how big a stellar mass black hole could be when it forms.

The discovery of larger black holes through gravitational wave detectors suggested that this wasn’t true. While there are ways for black holes to get bigger after they form—excessive feeding, mergers—it wasn’t clear that these events occurred often enough to explain the frequency of heavy black holes that we were seeing. And detecting them via gravitational waves doesn’t tell us anything about the history of how they got that large.

Which is why the discovery of Gaia BH3 (which is what the research team is using to avoid having to retype Gaia DR3 4318465066420528000 all the time) is so intriguing. The black hole is sitting calmly in a binary system, not doing anything in particular. But we know it’s there due to its gravitational influence.

Gaia is an ESA mission to map the location and movement of many of the Milky Way’s brighter stars by imaging them multiple times from different perspectives. It also gathers basic data on the stars’ light, allowing us to estimate things like age and composition. And, in addition to their movement across the galaxy, Gaia can measure their movement relative to Earth, a method that is useful for the detection of orbital interactions, such as the presence of companion stars or exoplanets.

The Gaia team was busy preparing for the fourth release of the data from the spacecraft and were running validation tests on the software used to detect binary star systems when they stumbled across Gaia BH3. While normally they’d publish its discovery at the same time as the data release, they consider the new object too important to wait: “We took the exceptional step of the publication of this paper based on preliminary data ahead of the official DR4 due to the unique nature of the discovery, which we believe should not be kept from the scientific community until the next release.”

Finding the invisible

Every star in our galaxy is in motion relative to every other. They orbit the center of our galaxy and may have a history that has imparted additional momentum—gravitational interactions with neighbors, having been part of a smaller galaxy that was consumed by the Milky Way, and so on. But that motion only changes on very long time scales. By contrast, any star in an orbit experiences regular changes in its motion in addition to its overall travel through the galaxy. As part of processing its data, the Gaia team attempts to identify both overall motion and any indications that a star is orbiting as part of a binary system.

The star that is orbiting Gaia BH3 is similar in mass to the Sun but shows the sort of periodic wobbles that indicate it’s in a mutual orbit with a companion. The companion itself, however, was completely invisible, which means it is almost certainly a black hole (the Gaia data had already been used to identify black holes this way). And, based on the mass and orbital motion of the visible star, it’s possible to estimate the mass of the invisible companion.

The estimate ended up being 32 solar masses, which is significantly larger than anything else identified in the Gaia dataset. So, the Gaia team wanted to confirm this wasn’t a software issue and used Earth-based telescopes to observe the same system. Three different observatories confirmed it was there, and the resulting mass estimates were slightly larger than those derived from the Gaia data alone: just under 33 solar masses.

Assuming it’s a single object and not two black holes orbiting each other closely, that makes it the largest non-supermassive black hole known in the Milky Way. And it places it in the mass range that had been difficult to explain via formations in supernovae.

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