astrophysics

40-years-later,-x-window-system-is-far-more-relevant-than-anyone-could-guess

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.”

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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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A supernova caused the BOAT gamma ray burst, JWST data confirms

astronomy, astrophysics, B.O.A.T., Gamma ray bursts, nucleosynthesis, Physics, Science, Webb telescope / /

Still the BOAT —

But astronomers are puzzled by the lack of signatures of expected heavy elements.

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Artist's visualization of GRB 221009A showing the narrow relativistic jets — emerging from a central black hole — that gave rise to the brightest gamma ray burst yet detected.

Enlarge / Artist’s visualization of GRB 221009A showing the narrow relativistic jets—emerging from a central black hole—that gave rise to the brightest gamma-ray burst yet detected.

Aaron M. Geller/Northwestern/CIERA/ ITRC&DS

In October 2022, several space-based detectors picked up a powerful gamma-ray burst so energetic that astronomers nicknamed it the BOAT (Brightest Of All Time). Now they’ve confirmed that the GRB came from a supernova, according to a new paper published in the journal Nature Astronomy. However, they did not find evidence of heavy elements like platinum and gold one would expect from a supernova explosion, which bears on the longstanding question of the origin of such elements in the universe.

As we’ve 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.

The October 2022 gamma-ray burst falls into the long category, lasting over 300 seconds. GRB 221009A triggered detectors aboard NASA’s Fermi Gamma-ray Space Telescope, the Neil Gehrels Swift Observatory, and Wind spacecraft, among others, just as gamma-ray astronomers had gathered for an annual meeting in Johannesburg, South Africa. The powerful signal came from the constellation Sagitta, traveling some 1.9 billion years to Earth.

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.

Swift’s X-ray Telescope captured the afterglow of GRB 221009A about an hour after it was first detected.

Enlarge / Swift’s X-ray Telescope captured the afterglow of GRB 221009A about an hour after it was first detected.

NASA/Swift/A. Beardmore (University of Leicester)

That’s why Peter Blanchard of Northwestern University and his fellow co-authors decided to wait six months before undertaking their own analysis, relying on data collected during the GRB’s later phase by the Webb Space Telescope’s Near Infrared Spectrograph. They augmented that spectral data with observations from ALMA (Atacama Large Millimeter/Submillimeter Array) in Chile so they could separate light from the supernova and the GRB afterglow. The most significant finding was the telltale signatures of key elements like calcium and oxygen that one would expect to find with a supernova.

Yet the supernova wasn’t brighter than other supernovae associated with less energetic GRBs, which is puzzling. “You might expect that the same collapsing star producing a very energetic and bright GRB would also produce a very energetic and bright supernova,” said Blanchard. “But it turns out that’s not the case. We have this extremely luminous GRB, but a normal supernova.” The authors suggest that this might have something to do with the shape and structure of the relativistic jet, which was much narrower than other GRB jets, resulting in a more focused and brighter beam of light.

The data held another surprise for astronomers. The only confirmed source of heavy elements in the universe to date is the merging of binary neutron stars. But per Blanchard, there are far too few neutron star mergers to account for the abundance of heavy elements, so there must be another source. One hypothetical additional source is a rapidly spinning massive star that collapses and explodes into a supernova. Alas, there was no evidence of heavy elements in the JWST spectral data regarding the BOAT.

“When we confirmed that the GRB was generated by the collapse of a massive star, that gave us the opportunity to test a hypothesis for how some of the heaviest elements in the universe are formed,” said Blanchard. “We did not see signatures of these heavy elements, suggesting that extremely energetic GRBs like the BOAT do not produce these elements. That doesn’t mean that all GRBs do not produce them, but it’s a key piece of information as we continue to understand where these heavy elements come from. Future observations with JWST will determine if the BOAT’s ‘normal’ cousins produce these elements.”

Nature Astronomy, 2024. DOI: 10.1038/s41550-024-02237-4  (About DOIs).

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Gravitational waves reveal “mystery object” merging with a neutron star

astrophysics, gravitational waves, LIGO, LIGO/VIRGO/KAGRA, neutron star merger, neutron stars, Physics, Science / /

mind the gap —

The so-called “mass gap” might be less empty than physicists previously thought.

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Artistic rendition of a black hole merging with a neutron star.

Enlarge / Artistic rendition of a black hole merging with a neutron star. LIGO/VIRGO/KAGRA detected a merger involving a neutron star and what might be a very light black hole falling within the “mass gap” range.

LIGO-India/ Soheb Mandhai

The LIGO/VIRGO/KAGRA collaboration searches the universe for gravitational waves produced by the mergers of black holes and neutron stars. It has now announced the detection of a signal indicating a merger between two compact objects, one of which has an unusual intermediate mass—heavier than a neutron star and lighter than a black hole. The collaboration provided specifics of their analysis of the merger and the “mystery object” in a draft manuscript posted to the physics arXiv, suggesting that the object might be a very low-mass black hole.

LIGO detects gravitational waves via laser interferometry, using high-powered lasers to measure tiny changes in the distance between two objects positioned kilometers apart. LIGO has detectors in Hanford, Washington state, and in Livingston, Louisiana. A third detector in Italy, Advanced VIRGO, came online in 2016. In Japan, KAGRA is the first gravitational-wave detector in Asia and the first to be built underground. Construction began on LIGO-India in 2021, and physicists expect it will turn on sometime after 2025.

To date, the collaboration has detected dozens of merger events since its first Nobel Prize-winning discovery. Early detected mergers involved either two black holes or two neutron stars, but in 2021, LIGO/VIRGO/KAGRA confirmed the detection of two separate “mixed” mergers between black holes and neutron stars.

Most objects involved in the mergers detected by the collaboration fall into two groups: stellar-mass black holes (ranging from a few solar masses to tens of solar masses) and supermassive black holes, like the one in the middle of our Milky Way galaxy (ranging from hundreds of thousands to billions of solar masses). The former are the result of massive stars dying in a core-collapse supernova, while the latter’s formation process remains something of a mystery. The range between the heaviest known neutron star and the lightest known black hole is known as the “mass gap” among scientists.

There have been gravitational wave hints of compact objects falling within the mass gap before. For instance, as reported previously, in 2019, LIGO/VIRGO picked up a gravitational wave signal from a black hole merger dubbed “GW190521,” that produced the most energetic signal detected thus far, showing up in the data as more of a “bang” than the usual “chirp.” Even weirder, the two black holes that merged were locked in an elliptical (rather than circular) orbit, and their axes of spin were tipped far more than usual compared to those orbits. And the new black hole resulting from the merger had an intermediate mass of 142 solar masses—smack in the middle of the mass gap.

Masses in the stellar graveyard.

Enlarge / Masses in the stellar graveyard.

xIGO-Virgo-KAGRA / Aaron Geller / Northwestern

That same year, the collaboration detected another signal, GW 190814, a compact binary merger involving a mystery object that also fell within the mass gap. With no corresponding electromagnetic signal to accompany the gravitational wave signal, astrophysicists were unable to determine whether that object was an unusually heavy neutron star or an especially light black hole. And now we have a new mystery object within the mass gap in a merger event dubbed “GW 230529.”

“While previous evidence for mass-gap objects has been reported both in gravitational and electromagnetic waves, this system is especially exciting because it’s the first gravitational-wave detection of a mass-gap object paired with a neutron star,” said co-author Sylvia Biscoveanu of Northwestern University. “The observation of this system has important implications for both theories of binary evolution and electromagnetic counterparts to compact-object mergers.”

See where this discovery falls within the mass gap.

Enlarge / See where this discovery falls within the mass gap.

Shanika Galaudage / Observatoire de la Côte d’Azur

LIGO/VIRGO/KAGRA started its fourth observing run last spring and soon picked up GW 230529’s signal. Scientists determined that one of the two merging objects had a mass between 1.2 to 2 times the mass of our sun—most likely a neutron star—while the other’s mass fell in the mass-gap range of 2.5 to 4.5 times the mass of our sun. As with GW 190814, there were no accompanying bursts of electromagnetic radiation, so the team wasn’t able to conclusively identify the nature of the more massive mystery object located some 650 million light-years from Earth, but they think it is probably a low-mass black hole. If so, the finding implies an increase in the expected rate of neutron star–black hole mergers with electromagnetic counterparts, per the authors.

“Before we started observing the universe in gravitational waves, the properties of compact objects like black holes and neutron stars were indirectly inferred from electromagnetic observations of systems in our Milky Way,” said co-author Michael Zevin, an astrophysicist at the Adler Planetarium. “The idea of a gap between neutron-star and black-hole masses, an idea that has been around for a quarter of a century, was driven by such electromagnetic observations. GW230529 is an exciting discovery because it hints at this ‘mass gap’ being less empty than astronomers previously thought, which has implications for the supernova explosions that form compact objects and for the potential light shows that ensue when a black hole rips apart a neutron star.”

arXiv, 2024. DOI: 10.48550/arXiv.2404.04248  (About DOIs).

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Astronomers have solved the mystery of why this black hole has the hiccups

astronomy, astrophysics, black holes, Physics, Science / /

David vs. Goliath —

Blame it on a smaller orbiting black hole repeatedly punching through the accretion disk.

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graphic of hiccuping black hole

Enlarge / Scientists have found a large black hole that “hiccups,” giving off plumes of gas.

Jose-Luis Olivares, MIT

In December 2020, astronomers spotted an unusual burst of light in a galaxy roughly 848 million light-years away—a region with a supermassive black hole at the center that had been largely quiet until then. The energy of the burst mysteriously dipped about every 8.5 days before the black hole settled back down, akin to having a case of celestial hiccups.

Now scientists think they’ve figured out the reason for this unusual behavior. The supermassive black hole is orbited by a smaller black hole that periodically punches through the larger object’s accretion disk during its travels, releasing a plume of gas. This suggests that black hole accretion disks might not be as uniform as astronomers thought, according to a new paper published in the journal Science Advances.

Co-author Dheeraj “DJ” Pasham of MIT’s Kavli Institute for Astrophysics and Space research noticed the community alert that went out after the All Sky Automated Survey for SuperNovae (ASAS-SN) detected the flare, dubbed ASASSN-20qc. He was intrigued and still had some allotted time on the X-ray telescope, called NICER (the Neutron star Interior Composition Explorer) on board the International Space Station. He directed the telescope to the galaxy of interest and gathered about four months of data, after which the flare faded.

Pasham noticed a strange pattern as he analyzed that four months’ worth of data. The bursts of energy dipped every 8.5 days in the X-ray regime, much like a star’s brightness can briefly dim whenever an orbiting planet crosses in front. Pasham was puzzled as to what kind of object could cause a similar effect in an entire galaxy. That’s when he stumbled across a theoretical paper by Czech physicists suggesting that it was possible for a supermassive black hole at the center of a galaxy to have an orbiting smaller black hole; they predicted that, under the right circumstances, this could produce just such a periodic effect as Pasham had observed in his X-ray data.

Computer simulation of an intermediate-mass black hole orbiting a supermassive black hole and driving periodic gas plumes that can explain the observations.

Computer simulation of an intermediate-mass black hole orbiting a supermassive black hole and driving periodic gas plumes that can explain the observations.

Petra Sukova, Astronomical Institute of the CAS

“I was super excited about this theory and immediately emailed to say, ‘I think we’re observing exactly what your theory predicted,” Pasham said. They joined forces to run simulations incorporating the data from NICER, and the results supported the theory. The black hole at the galaxy’s center is estimated to have a mass of 50 million suns. Since there was no burst before December 2020, the team thinks there was, at most, just a faint accretion disk around that black hole and a smaller orbiting black hole of between 100 to 10,000 solar masses that eluded detection because of that.

So what changed? Pasham et al. suggest that a nearby star got caught in the gravitational pull of the supermassive black hole in December 2020 and was ripped to shreds, known as a tidal disruption event (TDE). As previously reported, in a TDE, 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. The jets are one way astronomers can indirectly infer the presence of a black hole. Those outflow emissions typically occur soon after the TDE.

That seems to be what happened in the current system to cause the sudden flare in the primary supermassive black hole. Now it had a much brighter accretion disk, so when its smaller black hole partner passed through the disk, larger than usual gas plumes were emitted. As luck would have it, that plume just happened to be pointed in the direction of an observing telescope.

Astronomers have known about so-called “David and Goliath” binary black hole systems for a while, but “this is a different beast,” said Pasham. “It doesn’t fit anything that we know about these systems. We’re seeing evidence of objects going in and through the disk, at different angles, which challenges the traditional picture of a simple gaseous disk around black holes. We think there is a huge population of these systems out there.”

Science Advances, 2024. DOI: 10.1126/sciadv.adj8898  (About DOIs).

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Event Horizon Telescope captures stunning new image of Milky Way’s black hole

astronomy, astrophysics, black holes, Event Horizon Telescope, Physics, Science / /
A new image from the Event Horizon Telescope has revealed powerful magnetic fields spiraling from the edge of a supermassive black hole at the center of the Milky Way, Sagittarius A*.

Enlarge / A new image from the Event Horizon Telescope has revealed powerful magnetic fields spiraling from the edge of a supermassive black hole at the center of the Milky Way, Sagittarius A*.

EHT Collaboration

Physicists have been confident since the1980s that there is a supermassive black hole at the center of the Milky Way galaxy, similar to those thought to be at the center of most spiral and elliptical galaxies. It’s since been dubbed Sagittarius A* (pronounced A-star), or SgrAfor short. The Event Horizon Telescope (EHT) captured the first image of SgrAtwo years ago. Now the collaboration has revealed a new polarized image (above) showcasing the black hole’s swirling magnetic fields. The technical details appear in two new papers published in The Astrophysical Journal Letters. The new image is strikingly similar to another EHT image of a larger supermassive black hole, M87*, so this might be something that all such black holes share.

The only way to “see” a black hole is to image the shadow created by light as it bends in response to the object’s powerful gravitational field. As Ars Science Editor John Timmer reported in 2019, the EHT isn’t a telescope in the traditional sense. Instead, it’s a collection of telescopes scattered around the globe. The EHT is created by interferometry, which uses light in the microwave regime of the electromagnetic spectrum captured at different locations. These recorded images are combined and processed to build an image with a resolution similar to that of a telescope the size of the most distant locations. Interferometry has been used at facilities like ALMA (the Atacama Large Millimeter/submillimeter Array) in northern Chile, where telescopes can be spread across 16 km of desert.

In theory, there’s no upper limit on the size of the array, but to determine which photons originated simultaneously at the source, you need very precise location and timing information on each of the sites. And you still have to gather sufficient photons to see anything at all. So atomic clocks were installed at many of the locations, and exact GPS measurements were built up over time. For the EHT, the large collecting area of ALMA—combined with choosing a wavelength in which supermassive black holes are very bright—ensured sufficient photons.

In 2019, the EHT announced the first direct image taken of a black hole at the center of an elliptical galaxy, Messier 87, located in the constellation of Virgo some 55 million light-years away. This image would have been impossible a mere generation ago, and it was made possible by technological breakthroughs, innovative new algorithms, and (of course) connecting several of the world’s best radio observatories. The image confirmed that the object at the center of M87is indeed a black hole.

In 2021, the EHT collaboration released a new image of M87showing what the black hole looks like in polarized light—a signature of the magnetic fields at the object’s edge—which yielded fresh insight into how black holes gobble up matter and emit powerful jets from their cores. A few months later, the EHT was back with images of the “dark heart” of a radio galaxy known as Centaurus A, enabling the collaboration to pinpoint the location of the supermassive black hole at the galaxy’s center.

SgrAis much smaller but also much closer than M87*. That made it a bit more challenging to capture an equally sharp image because SgrAchanges on time scales of minutes and hours compared to days and weeks for M87*. Physicist Matt Strassler previously compared the feat to “taking a one-second exposure of a tree on a windy day. Things get blurred out, and it can be difficult to determine the true shape of what was captured in the image.”

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