Eruption of mega-magnetic star lights up nearby galaxy

While ESA's satellite INTEGRAL was observing the sky, it spotted a burst of gamma-rays -- high-energy photons -- coming from the nearby galaxy M82. Only a few hours later, ESA's XMM-Newton X-ray space telescope searched for an afterglow from the explosion but found none. An international team, including researchers from the University of Geneva (UNIGE), realised that the burst must have been an extra-galactic flare from a magnetar, a young neutron star with an exceptionally strong magnetic field. The discovery is published in the journal Nature.

On 15 November 2023, ESA's satellite INTEGRAL spotted a sudden explosion from a rare object. For only a tenth of a second, a short burst of energetic gamma-rays appeared in the sky. "The satellite data were received in the INTEGRAL Science Data Centre (ISDC), based on the Ecogia site of the UNIGE Astronomy Department, from where a gamma-ray burst alert was sent out to astronomers worldwide, only 13 seconds after its detection," explains Carlo Ferrigno, senior research associate in the Astronomy Department at UNIGE Faculty of Science, PI of the ISDC and co-author of the publication.

The IBAS (Integral Burst Alert System) software gave an automatic localisation coinciding with the galaxy M82, 12 million light-years away. This alert system was developed and is operated by scientists and engineers from the UNIGE in collaboration with international colleagues.

A curious signal from a nearby galaxy?

"We immediately realised that this was a special alert. Gamma-ray bursts come from far-away and anywhere in the sky, but this burst came from a bright nearby galaxy," explains Sandro Mereghetti of the National Institute for Astrophysics (INAF-IASF) in Milan, Italy, lead author of the publication and contributor of IBAS. The team immediately requested ESA's XMM-Newton space telescope to perform a follow-up observation of the burst's location as soon as possible. If this had been a short gamma-ray burst, caused by two colliding neutron stars, the collision would have created gravitational waves and have an afterglow in X-rays and visible light.

However, XMM-Newton's observations only showed the hot gas and stars in the galaxy. Using ground-based optical telescopes, including the Italian Telescopio Nazionale Galileo and the French Observatoire de Haute-Provence, they also looked for a signal in visible light, starting only a few hours after the explosion, but again did not find anything. With no signal in X-rays and visible light, and no gravitational waves measured by detectors on Earth (LIGO/VIRGO/KAGRA), the most certain explanation is that the signal came from a magnetar.

Magnetars: mega-magnetic stars, recently dead

"When stars more massive than eight times the Sun die, they explode in a supernova that leaves a black hole or neutron star behind. Neutron stars are very compact stellar remnants with more than the mass of the Sun packed into a sphere with the size of the Canton of Geneva. They rotate quickly and have strong magnetic fields." explains Volodymyr Savchenko, senior research associate in the Astronomy Department at UNIGE Faculty of Science, and co-author of the publication. Some young neutron stars have extra strong magnetic fields, more than 10,000 times that of typical neutron stars. These are called magnetars. They emit energy away in flares, and occasionally these flares are gigantic.

However, in the past 50 years of gamma-ray observations, only three giant flares have been identified as coming from magnetars in our galaxy. These outbursts are very strong: one that was detected in December 2004, came from 30,000 light-years from us but was still powerful enough to affect the upper layers of Earth's atmosphere, like the Solar flares, coming from much closer to us, do.

The flare detected by INTEGRAL is the first firm confirmation of a magnetar flare outside of the Milky Way. M82 is a bright galaxy where star formation takes place. In these regions, massive stars are born, live short turbulent lives and leave behind a neutron star. "The discovery of a magnetar in this region confirms that magnetars are likely young neutron stars," adds Volodymyr Savchenko. The search for more magnetars will continue in other extra-galactic star-forming regions, to?understand these extraordinary astronomical objects. If astronomers can find many more, they can start to understand how often these flares happen and how neutron stars lose energy in the process.

INTEGRAL, a key instrument in a race against time

Outbursts of such short duration can only be captured serendipitously when an observatory is already pointing in the right direction. This makes INTEGRAL with its large field of view, more than 3000 times greater than the sky area covered by the Moon, so important for these detections.

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No gamma rays seen coming from nearby supernova

A nearby supernova in 2023 offered astrophysicists an excellent opportunity to test ideas about how these types of explosions boost particles, called cosmic rays, to near light-speed. But surprisingly, NASA's Fermi Gamma-ray Space Telescope detected none of the high-energy gamma-ray light those particles should produce.

On May 18, 2023, a supernova erupted in the nearby Pinwheel galaxy (Messier 101), located about 22 million light-years away in the constellation Ursa Major. The event, named SN 2023ixf, is the most luminous nearby supernova discovered since Fermi launched in 2008.

"Astrophysicists previously estimated that supernovae convert about 10% of their total energy into cosmic ray acceleration," said Guillem Martí-Devesa, a researcher at the University of Trieste in Italy. "But we have never observed this process directly. With the new observations of SN 2023ixf, our calculations result in an energy conversion as low as 1% within a few days after the explosion. This doesn't rule out supernovae as cosmic ray factories, but it does mean we have more to learn about their production."

The paper, led by Martí-Devesa while at the University of Innsbruck in Austria, will appear in a future edition of Astronomy and Astrophysics.

Trillions of trillions of cosmic rays collide with Earth's atmosphere every day. Roughly 90% of them are hydrogen nuclei -- or protons -- and the remainder are electrons or the nuclei of heavier elements.

Scientists have been investigating cosmic ray origins since the early 1900s, but the particles can't be traced back to their sources. Because they're electrically charged, cosmic rays change course as they travel to Earth thanks to magnetic fields they encounter.

"Gamma rays, however, travel directly to us," said Elizabeth Hays, the Fermi project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Cosmic rays produce gamma rays when they interact with matter in their environment. Fermi is the most sensitive gamma-ray telescope in orbit, so when it doesn't detect an expected signal, scientists must explain the absence. Solving that mystery will build a more accurate picture of cosmic ray origins."

Astrophysicists have long suspected supernovae of being top cosmic ray contributors.

These explosions occur when a star at least eight times the Sun's mass runs out of fuel. The core collapses and then rebounds, propelling a shock wave outward through the star. The shock wave accelerates particles, creating cosmic rays. When cosmic rays collide with other matter and light surrounding the star, they generate gamma rays.

Supernovae greatly impact a galaxy's interstellar environment. Their blast waves and expanding cloud of debris may persist for more than 50,000 years. In 2013, Fermi measurements showed that supernova remnants in our own Milky Way galaxy were accelerating cosmic rays, which generated gamma-ray light when they struck interstellar matter. But astronomers say the remnants aren't producing enough high-energy particles to match scientists' measurements on Earth.

One theory proposes that supernovae may accelerate the most energetic cosmic rays in our galaxy in the first few days and weeks after the initial explosion.

But supernovae are rare, occurring only a few times a century in a galaxy like the Milky Way. Out to distances of around 32 million light-years, a supernova occurs, on average, just once a year.

After a month of observations, starting when visible light telescopes first saw SN 2023ixf, Fermi had not detected gamma rays.

"Unfortunately, seeing no gamma rays doesn't mean there are no cosmic rays," said co-author Matthieu Renaud, an astrophysicist at the Montpellier Universe and Particles Laboratory, part of the National Center for Scientific Research in France. "We have to go through all the underlying hypotheses regarding acceleration mechanisms and environmental conditions in order to convert the absence of gamma rays into an upper limit for cosmic ray production."

The researchers propose a few scenarios that may have affected Fermi's ability to see gamma rays from the event, like the way the explosion distributed debris and the density of material surrounding the star.

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Scientists discover the highest energy gamma-rays ever from a pulsar

Scientists using the H.E.S.S. observatory in Namibia have detected the highest energy gamma rays ever from a dead star called a pulsar. The energy of these gamma rays clocked in at 20 tera-electronvolts, or about ten trillion times the energy of visible light. This observation is hard to reconcile with the theory of the production of such pulsed gamma rays, as the international team reports in the journal Nature Astronomy.

Pulsars are the left-over corpses of stars that spectacularly exploded in a supernova. The explosions leave behind a tiny, dead star with a diameter of just some 20 kilometres, rotating extremely fast and endowed with an enormous magnetic field. "These dead stars are almost entirely made up of neutrons and are incredibly dense: a teaspoon of their material has a mass of more than five billion tonnes, or about 900 times the mass of the Great Pyramid of Giza," explains H.E.S.S. scientist Emma de Oña Wilhelmi, a co-author of the publication working at DESY.

Pulsars emit rotating beams of electromagnetic radiation, somewhat like cosmic lighthouses. If their beam sweeps across our solar system, we see flashes of radiation at regular time intervals. These flashes, also called pulses of radiation, can be searched for in different energy bands of the electromagnetic spectrum. Scientists think that the source of this radiation are fast electrons produced and accelerated in the pulsar's magnetosphere, while traveling towards its periphery. The magnetosphere is made up of plasma and electromagnetic fields that surround and co-rotate with the star. "On their outward journey, the electrons acquire energy and release it in the form of the observed radiation beams," says Bronek Rudak from the Nicolaus Copernicus Astronomical Center (CAMK PAN) in Poland, also a co-author.

The Vela pulsar, located in the Southern sky in the constellation Vela (sail of the ship), is the brightest pulsar in the radio band of the electromagnetic spectrum and the brightest persistent source of cosmic gamma rays in the giga-electronvolts (GeV) range. It rotates about eleven times per second. However, above a few GeV, its radiation ends abruptly, presumably because the electrons reach the end of the pulsar's magnetosphere and escape from it.

But this is not the end of the story: using deep observations with H.E.S.S., a new radiation component at even higher energies has now been discovered, with energies of up to tens of tera-electronvolts (TeV). "That is about 200 times more energetic than all radiation ever detected before from this object," says co-author Christo Venter from the North-West University in South Africa. This very high-energy component appears at the same phase intervals as the one observed in the GeV range. However, to attain these energies, the electrons might have to travel even farther than the magnetosphere, yet the rotational emission pattern needs to remain intact.

"This result challenges our previous knowledge of pulsars and requires a rethinking of how these natural accelerators work," says Arache Djannati-Atai from the Astroparticle & Cosmology (APC) laboratory in France, who led the research. "The traditional scheme according to which particles are accelerated along magnetic field lines within or slightly outside the magnetosphere cannot sufficiently explain our observations. Perhaps we are witnessing the acceleration of particles through the so-called magnetic reconnection process beyond the light cylinder, which still somehow preserves the rotational pattern? But even this scenario faces difficulties to explain how such extreme radiation is produced."

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Scientists help discover the highest-energy light coming from the sun

Sometimes, the best place to hide a secret is in broad daylight. Just ask the sun.

"The sun is more surprising than we knew," said Mehr Un Nisa, a postdoctoral research associate at Michigan State University. "We thought we had this star figured out, but that's not the case."

Nisa, who will soon be joining MSU's faculty, is the corresponding author of a new paper in the journal Physical Review Letters that details the discovery of the highest-energy light ever observed from the sun.

The international team behind the discovery also found that this type of light, known as gamma rays, is surprisingly bright. That is, there's more of it than scientists had previously anticipated.

Watching like a HAWC

Although the high-energy light doesn't reach the Earth's surface, these gamma rays create telltale signatures that were detected by Nisa and her colleagues working with the High-Altitude Water Cherenkov Observatory, or HAWC.

Funded by the National Science Foundation and the National Council of Humanities Science and Technology, HAWC is an important part of the story. Unlike other observatories, it works around the clock.

"We now have observational techniques that weren't possible a few years ago," said Nisa, who works in the Department of Physics and Astronomy in the College of Natural Science.

"In this particular energy regime, other ground-based telescopes couldn't look at the sun because they only work at night," she said. "Ours operates 24/7."

In addition to working differently from conventional telescopes, HAWC looks a lot different from the typical telescope.

Rather than a tube outfitted with glass lenses, HAWC uses a network of 300 large water tanks, each filled with about 200 metric tons of water. The network is nestled between two dormant volcano peaks in Mexico, more than 13,000 feet above sea level.

From this vantage point, it can observe the aftermath of gamma rays striking air in the atmosphere. Such collisions create what are called air showers, which are a bit like particle explosions that are imperceptible to the naked eye.

The energy of the original gamma ray is liberated and redistributed amongst new fragments consisting of lower energy particles and light. It's these particles -- and the new particles they create on their way down -- that HAWC can "see."

When the shower particles interact with water in HAWC's tanks, they create what's known as Cherenkov radiation that can be detected with the observatory's instruments.

Nisa and her colleagues began collecting data in 2015. In 2021, the team had accrued enough data to start examining the sun's gamma rays with sufficient scrutiny.

"After looking at six years' worth of data, out popped this excess of gamma rays," Nisa said. "When we first saw it, we were like, 'We definitely messed this up. The sun cannot be this bright at these energies.'"

Making history

The sun gives off a lot of light spanning a range of energies, but some energies are more abundant than others.

For example, through its nuclear reactions, the sun provides a ton of visible light -- that is, the light we see. This form of light carries an energy of about 1 electron volt, which is a handy unit of measure in physics.

The gamma rays that Nisa and her colleagues observed had about 1 trillion electron volts, or 1 tera electron volt, abbreviated 1 TeV. Not only was this energy level surprising, but so was the fact that they were seeing so much of it.

In the 1990s, scientists predicted that the sun could produce gamma rays when high-energy cosmic rays -- particles accelerated by a cosmic powerhouse like a black hole or supernova -- smash into protons in the sun. But, based on what was known about cosmic rays and the sun, the researchers also hypothesized it would be rare to see these gamma rays reach Earth.

At the time, though, there wasn't an instrument capable of detecting such high-energy gamma rays and there wouldn't be for a while. The first observation of gamma rays with energies of more than a billion electron volts came from NASA's Fermi Gamma-ray Space Telescope in 2011.

Over the next several years, the Fermi mission showed that not only could these rays be very energetic, but also that there were about seven times more of them than scientists had originally expected. And it looked like there were gamma rays left to discover at even higher energies.

When a telescope launches into space, there's a limit to how big and powerful its detectors can be. The Fermi telescope's measurements of the sun's gamma rays maxed out around 200 billion electron volts.

Theorists led by John Beacom and Annika Peter, both professors at Ohio State University, encouraged the HAWC Collaboration to take a look.

"They nudged us and said, 'We're not seeing a cutoff. You might be able to see something," Nisa said.

The HAWC Collaboration includes more than 30 institutions across North America, Europe and Asia, and a sizable portion of that is represented in the nearly 100 authors on the new paper. That includes three additional Spartans: graduate student Daniel Salazar-Gallegos, Professor Emeritus James Linnemann and Kirsten Tollefson, a professor of physics and astronomy and associate dean in the Graduate School at MSU.

Now, for the first time, the team has shown that the energies of the sun's rays extend into the TeV range, up to nearly 10 TeV, which does appear to be the maximum, Nisa said.

Currently, the discovery creates more questions than answers. Solar scientists will now scratch their heads over how exactly these gamma rays achieve such high energies and what role the sun's magnetic fields play in this phenomenon, Nisa said.

When it comes to the cosmos, though, that's part of the excitement. It tells us that there was something wrong, missing or perhaps both when it comes to how we understand our nearest and dearest star.

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NASA missions probe game-changing cosmic explosion

On Dec. 11, 2021, NASA's Neil Gehrels Swift Observatory and Fermi Gamma-ray Space Telescope detected a blast of high-energy light from the outskirts of a galaxy around 1 billion light-years away. The event has rattled scientists' understanding of gamma-ray bursts (GRBs), the most powerful events in the universe.

For the last few decades, astronomers have generally divided GRBs into two categories. Long bursts emit gamma rays for two seconds or more and originate from the formation of dense objects like black holes in the centers of massive collapsing stars. Short bursts emit gamma rays for less than two seconds and are caused by mergers of dense objects like neutron stars. Scientists sometimes observe short bursts with a following flare of visible and infrared light called a kilonova.

"This burst, named GRB 211211A, was paradigm-shifting as it is the first long-duration gamma-ray burst traced to a neutron star merger origin," said Jillian Rastinejad, a graduate student at Northwestern University in Evanston, Illinois, who led one team that studied the burst. "The high-energy burst lasted about a minute, and our follow-up observations led to the identification of a kilonova. This discovery has deep implications for how the universe's heavy elements came to be."

A classic short gamma-ray burst begins with two orbiting neutron stars, the crushed remnants of massive stars that exploded as supernovae. As the stars circle ever closer, they strip neutron-rich material from each other. They also generate gravitational waves, or ripples in space-time -- although none were detected from this event.

Eventually the neutron stars collide and merge, creating a cloud of hot debris emitting light across multiple wavelengths. Scientists hypothesize that jets of high-speed particles, launched by the merger, produce the initial gamma-ray flare before they collide with the wreckage. Heat generated by the radioactive decay of elements in the neutron-rich debris likely creates the kilonova's visible and infrared light. This decay results in the production of heavy elements like gold and platinum.

"Many years ago, Neil Gehrels, an astrophysicist and Swift's namesake, suggested that neutron star mergers could produce some long bursts," said Eleonora Troja, an astrophysicist at the University of Rome who led another team that studied the burst. "The kilonova we observed is the proof that connects mergers to these long-duration events, forcing us to rethink how black holes are formed."

Fermi and Swift detected the burst simultaneously, and Swift was able to rapidly identify its location in the constellation Boötes, enabling other facilities to quickly respond with follow-up observations. Their observations have provided the earliest look yet at the first stages of a kilonova.

Many research groups have delved into the observations collected by Swift, Fermi, the Hubble Space Telescope, and others. Some have suggested the burst's oddities could be explained by the merger of a neutron star with another massive object, like a black hole. The event was also relatively nearby, by gamma-ray burst standards, which may have allowed telescopes to catch the kilonova's fainter light. Perhaps some distant long bursts could also produce kilonovae, but we haven't been able to see them.

The light following the burst, called the afterglow emission, also exhibited unusual features. Fermi detected high-energy gamma rays starting 1.5 hours post-burst and lasting more than 2 hours. These gamma rays reached energies of up to 1 billion electron volts. (Visible light's energy measures between about 2 and 3 electron volts, for comparison.)

"This is the first time we've seen such an excess of high-energy gamma rays in the afterglow of a merger event. Normally that emission decreases over time," said Alessio Mei, a doctoral candidate at the Gran Sasso Science Institute in L'Aquila, Italy, who led a group that studied the data. "It's possible these high-energy gamma rays come from collisions between visible light from the kilonova and electrons in particle jets. The jets could be weakening ones from the original explosion or new ones powered by the resulting black hole or magnetar."

Scientists think neutron star mergers are a major source of the universe's heavy elements. They based their estimates on the rate of short bursts thought to occur across the cosmos. Now they'll need to factor long bursts into their calculations as well.

A team led by Benjamin Gompertz, an astrophysicist at the University of Birmingham in the United Kingdom, looked at the entire high-energy light curve, or the evolution of the event's brightness over time. The scientists noted features that might provide a key for identifying similar incidents -- long bursts from mergers -- in the future, even ones that are dimmer or more distant. The more astronomers can find, the more they can refine their understanding of this new class of phenomena.

On Dec. 7, 2022, papers led by Rastinejad, Troja, and Mei were published in the scientific journal Nature, and a paper led by Gompertz was published in Nature Astronomy.

"This result underscores the importance of our missions working together and with others to provide multiwavelength follow up of these kinds of phenomenon," said Regina Caputo, Swift project scientist, at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Similar coordinated efforts have hinted that some supernovae might produce short bursts, but this event is the final nail in the coffin for the simple dichotomy we've used for years. You never know when you might find something surprising."

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Meteorites plus gamma rays could have given Earth the building blocks for life

Even as detailed images of distant galaxies from the James Webb Space Telescope show us more of the greater universe, scientists still disagree about how life began here on Earth. One hypothesis is that meteorites delivered amino acids -- life's building blocks -- to our planet. Now, researchers reporting in ACS Central Science have experimentally shown that amino acids could have formed in these early meteorites from reactions driven by gamma rays produced inside the space rocks.

Ever since Earth was a newly formed, sterile planet, meteorites have been hurtling through the atmosphere at high speeds toward its surface. If the initial space debris had included carbonaceous chondrites -- a class of meteorite whose members contain significant amounts of water and small molecules, such as amino acids -- then it could have contributed to the evolution of life on Earth. However, the source of amino acids in meteorites has been hard to pinpoint. In previous lab experiments, Yoko Kebukawa and colleagues showed that reactions between simple molecules, such as ammonia and formaldehyde, can synthesize amino acids and other macromolecules, but liquid water and heat are required. Radioactive elements, such as aluminum-26 (26Al) -- which is known to have existed in early carbonaceous chondrites -- release gamma rays, a form of high-energy radiation, when they decay. This process could have provided the heat needed to make biomolecules. So, Kebukawa and a new team wanted to see whether radiation could have contributed to the formation of amino acids in early meteorites.

The researchers dissolved formaldehyde and ammonia in water, sealed the solution in glass tubes and then irradiated the tubes with high-energy gamma rays produced from the decay of cobalt-60. They found that the production of α-amino acids, such as alanine, glycine, α-aminobutyric acid and glutamic acid, and β-amino acids, such as β-alanine and β-aminoisobutyric acid, rose in the irradiated solutions as the total gamma-ray dose increased. Based on these results and the expected gamma ray dose from the decay of 26Al in meteorites, the researchers estimated that it would have taken between 1,000 and 100,000 years to produce the amount of alanine and β-alanine found in the Murchison meteorite, which landed in Australia in 1969. This study provides evidence that gamma ray-catalyzed reactions can produce amino acids, possibly contributing to the origin of life on Earth, the researchers say.

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Short gamma-ray bursts traced farther into distant universe

A Northwestern University-led team of astronomers has developed the most extensive inventory to date of the galaxies where short gamma-ray bursts (SGRBs) originate.

Using several highly sensitive instruments and sophisticated galaxy modeling, the researchers pinpointed the galactic homes of 84 SGRBs and probed the characteristics of 69 of the identified host galaxies. Among their findings, they discovered that about 85% of the studied SGRBs come from young, actively star-forming galaxies.

The astronomers also found that more SGRBs occurred at earlier times, when the universe was much younger -- and with greater distances from their host galaxies' centers -- than previously known. Surprisingly, several SGRBs were spotted far outside their host galaxies -- as if they were "kicked out," a finding that raises questions as to how they were able to travel so far away.

"This is the largest catalog of SGRB host galaxies to ever exist, so weexpect it to be the gold standard for many years to come," said Anya Nugent, a Northwestern graduate student who led the study focused on modeling host galaxies. "Building this catalog and finally having enough host galaxies to see patterns and draw significant conclusions is exactly what the field needed to push our understanding of these fantastic events and what happens to stars after they die."

The team will publish two papers, detailing the new catalog. Both papers will publish on Monday, Nov. 21 in The Astrophysical Journal. Because SGRBs are among the brightest explosions in the universe, the team calls its catalog BRIGHT (Broadband Repository for Investigating Gamma-ray burst Host Traits). All of BRIGHT's data and modeling products are publicly available online for community use.

Nugent is a graduate student in physics and astronomy at Northwestern's Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). She is advised by Wen-fai Fong, an assistant professor of physics and astronomy at Weinberg and a key member of CIERA, who led a second study focused on SGRB host observations.

Benchmark for future comparisons

When two neutron stars collide, they generate momentary flashes of intense gamma-ray light, known as SGRBs. While the gamma rays last mere seconds, the optical light can continue for hours before fading below detection levels (an event called an afterglow). SGRBs are some of the most luminous explosions in the universe with, at most, a dozen detected and pinpointed each year. They currently represent the only way to study and understand a large population of merging neutron star systems.

Since NASA's Neil Gehrels Swift Observatory first discovered an SGRB afterglow in 2005, astronomers have spent the last 17 years trying to understand which galaxies produce these powerful bursts. Stars within a galaxy can give insight into the environmental conditions needed to produce SGRBs and can connect the mysterious bursts to their neutron-star merger origins. So far, only one SGRB (GRB 170817A) has a confirmed neutron-star merger origin -- as it was detected just seconds after gravitational wave detectors observed the binary neutron-star merger (GW170817).

"In a decade, the next generation of gravitational wave observatories will be able to detect neutron star mergers out to the same distances as we do SGRBs today," Fong said. "Thus, our catalog will serve as a benchmark for comparison to future detections of neutron star mergers."

"The catalog can really make impacts beyond just a single class of transients like SGRBs," said Yuxin "Vic" Dong, study co-author and astrophysics Ph.D. student at Northwestern. "With the wealth of data and results presented in the catalog, I believe a variety of research projects will make use of it, maybe even in ways we have yet not thought of."

Insight into neutron-star systems

To create the catalog, the researchers used several highly sensitive instruments at W.M. Keck Observatory, the Gemini Observatories, the MMT Observatory, the Large Binocular Telescope Observatory and the Magellan Telescopes at Las Campanas Observatory to capture deep imaging and spectroscopy of some of the faintest galaxies identified in the survey of SGRB hosts. The team also used data from two of NASA's Great Observatories, the Hubble Space Telescope and Spitzer Space Telescope.

Prior to these new studies, astronomers characterized host galaxies from only a couple dozen SGRBs. The new catalog is quadruple the number of existing samples. With the advantage of a much larger dataset, the catalog shows that SGRB host galaxies can be either young and star-forming or old and approaching death. This means neutron-star systems form in a broad range of environments and many of them have quick formation-to-merger timescales. Because neutron-star mergers create heavy elements like gold and platinum, the catalog's data also will deepen scientists' understanding of when precious metals were first created in the universe.

"We suspect that the younger SGRBs we found in younger host galaxies come from binary stellar systems that formed in a star formation 'burst' and are so tightly bound that they can merge very fast," Nugent said. "Long-standing theories have suggested there must be ways to merge neutron stars quickly, but, until now, we have not been able to witness them. We find evidence for older SGRBs in the galaxies that are much older and believe the stars in those galaxies either took a longer time to form a binary or were a binary system that was further separated. Hence, those took longer to merge."

Potential of JWST

With the ability to detect the faintest host galaxies from very early times in the universe, NASA's new infrared flagship observatory, the James Webb Space Telescope (JWST), is poised to further advance the understanding of neutron star mergers and how far back in time they began.

"I'm most excited about the possibility of using JWST to probe deeper into the homes of these rare, explosive events," Nugent said. "JWST's ability to observe faint galaxies in the universe could uncover more SGRB host galaxies that are currently evading detection, perhaps even revealing a missing population and a link to the early universe."

"I started observations for this project 10 years ago, and it was so gratifying to be able to pass the torch onto the next generation of researchers," Fong said. "It is one of my career's greatest joys to see years of work come to life in this catalog, thanks to the young researchers who really took this study to the next level."

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Out with a bang: Explosive neutron star merger captured for the first time in millimeter light

Scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) -- an international observatory co-operated by the US National Science Foundation's National Radio Astronomy Observatory (NRAO) -- have for the first time recorded millimeter-wavelength light from a fiery explosion caused by the merger of a neutron star with another star. The team also confirmed this flash of light to be one of the most energetic short-duration gamma-ray bursts ever observed, leaving behind one of the most luminous afterglows on record. The results of the research will be published in an upcoming edition of The Astrophysical Journal Letters.

Gamma-ray bursts (GRBs) are the brightest and most energetic explosions in the Universe, capable of emitting more energy in a matter of seconds than our Sun will emit during its entire lifetime. GRB 211106A belongs to a GRB sub-class known as short-duration gamma-ray bursts. These explosions -- which scientists believe are responsible for the creation of the heaviest elements in the Universe, such as platinum and gold -- result from the catastrophic merger of binary star systems containing a neutron star. "These mergers occur because of gravitational wave radiation that removes energy from the orbit of the binary stars, causing the stars to spiral in toward each other," said Tanmoy Laskar, who will soon commence work as an Assistant Professor of Physics and Astronomy at the University of Utah. "The resulting explosion is accompanied by jets moving at close to the speed of light. When one of these jets is pointed at Earth, we observe a short pulse of gamma-ray radiation or a short-duration GRB."

A short-duration GRB usually lasts only a few tenths of a second. Scientists then look for an afterglow, an emission of light caused by the interaction of the jets with surrounding gas. Even still, they're difficult to detect; only half-a-dozen short-duration GRBs have been detected at radio wavelengths, and until now none had been detected in millimeter wavelengths. Laskar, who led the research while an Excellence Fellow at Radboud University in The Netherlands, said that the difficulty is the immense distance to GRBs, and the technological capabilities of telescopes. "Short-duration GRB afterglows are very luminous and energetic. But these explosions take place in distant galaxies which means the light from them can be quite faint for our telescopes on Earth. Before ALMA, millimeter telescopes were not sensitive enough to detect these afterglows."

At roughly 20 billion light-years from Earth, GRB 211106A is no exception. The light from this short-duration gamma-ray burst was so faint that while early X-ray observations with NASA's Neil Gehrels Swift Observatory saw the explosion, the host galaxy was undetectable at that wavelength, and scientists weren't able to determine exactly where the explosion was coming from. "Afterglow light is essential for figuring out which galaxy a burst comes from and for learning more about the burst itself. Initially, when only the X-ray counterpart had been discovered, astronomers thought that this burst might be coming from a nearby galaxy," said Laskar, adding that a significant amount of dust in the area also obscured the object from detection in optical observations with the Hubble Space Telescope.

Each wavelength added a new dimension to scientists' understanding of the GRB, and millimeter, in particular, was critical to uncovering the truth about the burst. "The Hubble observations revealed an unchanging field of galaxies. ALMA's unparalleled sensitivity allowed us to pinpoint the location of the GRB in that field with more precision, and it turned out to be in another faint galaxy, which is further away. That, in turn, means that this short-duration gamma-ray burst is even more powerful than we first thought, making it one of the most luminous and energetic on record," said Laskar.

Wen-fai Fong, an Assistant Professor of Physics and Astronomy at Northwestern University added, "This short gamma-ray burst was the first time we tried to observe such an event with ALMA. Afterglows for short bursts are very difficult to come by, so it was spectacular to catch this event shining so bright. After many years of observing these bursts, this surprising discovery opens up a new area of study, as it motivates us to observe many more of these with ALMA, and other telescope arrays, in the future."

Joe Pesce, National Science Foundation Program Officer for NRAO/ALMA said, "These observations are fantastic on many levels. They provide more information to help us understand the enigmatic gamma-ray bursts (and neutron-star astrophysics in general), and they demonstrate how important and complementary multi-wavelength observations with space- and ground-based telescopes are in understanding astrophysical phenomena."

And there's plenty of work still to be done across multiple wavelengths, both with new GRBs and with GRB 211106A, which could uncover additional surprises about these bursts. "The study of short-duration GRBs requires the rapid coordination of telescopes around the world and in space, operating at all wavelengths," said Edo Berger, Professor of Astronomy at Harvard University. "In the case of GRB 211106A, we used some of the most powerful telescopes available -- ALMA, the National Science Foundation's Karl G. Jansky Very Large Array (VLA), NASA's Chandra X-ray Observatory, and the Hubble Space Telescope. With the now-operational James Webb Space Telescope (JWST), and future 20-40 meter optical and radio telescopes such as the next generation VLA (ngVLA) we will be able to produce a complete picture of these cataclysmic events and study them at unprecedented distances."

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Falling stardust, wobbly jets explain blinking gamma ray bursts

A Northwestern University-led team of astrophysicists has developed the first-ever full 3D simulation of an entire evolution of a jet formed by a collapsing star, or a "collapsar."

Because these jets generate gamma ray bursts (GRBs) -- the most energetic and luminous events in the universe since the Big Bang -- the simulations have shed light on these peculiar, intense bursts of light. Their new findings include an explanation for the longstanding question of why GRBs are mysteriously punctuated by quiet moments -- blinking between powerful emissions and an eerily quiet stillness. The new simulation also shows that GRBs are even rarer than previously thought.

The new study will be published on June 29 in Astrophysical Journal Letters. It marks the first full 3D simulation of the entire evolution of a jet -- from its birth near the black hole to its emission after escaping from the collapsing star. The new model also is the highest-ever resolution simulation of a large-scale jet.

"These jets are the most powerful events in the universe," said Northwestern's Ore Gottlieb, who led the study. "Previous studies have tried to understand how they work, but those studies were limited by computational power and had to include many assumptions. We were able to model the entire evolution of the jet from the very beginning -- from its birth by a black hole -- without assuming anything about the jet's structure. We followed the jet from the black hole all the way to the emission site and found processes that have been overlooked in previous studies."

Gottlieb is a Rothschild Fellow in Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). He coauthored the paper with CIERA member Sasha Tchekhovskoy, an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences.

Weird wobbling

The most luminous phenomenon in the universe, GRBs emerge when the core of a massive star collapses under its own gravity to form a black hole. As gas falls into the rotating black hole, it energizes -- launching a jet into the collapsing star. The jet punches the star until finally escaping from it, accelerating at speeds close to the speed of light. After breaking free from the star, the jet generates a bright GRB.

"The jet generates a GRB when it reaches about 30 times the size of the star -- or a million times the size of the black hole," Gottlieb said. "In other words, if the black hole is the size of a beach ball, the jet needs to expand over the entire size of France before it can produce a GRB."

Due to the enormity of this scale, previous simulations have been unable to model the full evolution of the jet's birth and subsequent journey. Using assumptions, all previous studies found that the jet propagates along one axis and never deviates from that axis.

But Gottlieb's simulation showed something very different. As the star collapses into a black hole, material from that star falls onto the disk of magnetized gas that swirls around the black hole. The falling material causes the disk to tilt, which, in turn, tilts the jet. As the jet struggles to realign with its original trajectory, it wobbles inside the collapsar.

This wobbling provides a new explanation for why GRBs blink. During the quiet moments, the jet doesn't stop -- its emission beams away from Earth, so telescopes simply cannot observe it.

"Emission from GRBs is always irregular," Gottlieb said. "We see spikes in emission and then a quiescent time that lasts for a few seconds or more. The entire duration of a GRB is about one minute, so these quiescent times are a non-negligible fraction of the total duration. Previous models were not able to explain where these quiescent times were coming from. This wobbling naturally gives an explanation to that phenomenon. We observe the jet when its pointing at us. But when the jet wobbles to point away from us, we cannot see its emission. This is part of Einstein's theory of relativity."

Rare becomes rarer

These wobbly jets also provide new insights into the rate and nature of GRBs. Although previous studies estimated that about 1% of collapsars produce GRBs, Gottlieb believes that GRBs are actually much rarer.

If the jet were constrained to moving along one axis, then it would only cover a thin slice of the sky -- limiting the likelihood of observing it. But the wobbly nature of the jet means that astrophysicists can observe GRBs at different orientations, increasing the likelihood of spotting them. According to Gottlieb's calculations, GRBs are 10 times more observable than previously thought, which means that astrophysicists are missing 10 times fewer GRBs than previously thought.

"The idea is that we observe GRBs on the sky in a certain rate, and we want to learn about the true rate of GRBs in the universe," Gottlieb explained. "The observed and true rates are different because we can only see the GRBs that are pointing at us. That means we need to assume something about the angle that these jets cover on the sky, in order to infer the true rate of GRBs. That is, what fraction of GRBs we are missing. Wobbling increases the number of detectable GRBs, so the correction from the observed to true rate is smaller. If we miss fewer GRBs, then there are fewer GRBs overall in the sky."

If this is true, Gottlieb posits, then most of the jets either fail to be launched at all or never succeed in escaping from the collapsar to produce a GRB. Instead, they remain buried inside.

Mixed energy

The new simulations also revealed that some of the magnetic energy in the jets partially converts to thermal energy. This suggests that the jet has a hybrid composition of magnetic and thermal energies, which produce the GRB. In a major step forward in understanding the mechanisms that power GRBs, this is the first time researchers have inferred the jet composition of GRBs at the time of emission.

"Studying jets enables us to 'see' what happens deep inside the star as it collapses," Gottlieb said. "Otherwise, it's difficult to learn what happens in a collapsed star because light cannot escape from the stellar interior. But we can learn from the jet emission -- the history of the jet and the information that it carries from the systems that launch them."

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Spinning stars shed new light on strange signal coming from galactic center

Researchers from The Australian National University (ANU) have found an alternative explanation for a mysterious gamma-ray signal coming from the centre of the galaxy, which was long claimed as a signature of dark matter.

Gamma-rays are the form of electromagnetic radiation with the shortest wavelength and highest energy.

Co-author of the study Associate Professor Roland Crocker said this particular gamma-ray signal -- known as the Galactic Centre Excess -- may actually come from a specific type of rapidly-rotating neutron star, the super-dense stellar remnants of some stars much more massive than our sun.

The Galactic Centre Excess is an unexpected concentration of gamma-rays emerging from the centre of our galaxy that has long puzzled astronomers.

"Our work does not throw any doubt on the existence of the signal, but offers another potential source," Associate Professor Crocker said.

"It is based on millisecond pulsars -- neutron stars that spin really quickly -- around 100 times a second.

"Scientists have previously detected gamma-ray emissions from individual millisecond pulsars in the neighbourhood of the solar system, so we know these objects emit gamma-rays. Our model demonstrates that the integrated emission from a whole population of such stars, around 100,000 in number, would produce a signal entirely compatible with the Galactic Centre Excess."

The discovery may mean scientists have to re-think where they look for clues about dark matter.

"The nature of dark matter is entirely unknown, so any potential clues garner a lot of excitement," Associate Professor Crocker said.

"But our results point to another important source of gamma-ray production.

"For instance, the gamma-ray signal from Andromeda, the next closest large galaxy to our own may be mostly due to millisecond pulsars."

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Nova outbursts are apparently a source for cosmic rays

Light on, light off -- this is how one could describe the behavior of the nova, which goes by the name RS Ophiuchi (RS Oph). Every 15 years or so, a dramatic explosion occurs in the constellation of the Serpent Bearer. Birthplaces of a nova are systems in which two very different stars live in a parasitic relationship: A white dwarf, a small, burned-out and tremendously dense star -- a teaspoon of its matter weighs about 1 ton -- orbits a red giant, an old star that will soon burn up.

The dying giant star feeds the white dwarf with matter shedding its outer hydrogen layer as the gas flows onto the nearby white dwarf. This flow of matter continues, until the white dwarf over(h)eats itself. The temperature and pressure in the newly gained stellar shells become too large and are flung away in a gigantic thermonuclear explosion. The dwarf star remains intact and the cycle begins again -- until the spectacle repeats itself.

Explosion in the high-energy range

It had been speculated that such explosions involve high energies. The two MAGIC telescopes recorded gamma rays with the value of 250 gigaelectronvolts (GeV), among the highest energies ever measured in a nova. By comparison, the radiation is a hundred billion times more energetic than visible light.

MAGIC was able to make its observations following initial alerts from other instruments measuring at different wavelengths. "The spectacular eruption of the RS Ophiuchi shows that the MAGIC telescopes' fast response really pays off: It takes them no more than 30 seconds to move to a new target," said David Green, a scientist at the Max Planck Institute for Physics and one of the authors of the paper.

Accelerated protons as a part of cosmic rays

After the explosion, several shock fronts propagated through the stellar wind from the Red Giant and the interstellar medium surrounding the binary system. These shock waves work like a giant power plant in which particles are accelerated to near the speed of light. The combined measurements suggest that the gamma rays emanate from energetic protons, nuclei of hydrogen atoms.

"This also makes nova outbursts a source of cosmic rays," explains David Green. "However, they tend to play the role of local heroes -- meaning to only contribute to the cosmic rays in the close neighborhood. The big players for cosmic rays are supernova remnants. The shock fronts created from stellar explosions are far more violent compared to novae.

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Did rapid spin delay 2017 collapse of merged neutron stars into black hole?

When two neutron stars spiral into one another and merge to form a black hole -- an event recorded in 2017 by gravitational wave detectors and telescopes worldwide -- does it immediately become a black hole? Or does it take a while to spin down before gravitationally collapsing past the event horizon into a black hole?

Ongoing observations of that 2017 merger by the Chandra X-ray Observatory, an orbiting telescope, suggests the latter: that the merged object stuck around, likely for a mere second, before undergoing ultimate collapse.

The evidence is in the form of an X-ray afterglow from the merger, dubbed GW170817, that would not be expected if the merged neutron stars collapsed immediately to a black hole. The afterglow can be explained as a rebound of material off the merged neutron stars, which plowed through and heated the material around the binary neutron stars. This hot material has now kept the remnant glowing steadily more than four years after the merger threw material outward in what's referred to as a kilonova. X-ray emissions from a jet of material that was detected by Chandra shortly after the merger would otherwise be dimming by now.

While the excess X-ray emissions observed by Chandra could come from debris in an accretion disk swirling around and eventually falling into the black hole, astrophysicist Raffaella Margutti of the University of California, Berkeley, favors the delayed collapse hypothesis, which is predicted theoretically.

"If the merged neutron stars were to collapse directly to a black hole with no intermediate stage, it would be very hard to explain this X-ray excess that we see right now, because there would be no hard surface for stuff to bounce off and fly out at high velocities to create this afterglow," said Margutti, UC Berkeley associate professor of astronomy and of physics. "It would just fall in. Done. The true reason why I'm excited scientifically is the possibility that we are seeing something more than the jet. We might finally get some information about the new compact object."

Margutti and her colleagues, including first author Aprajita Hajela, who was Margutti's graduate student when she was at Northwestern University before moving to UC Berkeley, report their analysis of the X-ray afterglow in a paper recently accepted for publication in The Astrophysical Journal Letters.

The radioactive glow of a kilonova

Gravitational waves from the merger were first detected on Aug. 17, 2017, by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo collaboration. Satellite- and ground-based telescopes quickly followed up to record a burst of gamma rays and visible and infrared emissions that together confirmed the theory that many heavy elements are produced in the aftermath of such mergers inside hot ejecta that produces a bright kilonova. The kilonova glows because of light emitted during the decay of radioactive elements, like platinum and gold, that are produced in the merger debris.

Chandra, too, pivoted to observe GW170817, but saw no X-rays until nine days later, suggesting that the merger also produced a narrow jet of material that, upon colliding with the material around the neutron stars, emitted a cone of X-rays that initially missed Earth. Only later did the head of the jet expand and begin emitting X-rays in a broader jet visible from Earth.

The X-ray emissions from the jet increased for 160 days after the merger, after which they steadily grew fainter as the jet slowed down and expanded. But Hajela and her team noticed that from March 2020 -- about 900 days after the merger -- until the end of 2020, the decline stopped, and the X-ray emissions remained approximately constant in brightness.

"The fact that the X-rays stopped fading quickly was our best evidence yet that something in addition to a jet is being detected in X-rays in this source," Margutti said. "A completely different source of X-rays appears to be needed to explain what we're seeing."

The researchers suggest that the excess X-rays are produced by a shock wave distinct from the jets produced by the merger. This shock was a result of the delayed collapse of the merged neutron stars, likely because its rapid spin very briefly counteracted the gravitational collapse. By sticking around for an extra second, the material around the neutron stars got an extra bounce that produced a very fast tail of kilonova ejecta that created the shock.

"We think the kilonova afterglow emission is produced by shocked material in the circumbinary medium," Margutti said. "It is material that was in the environment of the two neutron stars that was shocked and heated up by the fastest edge of the kilonova ejecta, which is driving the shock wave."

The radiation is reaching us only now because it took time for the heavy kilonova ejecta to be decelerated in the low-density environment and for the kinetic energy of the ejecta to be converted into heat by shocks, she said. This is the same process that produces radio and X-rays for the jet, but because the jet is much, much lighter, it is immediately decelerated by the environment and shines in the X-ray and radio from the very earliest times.

An alternative explanation, the researchers note, is that the X-rays come from material falling towards the black hole that formed after the neutron stars merged.

"This would either be the first time we've seen a kilonova afterglow or the first time we've seen material falling onto a black hole after a neutron star merger," said co-author Joe Bright, a UC Berkeley postdoctoral researcher. "Either outcome would be extremely exciting."

Chandra is now the only observatory still able to detect light from this cosmic collision. Follow-up observations by Chandra and radio telescopes could distinguish between the alternative explanations, however. If it is a kilonova afterglow, radio emission is expected to be detected again in the next few months or years. If the X-rays are being produced by matter falling onto a newly formed black hole, then the X-ray output should stay steady or decline rapidly, and no radio emission will be detected over time.

Margutti hopes that LIGO, Virgo and other telescopes will capture gravitational waves and electromagnetic waves from more neutron star mergers so that the series of events preceding and following the merger can be pinned down more precisely and help reveal the physics of black hole formation. Until then, GW170817 is the only example available for study.

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Cosmic 'spider' found to be source of powerful gamma-rays

Using the 4.1-meter SOAR Telescope in Chile, astronomers have discovered the first example of a binary system where a star in the process of becoming a white dwarf is orbiting a neutron star that has just finished turning into a rapidly spinning pulsar. The pair, originally detected by the Fermi Gamma-ray Space Telescope, is a "missing link" in the evolution of such binary systems.

A bright, mysterious source of gamma rays has been found to be a rapidly spinning neutron star -- dubbed a millisecond pulsar -- that is orbiting a star in the process of evolving into an extremely-low-mass white dwarf. These types of binary systems are referred to by astronomers as "spiders" because the pulsar tends to "eat" the outer parts of the companion star as it turns into a white dwarf.

The duo was detected by astronomers using the 4.1-meter SOAR Telescope on Cerro Pachón in Chile, part of Cerro Tololo Inter-American Observatory (CTIO), a Program of NSF's NOIRLab.

NASA's Fermi Gamma-ray Space Telescope has been cataloging objects in the Universe that produce copious gamma rays since its launch in 2008, but not all of the sources of gamma rays that it detects have been classified. One such source, called 4FGL J1120.0-2204 by astronomers, was the second brightest gamma-ray source in the entire sky that had gone unidentified, until now.

Astronomers from the United States and Canada, led by Samuel Swihart of the US Naval Research Laboratory in Washington, D.C., used the Goodman Spectrograph on the SOAR Telescope to determine the true identity of 4FGL J1120.0-2204. The gamma-ray source, which also emits X-rays, as observed by NASA's Swift and ESA's XMM-Newton space telescopes, has been shown to be a binary system consisting of a "millisecond pulsar" that spins hundreds of times per second, and the precursor to an extremely-low-mass white dwarf. The pair are located over 2600 light-years away.

"Michigan State University's dedicated time on the SOAR Telescope, its location in the southern hemisphere and the precision and stability of the Goodman spectrograph, were all important aspects of this discovery," says Swihart.

"This is a great example of how mid-sized telescopes in general, and SOAR in particular, can be used to help characterize unusual discoveries made with other ground and space-based facilities," notes Chris Davis, NOIRLab Program Director at US National Science Foundation. "We anticipate that SOAR will play a crucial role in the follow-up of many other time-variable and multi-messenger sources over the coming decade."

The optical spectrum of the binary system measured by the Goodman spectrograph showed that light from the proto-white dwarf companion is Doppler shifted -- alternately shifted to the red and the blue -- indicating that it orbits a compact, massive neutron star every 15 hours.

"The spectra also allowed us to constrain the approximate temperature and surface gravity of the companion star," says Swihart, whose team was able to take these properties and apply them to models describing how binary star systems evolve. This allowed them to determine that the companion is the precursor to an extremely-low-mass white dwarf, with a surface temperature of 8200 °C (15,000 °F), and a mass of just 17% that of the Sun.

When a star with a mass similar to that of the Sun or less reaches the end of its life, it will run out of the hydrogen used to fuel the nuclear fusion processes in its core. For a time, helium takes over and powers the star, causing it to contract and heat up, and prompting its expansion and evolution into a red giant that is hundreds of millions of kilometers in size. Eventually, the outer layers of this swollen star can be accreted onto a binary companion and nuclear fusion halts, leaving behind a white dwarf about the size of Earth and sizzling at temperatures exceeding 100,000 °C (180,000 °F).

The proto-white dwarf in the 4FGL J1120.0-2204 system hasn't finished evolving yet. "Currently it's bloated, and is about five times larger in radius than normal white dwarfs with similar masses," says Swihart. "It will continue cooling and contracting and, in about two billion years, it will look identical to many of the extremely low mass white dwarfs that we already know about."

Millisecond pulsars twirl hundreds of times every second. They are spun up by accreting matter from a companion, in this case from the star that became the white dwarf. Most millisecond pulsars emit gamma rays and X-rays, often when the pulsar wind, which is a stream of charged particles emanating from the rotating neutron star, collides with material emitted from a companion star.

About 80 extremely low-mass white dwarfs are known, but "this is the first precursor to an extremely low-mass white dwarf found that is likely orbiting a neutron star," says Swihart. Consequently, 4FGL J1120.0-2204 is a unique look at the tail-end of this spin-up process. All the other white dwarf-pulsar binaries that have been discovered are well past the spinning-up stage.

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Gamma rays and neutrinos from mellow supermassive black holes

The Universe is filled with energetic particles, such as X rays, gamma rays, and neutrinos. However, most of the high-energy cosmic particles' origins remain unexplained.

Now, an international research team has proposed a scenario that explains these; black holes with low activity act as major factories of high-energy cosmic particles.

Details of their research were published in the journal Nature Communications.

Gamma rays are high-energy photons that are many orders of magnitude more energetic than visible light. Space satellites have detected cosmic gamma rays with energies of megaelectron to gigaelectron volts.

Neutrinos are subatomic particles whose mass is nearly zero. They rarely interact with ordinary matter. Researchers at the IceCube Neutrino Observatory have also measured high-energy cosmic neutrinos.

Both gamma rays and neutrinos should be created by powerful cosmic-ray accelerators or surrounding environments in the Universe. However, their origins are still unknown. It is widely believed that active supermassive black holes (so-called active galactic nuclei), especially those with powerful jets, are the most promising emitters of high-energy gamma rays and neutrinos. However, recent studies have revealed that they do not explain the observed gamma rays and neutrinos, suggesting that other source classes are necessary.

The new model shows that not only active black holes but also non-active, "mellow" ones are important, acting as gamma-ray and neutrino factories.

All galaxies are expected to contain supermassive black holes at their centers. When matter falls into a black hole, a huge amount of gravitational energy is released. This process heats the gas, forming high-temperature plasma. The temperature can reach as high as tens of billions of Celsius degrees for low-accreting black holes because of inefficient cooling, and the plasma can generate gamma rays in the megaelectron volt range.

Such mellow black holes are dim as individual objects, but they are numerous in the Universe. The research team found that the resulting gamma rays from low-accreting supermassive black holes may contribute significantly to the observed gamma rays in the megaelectron volt range.

In the plasma, protons can be accelerated to energies roughly 10,000 times higher than those achieved by the Large Hadron Collider -- the largest human-made particle accelerator. The sped-up protons produce high-energy neutrinos through interactions with matter and radiation, which can account for the higher-energy part of the cosmic neutrino data. This picture can be applied to active black holes as demonstrated by previous research. The supermassive black holes including both active and non-active galactic nuclei can explain a large fraction of the observed IceCube neutrinos in a wide energy range.

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Astrophysicists solve 'empty sky' gamma-ray mystery

Star-forming galaxies are responsible for creating gamma-rays that until now had not been associated with a known origin, researchers from The Australian National University (ANU) have confirmed.

Lead author Dr Matt Roth, from the ANU Research School of Astronomy and Astrophysics, said until now it has been unclear what created gamma-rays -- one of the most energetic forms of light in the Universe -- that appear in patches of seemingly 'empty sky'.

The discovery could offer clues to help astronomers solve other mysteries of the Universe, such as what kind of particles make up Dark Matter -- one of the holy grails of astrophysics.

"It's a significant milestone to finally discover the origins of this gamma-ray emission, solving a mystery of the Universe astronomers have been trying to decipher since the 1960s," Dr Roth said.

"There are two obvious sources that produce large amounts of gamma-rays seen in the Universe. One when gas falls into the supermassive black holes which are found at the centres of all galaxies -- called an active galactic nucleus (AGN) -- and the other associated with star formation in the disks of galaxies.

"We modelled the gamma-ray emission from all the galaxies in the Universe and compared our results with the predictions for other sources and found that it is star-forming galaxies that produce the majority of this diffuse gamma-ray radiation and not the AGN process."

ANU researchers were able to pinpoint what created these mysterious gamma-rays after obtaining a better understanding of how cosmic rays -- particles that travel at speeds very close to the speed of light -- move through the gas between the stars. Cosmic rays are important because they create large amounts of gamma-ray emission in star-forming galaxies when they collide with the interstellar gas.

Data from NASA's Hubble Space Telescope and Fermi Gamma-Ray Space Telescope was a key resource used to uncover the unknown origins of the gamma-rays. Researchers analysed information about many galaxies such as their star-formation rates, total masses, physical size and distances from Earth.

"Our model can also be used to make predictions for radio emission -- the electromagnetic radiation that has a frequency similar to a car radio -- from star-forming galaxies, which could help researchers understand more about the internal structure of galaxies," Dr Roth said.

"We are currently looking at producing maps of the gamma-ray sky that can be used to inform upcoming gamma-ray observations from next-generation telescopes. This includes the Cherenkov Telescope Array, which Australia is involved in.

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Galactic gamma ray bursts predicted last year show up right on schedule

Magnetars are bizarre objects -- massive, spinning neutron stars with magnetic fields among the most powerful known, capable of shooting off brief bursts of radio waves so bright they're visible across the universe.

A team of astrophysicists has now found another peculiarity of magnetars: They can emit bursts of low energy gamma rays in a pattern never before seen in any other astronomical object.

It's unclear why this should be, but magnetars themselves are poorly understood, with dozens of theories about how they produce radio and gamma ray bursts. The recognition of this unusual pattern of gamma ray activity could help theorists figure out the mechanisms involved.

"Magnetars, which are connected with fast radio bursts and soft gamma repeaters, have something periodic going on, on top of randomness," said astrophysicist Bruce Grossan, an astrophysicist at the University of California, Berkeley's Space Sciences Laboratory (SSL). "This is another mystery on top of the mystery of how the bursts are produced."

The researchers -- Grossan and theoretical physicist and cosmologist Eric Linder from UC Berkeley and postdoctoral fellow Mikhail Denissenya from Nazarbayev University in Kazakhstan -- discovered the pattern in bursts from a soft gamma repeater, SGR1935+2154, that is a magnetar, a prolific source of soft or lower energy gamma ray bursts and the only known source of fast radio bursts within our Milky Way galaxy. They found that the object emits bursts randomly, but only within regular four-month windows of time, each active window separated by three months of inactivity.

On March 19, the team uploaded a preprint claiming "periodic windowed behavior" in soft gamma bursts from SGR1935+2154 and predicted that these bursts would start up again after June 1 -- following a three month hiatus -- and could occur throughout a four-month window ending Oct. 7.

On June 24, three weeks into the window of activity, the first new burst from SGR1935+2154 was observed after the predicted three month gap, and nearly a dozen more bursts have been observed since, including one on July 6, the day the paper was published online in the journal Physical Review D.

"These new bursts within this window means that our prediction is dead on," said Grossan, who studies high energy astronomical transients. "Probably more important is that no bursts were detected between the windows since we first published our preprint."

Linder likens the non-detection of bursts in three-month windows to a key clue -- the "curious incident" that a guard dog did not bark in the nighttime -- that allowed Sherlock Holmes to solve a murder in the short story "The Adventure of Silver Blaze."

"Missing or occasional data is a nightmare for any scientist," noted Denissenya, the first author of the paper and a member of the Energetic Cosmos Laboratory at Nazarbayev University that was founded several years ago by Grossan, Linder and UC Berkeley cosmologist and Nobel laureate George Smoot. "In our case, it was crucial to realize that missing bursts or no bursts at all carry information."

The confirmation of their prediction startled and thrilled the researchers, who think this may be a novel example of a phenomenon -- periodic windowed behavior -- that could characterize emissions from other astronomical objects.

Mining data from 27-year-old satellite

Within the last year, researchers suggested that the emission of fast radio bursts -- which typically last a few thousandths of a second -- from distant galaxies might be clustered in a periodic windowed pattern. But the data were intermittent, and the statistical and computational tools to firmly establish such a claim with sparse data were not well developed.

Grossan convinced Linder to explore whether advanced techniques and tools could be used to demonstrate that periodically windowed -- but random, as well, within an activity window -- behavior was present in the soft gamma ray burst data of the SGR1935+2154 magnetar. The Konus instrument aboard the WIND spacecraft, launched in 1994, has recorded soft gamma ray bursts from that object -- which also exhibits fast radio bursts -- since 2014 and likely never missed a bright one.

Linder, a member of the Supernova Cosmology Project based at Lawrence Berkeley National Laboratory, had used advanced statistical techniques to study the clustering in space of galaxies in the universe, and he and Denissenya adapted these techniques to analyze the clustering of bursts in time. Their analysis, the first to use such techniques for repeated events, showed an unusual windowed periodicity distinct from the very precise repetition produced by bodies rotating or in orbit, which most astronomers think of when they think of periodic behavior.

"So far, we have observed bursts over 10 windowed periods since 2014, and the probability is 3 in 10,000 that while we think it is periodic windowed, it is actually random," he said, meaning there's a 99.97% chance they're right. He noted that a Monte Carlo simulation indicated that the chance they're seeing a pattern that isn't really there is likely well under 1 in a billion.

The recent observation of five bursts within their predicted window, seen by WIND and other spacecraft monitoring gamma ray bursts, adds to their confidence. However, a single future burst observed outside the window would disprove the whole theory, or cause them to redo their analysis completely.

"The most intriguing and fun part for me was to make predictions that could be tested in the sky. We then ran simulations against real and random patterns and found it really did tell us about the bursts," Denissenya said.

As for what causes this pattern, Grossan and Linder can only guess. Soft gamma ray bursts from magnetars are thought to involve starquakes, perhaps triggered by interactions between the neutron star's crust and its intense magnetic field. Magnetars rotate once every few seconds, and if the rotation is accompanied by a precession -- a wobble in the rotation -- that might make the source of burst emission point to Earth only within a certain window. Another possibility, Grossan said, is that a dense, rotating cloud of obscuring material surrounds the magnetar but has a hole that only periodically allows bursts to come out and reach Earth.

"At this stage of our knowledge of these sources, we can't really say which it is," Grossan said. "This is a rich phenomenon that will likely be studied for some time."

Linder agrees and points out that the advances were made by the cross-pollination of techniques from high energy astrophysics observations and theoretical cosmology.

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Front-row view reveals exceptional cosmic explosion

Scientists have gained the best view yet of the brightest explosions in the universe: A specialised observatory in Namibia has recorded the most energetic radiation and longest gamma-ray afterglow of a so-called gamma-ray burst (GRB) to date. The observations with the High Energy Stereoscopic System (H.E.S.S.) challenge the established idea of how gamma-rays are produced in these colossal stellar explosions which are the birth cries of black holes, as the international team reports in the journal Science.

"Gamma-ray bursts are bright X-ray and gamma-ray flashes observed in the sky, emitted by distant extragalactic sources," explains DESY scientist Sylvia Zhu, one of the authors of the paper. "They are the biggest explosions in the universe and associated with the collapse of a rapidly rotating massive star to a black hole. A fraction of the liberated gravitational energy feeds the production of an ultrarelativistic blast wave. Their emission is divided into two distinct phases: an initial chaotic prompt phase lasting tens of seconds, followed by a long-lasting, smoothly fading afterglow phase."

On 29 August 2019 the satellites Fermi and Swift detected a gamma-ray burst in the constellation of Eridanus. The event, catalogued as GRB 190829A according to its date of occurrence, turned out to be one of the nearest gamma-ray bursts observed so far, with a distance of about one billion lightyears. For comparison: The typical gamma-ray burst is about 20 billion lightyears away. "We were really sitting in the front row when this gamma-ray burst happened," explains co-author Andrew Taylor from DESY. The team caught the explosion's afterglow immediately when it became visible to the H.E.S.S. telescopes. "We could observe the afterglow for several days and to unprecedented gamma-ray energies," reports Taylor.

The comparatively short distance to this gamma-ray burst allowed detailed measurements of the afterglow's spectrum, which is the distribution of "colours" or photon energies of the radiation, in the very-high energy range. "We could determine GRB 190829A's spectrum up to an energy of 3.3 tera-electronvolts, that's about a trillion times as energetic as the photons of visible light," explains co-author Edna Ruiz-Velasco from the Max Planck Institute for Nuclear Physics in Heidelberg. "This is what's so exceptional about this gamma-ray burst -- it happened in our cosmic backyard where the very-high-energy photons were not absorbed in collisions with background light on their way to Earth, as it happens over larger distances in the cosmos."

The team could follow the afterglow up to three days after the initial explosion. The result came as a surprise: "Our observations revealed curious similarities between the X-ray and very-high energy gamma-ray emission of the burst's afterglow," reports Zhu. Established theories assume that the two emission components must be produced by separate mechanisms: the X-ray component originates from ultra-fast electrons that are deflected in the strong magnetic fields of the burst's surroundings. This "synchrotron" process is quite similar to how particle accelerators on Earth produce bright X-rays for scientific investigations.

However, according to existing theories it seemed very unlikely that even the most powerful explosions in the universe could accelerate electrons enough to directly produce the observed very-high-energy gamma rays. This is due to a "burn-off limit," which is determined by the balance of acceleration and cooling of particles within an accelerator. Producing very-high energy gamma-rays requires electrons with energies well beyond the burn-off limit. Instead, current theories assume that in a gamma-ray burst, fast electrons collide with synchrotron photons and thereby boost them to gamma-ray energies in a process dubbed synchrotron self-Compton.

But the observations of GRB 190829A's afterglow now show that both components, X-ray and gamma ray, faded in sync. Also, the gamma-ray spectrum clearly matched an extrapolation of the X-ray spectrum. Together, these results are a strong indication that X-rays and very-high-energy gamma rays in this afterglow were produced by the same mechanism. "It is rather unexpected to observe such remarkably similar spectral and temporal characteristics in the X-ray and very-high energy gamma-ray energy bands, if the emission in these two energy ranges had different origins," says co-author Dmitry Khangulyan from Rikkyo University in Tokyo. This poses a challenge for the synchrotron self-Compton origin of the very-high energy gamma-ray emission.

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Not just for finding planets: Exoplanet-hunter TESS telescope spots bright gamma-ray burst

NASA has a long tradition of unexpected discoveries, and the space program's TESS mission is no different. SMU astrophysicist and her team have discovered a particularly bright gamma-ray burst using a NASA telescope designed to find exoplanets -- those occurring outside our solar system -- particularly those that might be able to support life.

It's the first time a gamma-ray burst has been found this way.

Gamma-ray bursts are the brightest explosions in the universe, typically associated with the collapse of a massive star and the birth of a black hole. They can produce as much radioactive energy as the sun will release during its entire 10-billion-year existence.

Krista Lynne Smith, an assistant professor of physics at Southern Methodist University, and her team confirmed the blast -- called GRB 191016A -- happened on Oct. 16 and also determined its location and duration. A study on the discovery has been published in The Astrophysical Journal.

"Our findings prove this TESS telescope is useful not just for finding new planets, but also for high-energy astrophysics," said Smith, who specializes in using satellites like TESS (Transiting Exoplanet Survey Satellite) to study supermassive black holes and gas that surrounds them. Such studies shed light on the behavior of matter in the deeply warped spacetime around black holes and the processes by which black holes emit powerful jets into their host galaxies.

Smith calculated that GRB 191016A had a peak magnitude of 15.1, which means it was 10,000 times fainter than the faintest stars we can see with the naked eyes.

That may sound quite dim, but the faintness has to do with how far away the burst occurred. It is estimated that light from GRB 191016A's galaxy had been travelling 11.7 billion years before becoming visible in the TESS telescope.

Most gamma ray bursts are dimmer -- closer to 160,000 times fainter than the faintest stars.

The burst reached its peak brightness sometime between 1,000 and 2,600 seconds, then faded gradually until it fell below the ability of TESS to detect it some 7000 seconds after it first went off.

This gamma-ray burst was first detected by a NASA's satellite called Swift-BAT, which was built to find these bursts. But because GRB 191016A occurred too close to the moon, the Swift-BAT couldn't do the necessary follow-up it normally would have to learn more about it until hours later.

NASA's TESS happened to be looking at that same part of the sky. That was sheer luck, as TESS turns its attention to a new strip of the sky every month.

While exoplanet researchers at a ground-base for TESS could tell right away that a gamma-ray burst had happened, it would be months before they got any data from the TESS satellite on it. But since their focus was on new planets, these researchers asked if any other scientists at a TESS conference in Sydney, Australia were interested in doing more digging on the blast.

Smith was one of the few high-energy astrophysics specialists there at that time and quickly volunteered.

"The TESS satellite has a lot of potential for high-energy applications, and this was too good an example to pass up," she said. High-energy astrophysics studies the behavior of matter and energy in extreme environments, including the regions around black holes, powerful relativistic jets, and explosions like gamma-ray bursts.

TESS is an optical telescope that collects light curves on everything in its field of view, every half hour. Light curves are a graph of light intensity of a celestial object or region as a function of time. Smith analyzed three of these light curves to be able to determine how bright the burst was.

She also used data from ground-based observatories and the Swift gamma-ray satellite to determine the burst's distance and other qualities about it.

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True identity of mysterious gamma-ray source revealed

 An international research team including members from The University of Manchester has shown that a rapidly rotating neutron star is at the core of a celestial object now known as PSR J2039?5617

The international collaboration used novel data analysis methods and the enormous computing power of the citizen science project Einstein@Home to track down the neutron star's faint gamma-ray pulsations in data from NASA's Fermi Space Telescope. Their results show that the pulsar is in orbit with a stellar companion about a sixth of the mass of our Sun. The pulsar is slowly but surely evaporating this star. The team also found that the companion's orbit varies slightly and unpredictably over time. Using their search method, they expect to find more such systems with Einstein@Home in the future.

Searching for the so-called 'Spider' pulsar systems -- rapidly spinning neutron stars whose high-energy outflows are destroying their binary companion star, required 10 years of precise data. The pulsars have been given arachnid names of 'Black widows' or 'Redbacks', after species of spider where the females have been seen to kill the smaller males after mating.

New research published in, Monthly Notices of the Royal Astronomical Society, details how researchers found a neutron star rotating 377 times a second in an exotic binary system using data from NASA's Fermi Space Telescope.

The astronomer's findings were uniquely boosted by the Einstein@Home project, a network of thousands of civilian volunteers lending their home computing power to the efforts of the Fermi Telescope's work.

The group's search required combing very finely through the data in order not to miss any possible signals. The computing power required is enormous. The search would have taken 500 years to complete on a single computer core. By using a part of the Einstein@Home resources it was done in 2 months.

With the computing power donated by the Einstein@Home volunteers, the team discovered gamma-ray pulsations from the rapidly rotating neutron star. This gamma-ray pulsar, now known as J2039?5617, rotates about 377 times each second.

"It had been suspected for years that there is a pulsar, a rapidly rotating neutron star, at the heart of the source we now know as PSR J2039?5617," says Lars Nieder, a PhD student at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hannover. "But it was only possible to lift the veil and discover the gamma-ray pulsations with the computing power donated by tens of thousands of volunteers to Einstein@Home," he adds.

The celestial object has been known since 2014 as a source of X-rays, gamma rays, and light. All evidence obtained so far pointed at a rapidly rotating neutron star in orbit with a light-weight star being at the heart of the source. But clear proof was missing.

The first step to solving this riddle were new observations of the stellar companion with optical telescopes. They provided precise knowledge about the binary system without which a gamma-ray pulsar search (even with Einstein@Home's huge computing power) would be unfeasible.

The system's brightness varies during an orbital period depending on which side of the neutron star's companion is facing the Earth. "For J2039-5617, there are two main processes at work," explains Dr. Colin Clark from Jodrell Bank Centre for Astrophysics, lead author of the study. "The pulsar heats up one side of the light-weight companion, which appears brighter and more bluish. Additionally, the companion is distorted by the pulsar's gravitational pull causing the apparent size of the star to vary over the orbit. These observations allowed the team to get the most precise measurement possible of the binary star's 5.5-hour orbital period, as well as other properties of the system."

With this information and the precise sky position from Gaia data, the team used the aggregated computing power of the distributed volunteer computing project Einstein@Home for a new search of about 10 years of archival observations of NASA's Fermi Gamma-ray Space Telescope. Improving on earlier methods they had developed for this purpose, they enlisted the help of tens of thousands of volunteers to search Fermi data for periodic pulsations in the gamma-ray photons registered by the Large Area Telescope onboard the space telescope. The volunteers donated idle compute cycles on their computers' CPUs and GPUs to Einstein@Home.

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Gran Telescopio Canarias finds the farthest black hole that belongs to a rare family of galaxies

 An international team of astronomers has identified one of the rarest known classes of gamma-ray emitting galaxies, called BL Lacertae, within the first 2 billion years of the age of the Universe. The team, that has used one of the largest optical telescope in the world, Gran Telescopio Canarias (GTC), located at the Observatorio del Roque de los Muchachos (Garafía, La Palma), consists of researchers from the Universidad Complutense de Madrid (UCM, Spain), DESY (Germany), University of California Riverside and Clemson University (USA). The finding is published in The Astrophysical Journal Letters.

Only a small fraction of the galaxies emits gamma rays, which is the most extreme form of light. Astronomers believe that these highly energetic photons originate from the vicinity of a supermassive black hole residing at the centers of these galaxies. When this happens, they are known as active galaxies. The black hole swallows matter from its surroundings and emits jets or, in other words, collimated streams of matter and radiation. Few of these active galaxies (less than 1%) have their jets pointing by chance toward Earth. Scientists call them blazars and are one of the most powerful sources of radiation in the universe.

Blazars come in two flavors: BL Lacertae (BL Lac) and flat-spectrum radio-quasars (FSRQs). Our current understanding about these mysterious astronomical objects is that FSRQs are relatively young active galaxies, rich in dust and gas that surround the central black hole. As time passes, the amount of matter available to feed the black hole is consumed and the FSRQ evolves to become a BL Lac object. "In other words, BL Lacs may represent the elderly and evolved phase of a blazar's life, while FSRQs resemble an adult," explains Vaidehi Paliya, a DESY researcher who participated in this program.

"Since the speed of light is limited, the farther we look, the earlier in the age of the Universe we investigate," says Alberto Domínguez of the Institute of Physics of Particles and the Cosmos (IPARCOS) at UCM and co-author of the study. Astronomers believe that the current age of the Universe is around 13.8 billion years. The most distant FSRQ was identified at a distance when the age of the universe was merely 1 billion years. For a comparison, the farthest BL Lac that is known was found when the age of the Universe was around 2.5 billion years. Therefore, the hypothesis of the evolution from FSRQ to BL Lacs appears to be valid.

Now, the team of international scientists has discovered a new BL Lac object, named 4FGL J1219.0+3653, much farther away than the previous record holder. "We have discovered a BL Lac existing even 800 million years earlier, this is when the Universe was less than 2 billion years old," states Cristina Cabello, a graduate student at IPARCOS-UCM. "This finding challenges the current scenario that BL Lacs are actually an evolved phase of FSRQ," adds Nicolás Cardiel, a professor at IPARCOS-UCM. Jesús Gallego, also a professor at the same institution and a co-author of the study concludes: "This discovery has challenged our knowledge of the cosmic evolution of blazars and active galaxies in general."

The researchers have used the OSIRIS and EMIR instruments, designed and built by the Instituto de Astrofísica de Canarias (IAC) and mounted on GTC, also known as Grantecan. "These results are a clear example of how the combination of the large collecting area of ??GTC, the world's largest optical-infrared telescope, together with the unique capabilities of complementary instruments installed in the telescope are providing breakthrough results to improve our understanding of the Universe," underlines Romano Corradi, director of Grantecan.

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