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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Telescope Array detects second highest-energy cosmic ray ever

In 1991, the University of Utah Fly's Eye experiment detected the highest-energy cosmic ray ever observed. Later dubbed the Oh-My-God particle, the cosmic ray's energy shocked astrophysicists. Nothing in our galaxy had the power to produce it, and the particle had more energy than was theoretically possible for cosmic rays traveling to Earth from other galaxies. Simply put, the particle should not exist.

The Telescope Array has since observed more than 30 ultra-high-energy cosmic rays, though none approaching the Oh-My-God-level energy. No observations have yet revealed their origin or how they are able to travel to the Earth.

On May 27, 2021, the Telescope Array experiment detected the second-highest extreme-energy cosmic ray. At 2.4 x 1020eV, the energy of this single subatomic particle is equivalent to dropping a brick on your toe from waist height. Led by the University of Utah (the U) and the University of Tokyo, the Telescope Array consists of 507 surface detector stations arranged in a square grid that covers 700 km2 (~270 miles2) outside of Delta, Utah in the state's West Desert. The event triggered 23 detectors at the north-west region of the Telescope Array, splashing across 48 km2 (18.5 mi2). Its arrival direction appeared to be from the Local Void, an empty area of space bordering the Milky Way galaxy.

"The particles are so high energy, they shouldn't be affected by galactic and extra-galactic magnetic fields. You should be able to point to where they come from in the sky," said John Matthews, Telescope Array co-spokesperson at the U and co-author of the study. "But in the case of the Oh-My-God particle and this new particle, you trace its trajectory to its source and there's nothing high energy enough to have produced it. That's the mystery of this -- what the heck is going on?"

In their observation that published on Nov. 24, 2023, in the journal Science, an international collaboration of researchers describe the ultra-high-energy cosmic ray, evaluate its characteristics, and conclude that the rare phenomena might follow particle physics unknown to science. The researchers named it the Amaterasu particle after the sun goddess in Japanese mythology. The Oh-My-God and the Amaterasu particles were detected using different observation techniques, confirming that while rare, these ultra-high energy events are real.

"These events seem like they're coming from completely different places in the sky. It's not like there's one mysterious source," said John Belz, professor at the U and co-author of the study. "It could be defects in the structure of spacetime, colliding cosmic strings. I mean, I'm just spit-balling crazy ideas that people are coming up with because there's not a conventional explanation."

Natural particle accelerators

Cosmic rays are echoes of violent celestial events that have stripped matter to its subatomic structures and hurled it through universe at nearly the speed of light. Essentially cosmic rays are charged particles with a wide range of energies consisting of positive protons, negative electrons, or entire atomic nuclei that travel through space and rain down onto Earth nearly constantly.

Cosmic rays hit Earth's upper atmosphere and blasts apart the nucleus of oxygen and nitrogen gas, generating many secondary particles. These travel a short distance in the atmosphere and repeat the process, building a shower of billions of secondary particles that scatter to the surface. The footprint of this secondary shower is massive and requires that detectors cover an area as large as the Telescope Array. The surface detectors utilize a suite of instrumentation that gives researchers information about each cosmic ray; the timing of the signal shows its trajectory and the amount of charged particles hitting each detector reveals the primary particle's energy.

Because particles have a charge, their flight path resembles a ball in a pinball machine as they zigzag against the electromagnetic fields through the cosmic microwave background. It's nearly impossible to trace the trajectory of most cosmic rays, which lie on the low- to middle-end of the energy spectrum. Even high-energy cosmic rays are distorted by the microwave background. Particles with Oh-My-God and Amaterasuenergy blast through intergalactic space relatively unbent. Only the most powerful of celestial events can produce them.

"Things that people think of as energetic, like supernova, are nowhere near energetic enough for this. You need huge amounts of energy, really high magnetic fields to confine the particle while it gets accelerated," said Matthews.

Ultra-high-energy cosmic rays must exceed 5 x 1019 eV. This means that a single subatomic particle carries the same kinetic energy as a major league pitcher's fast ball and has tens of millions of times more energy than any human-made particle accelerator can achieve. Astrophysicists calculated this theoretical limit, known as the Greisen-Zatsepin-Kuzmin (GZK) cutoff, as the maximum energy a proton can hold traveling over long distances before the effect of interactions of the microwave background radiation take their energy. Known source candidates, such as active galactic nuclei or black holes with accretion disks emitting particle jets, tend to be more than 160 million light years away from Earth. The new particle's 2.4 x 1020 eV and the Oh-My-God particle's 3.2 x 1020 eV easily surpass the cutoff.

Researchers also analyze cosmic ray composition for clues of its origins. A heavier particle, like iron nuclei, are heavier, have more charge and are more susceptible to bending in a magnetic field than a lighter particle made of protons from a hydrogen atom. The new particle is likely a proton. Particle physics dictates that a cosmic ray with energy beyond the GZK cutoff is too powerful for the microwave background to distort its path, but back tracing its trajectory points towards empty space.

"Maybe magnetic fields are stronger than we thought, but that disagrees with other observations that show they're not strong enough to produce significant curvature at these ten-to-the-twentieth electron volt energies," said Belz. "It's a real mystery."

Expanding the footprint

The Telescope Array is uniquely positioned to detect ultra-high-energy cosmic rays. It sits at about 1,200 m (4,000 ft), the elevation sweet-spot that allows secondary particles maximum development, but before they start to decay. Its location in Utah's West Desert provides ideal atmospheric conditions in two ways: the dry air is crucial because humidity will absorb the ultraviolet light necessary for detection; and the region's dark skies are essential, as light pollution will create too much noise and obscure the cosmic rays.

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Helium nuclei research advances our understanding of cosmic ray origin and propagation

The CALorimetric Electron Telescope (CALET), aboard the Kibo's Exposed Facility (EF) of the International Space Station, has been on a mission to measure the flux of cosmic ray particles since 2015. In a new study, an international team of researchers report the results of a direct measurement of the cosmic ray helium spectrum using the data collected by CALET. In contrast to the single power-law that was previously believed to exist, analysis of the flux data collected between 2015 and 2022 reveals that the energy distribution of cosmic ray helium nuclei follows a Double Broken Power Law.

Much of our understanding of the Universe and its mysterious phenomena is based on theoretical interpretations. In order to deepen the understanding of distant objects and energetic phenomena, astronomers are looking at cosmic rays, which are high-energy charged particles composed of protons, electrons, atomic nuclei, and other subatomic particles. Such studies have revealed that cosmic rays contain all the elements known to us in the periodic table, suggesting that these elements originate from stars and high-energy events such as supernovae. Additionally, due to their charged nature, the path of cosmic rays through space is influenced by the magnetic fields of interstellar phenomena and objects.

Detailed observations of cosmic rays can, thus, not only shed light on the origins of these particles but also decode the existence of high-energy objects and phenomena such as supernova remnants, pulsars, and even dark matter. In an effort to better observe high-energy radiations, Japan, Italy, and USA collaboratively established the CALorimetric Electron Telescope (CALET) on the International Space Station in 2015.

In 2018, observations of the cosmic ray proton spectrum from 50 GeV to 10 TeV revealed that the particle flux of protons at high energies was significantly higher than expected. These results deviated from the conventional cosmic ray acceleration and propagation models that assume a "single power-law distribution," wherein the number of particles decrease with increasing energy.

Consequently, in a study published in 2022, the CALET team, including researchers from Waseda University, found cosmic ray protons in the energy range of 50 GeV to 60 TeV to follow a "Double Broken Power Law." This law assumes that the number of high-energy particles initially increase until 10 TeV (known as spectral hardening) and then decrease with an increase in energy (known as spectral softening).

Extending these observations further, the team has now found similar trends of spectral hardening and softening in the cosmic ray helium spectrum captured over a broad range of energy, from 40 GeV to 250 TeV.

The study, published in the journal Physical Review Letters on 27 April, 2023, was led by Associate Professor Kazuyoshi Kobayashi from Waseda University, Japan, along with contributions from Professor Emeritus Shoji Torii, Principal Investigator of the CALET project, also affiliated with Waseda University, and Research Assistant Paolo Brogi from the University of Siena in Italy.

"CALET has successfully observed energy spectral structure of cosmic ray helium, especially spectral hardening starting from around 1.3 TeV, and the tendency of softening starting from around 30 TeV," says Kobayashi.

These observations are based on data collected by CALET aboard the International Space Station (ISS) between 2015 to 2022. Representing the largest energy range to date for cosmic helium nuclei particles, these observations provide additional evidence for deviation of the particle flux from the single power-law model. The researchers noticed that deviation from the expected power-law distribution was more than eight standard deviations away from the mean, indicating a very low probability of this deviation occurring by chance.

Notably, the initial spectral hardening observed in this data suggests that there may be unique sources or mechanisms that are responsible for accelerating and propagating the helium nuclei to high energies. The discovery of these spectral features is also supported by recent observations from the Dark Matter Particle Explorer, and questions our current understanding of the origin and nature of cosmic rays.

"These results would significantly contribute to the understanding of cosmic ray acceleration in the supernova remnant and propagation mechanism," says Torii.

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Cosmic ray protons reveal new spectral structures at high energies

Discovered in 1912, cosmic rays have been studied extensively and our current understanding of them is compiled into what is called the Standard Model. Recently, this understanding has been challenged by the detection of unexpected spectral structures in the cosmic ray proton energy spectrum. Now, scientists take this further with high-statistics and low-uncertainty measurement of these protons over a broader energy range using the CALorimetric Electron Telescope, confirming the presence of such structures.

Cosmic rays constitute high-energy protons and atomic nuclei that originate from stars (both within our galaxy and from other galaxies) and are accelerated by supernovae and other high-energy astrophysical objects. Our current understanding of the Galactic cosmic ray energy spectrum suggests that it follows a power-law dependence, in that the spectral index of protons detected within a certain energy range goes down by power law as energy increases. But recent observations made using magnetic spectrometers for low energy levels and calorimeters for high energy levels has hinted at a deviation from this power-law variation, with the spectral index of protons becoming larger around an energy of few hundred GeV at energies up to 10 TeV. Following this "spectral hardening," characterized by a smaller absolute value of the spectral index, a "spectral softening" has been detected above 10 TeV using the CALorimetric Electron Telescope (CALET), a space telescope installed at the International Space Station. However, better measurements with high statistics and low uncertainty need to be performed over a broad energy spectrum for the confirmation of these spectral structures.

This is exactly what a team of international researchers led by Associate Professor Kazuyoshi Kobayashi from Waseda University in Japan set out to do. "With the data collected by CALET over roughly 6.2 years, we have put forth a detailed spectral structure of the cosmic ray protons. The novelty of our data lies in the high-statistics measurement over a broader energy range of 50 GeV to 60 TeV," elaborates Kobayashi. The findings of their study, which included contributions from Professor Emeritus Shoji Torii from Waseda University (PI, or Principal Investigator, of CALET project) and Professor Pier Simone Marrocchesi from University of Siena in Italy, was published in the journal Physical Review Letters on 1 September 2022.

The new observations confirmed the presence of spectral hardening and softening below and above 10 TeV, suggesting that the proton energy spectrum is not consistent with a single power law variation for the entire range. Moreover, the spectral softening starting at around 10 TeV is consistent with a previous measurement reported by the Dark Matter Particle Explorer (DAMPE) space telescope. Interestingly enough, the transition by spectral softening was found to be sharper than that by spectral hardening.

The variations and the uncertainty in the new CALET data were controlled using Monte Carlo simulations. The statistics was improved by a factor of around 2.2 and the spectral hardening feature was confirmed with a higher significance of more than 20 sigmas.

Talking about the significance of this research, Kobayashi remarks, "This result will significantly contribute to our understanding of cosmic ray acceleration by supernovae and the propagation mechanism of cosmic rays. The next step would be to extend our measurement of the proton spectra to even higher energies with reduced systematic uncertainties. This should be accompanied by a shift in the theoretical understanding to accommodate the new observations."

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Researchers hunt for one-pole magnets by combining cosmic rays and particle accelerators

Some of the world's most powerful particle accelerators have helped researchers draw new leading limits on the existence of long theorized magnetic monopoles from the collisions of energetic cosmic rays bombarding the Earth's atmosphere, reports a new study published in Physical Review Letters.

Magnets are intimately familiar to everyone, with wide-ranging applications within daily life, from TVs and computers to kids toys. However, breaking any magnet, such as a navigation compass needle consisting of north and south poles in half, will result in just two smaller two-pole magnets. This mystery has eluded researchers for decades since 1931, when physicist Paul Dirac theorized the existence of one-pole "magnetic monopoles'' -- particles comparable to electrons but with a magnetic charge.

To explore whether magnetic monopoles exist, an international team of researchers, including the University of Tokyo's Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Fellow Volodymyr Takhistov, studied available data from a variety of terrestrial experiments and have carried out the most sensitive searches to date for monopoles over a broad range of possible masses. The researchers focused on an unusual source of monopoles -- atmospheric collisions of cosmic rays that have been occurring for eons.

The interdisciplinary research required bringing together expertise from several distinct corners of science -- including accelerator physics, neutrino interactions and cosmic rays.

Cosmic ray collisions with the atmosphere have already played a central role in advancing science, especially the exploration of ghostly neutrinos. This lead to Kavli IPMU Senior Fellow Takaaki Kajita's 2015 Nobel Prize in Physics for the discovery by the Super-Kamiokande experiment that neutrinos oscillate in flight, implying that they have mass.

Partially inspired by the results of Super-Kamiokande, the team set to work on monopoles. Particularly intriguing were light monopoles with masses around the electroweak scale, which can be readily accessible to conventional particle accelerators.

By carrying out simulations of cosmic ray collisions, analogously to particle collisions at the LHC at CERN, the researchers obtained a persistent beam of light monopoles raining down upon different terrestrial experiments.

This unique source of monopoles is especially interesting, as it is independent of any pre-existing monopoles such as those potentially left over as relics from the early Universe, and covers a broad range of energies.

By re-analyzing data from a wide range of previous experimental monopole searches, the researchers identified novel limits on monopoles across a wide range of masses, including those beyond the reach of conventional collider monopole searches.

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New method to synchronize devices on Earth makes use of cosmic rays

Various technologies, networks and institutions benefit from or require accurate time keeping to synchronize their activities. Current ways of synchronizing time have some drawbacks that a new proposed method seeks to address. The cosmic time synchronizer works by synchronizing devices around cosmic ray events detected by those devices. This could bring accurate timing abilities to remote sensing stations, or even underwater, places that other methods cannot serve. Early tests show promise, but the real challenge may lie in the adoption of this new technique.

Humanity is intimately connected with the idea of time. Historically, we used the cosmos itself -- stars, the sun, and the moon -- to measure time and coordinate our activities. It's fitting, then, that researchers are looking out to the cosmos again to further develop our ability to keep time. Professor Hiroyuki Tanaka from Muographix at the University of Tokyo devised and tested a way to synchronize multiple devices, so they agree upon the time, that makes use of cosmic rays from deep space. Appropriately, it's called cosmic time synchronization (CTS).

"It's relatively easy to keep time accurately these days. For example, atomic clocks have been doing this for decades now," said Tanaka. "However, these are large and expensive devices that are very easy to disrupt. This is one reason I have been working on an improved way to keep time. The other is that, related to time measurement, position measurement could also be made better. So really, CTS is a precursor to a potential replacement for GPS, but that's still a little further down the line."

The reason it's critical for devices to have a shared sense of time is that certain devices are increasingly important in many aspects of life. Computer networks responsible for financial transactions must agree upon time so that the order of transactions can be ensured. There are sensors that work in unison to observe various physical phenomena which need to agree upon time so that, for example, the origin of a particular reading can be determined. Such sensors could even potentially be part of some kind of disaster warning system.

CTS works thanks to cosmic rays from deep space that strike the atmosphere around 15 kilometers up, creating showers of particles including muons. The muons travel close to the speed of light, reaching the ground almost immediately, they can easily penetrate water or rock, and spread out as they travel to cover a few square kilometers of ground. Independent CTS-enabled devices under the same particle shower can detect the incoming muons, which will have a specific signature unique to the cosmic ray event that generated them. By sharing this information, CTS devices can confer with one another and synchronize their clocks according to when the cosmic ray event took place. The ultrahigh-energy cosmic ray strikes occur frequently enough, about a hundred times per hour over every square kilometer of Earth, for CTS devices to work together in real time.

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Unveiling a century-old mystery: Where the Milky Way's cosmic rays come from

Astronomers have succeeded for the first time in quantifying the proton and electron components of cosmic rays in a supernova remnant. At least 70% of the very-high-energy gamma rays emitted from cosmic rays are due to relativistic protons, according to the novel imaging analysis of radio, X-ray, and gamma-ray radiation. The acceleration site of protons, the main components of cosmic rays, has been a 100-year mystery in modern astrophysics, this is the first time that the amount of cosmic rays being produced in a supernova remnant has been quantitatively shown and is an epoch-making step in the elucidation of the origin of cosmic rays.

The origin of cosmic rays, the particles with the highest energy in the universe, has been a great mystery since their discovery in 1912. Because cosmic rays promote the chemical evolution of interstellar matter, understanding their origin is critical in understanding the evolution of our Galaxy. The cosmic rays are thought to be accelerated by supernova remnants (the after-effects of supernova explostions) in our Galaxy and traveled to the Earth at almost the speed of light. Recent progress in gamma-ray observations has revealed that many supernova remnants emit gamma-rays at teraelectronvolts (TeV) energies. If gamma rays are produced by protons, which are the main component of cosmic rays, then the supernova remnant origin of cosmic rays can be verified. However, gamma rays are also produced by electrons, it is necessary to determine whether the proton or electron origin is dominant, and to measure the ratio of the two contributions. The results of this study provide compelling evidence of gamma rays originating from the proton component, which is the main component of cosmic rays, and clarify that Galactic cosmic rays are produced by supernova remnants.

The originality of this research is that gamma-ray radiation is represented by a linear combination of proton and electron components. Astronomers knew a relation that the intensity of gamma-ray from protons is proportional to the interstellar gas density obtained by radio-line imaging observations. On the other hand, gamma-rays from electrons are also expected to be proportional to X-ray intensity from electrons. Therefore, they expressed the total gamma-ray intensity as the sum of two gamma-ray components, one from the proton origin and the other from the electron origin. This led to a unified understanding of three independent observables. This method was first proposed in this study. As a result, it was shown that gamma rays from protons and electrons account for 70% and 30% of the total gamma-rays, respectively. This is the first time that the two origins have been quantified. The results also demonstrate that gamma rays from protons are dominated in interstellar gas-rich regions, whereas gamma rays from electrons are enhanced in the gas-poor region. This confirms that the two mechanisms work together and supporting the predictions of previous theoretical studies.

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Cosmic rays may be key to understanding galactic dynamics

Cosmic rays are charged subnuclear particles that move close to the speed of light, constantly raining down on the Earth. These particles are relativistic, as defined by Albert Einstein's special relativity, and manage to generate a magnetic field that controls the way they move within the galaxy.

Gas within the interstellar medium is composed of atoms, mostly hydrogen and mostly ionized, meaning its protons and electrons are separated. While moving around within this gas, cosmic rays kickstart the background protons, which causes a collective plasma wave movement akin to the ripples on a lake when you toss in a stone.

The big question is how cosmic rays deposit their momentum into the background plasma that composes the interstellar medium. In Physics of Plasmas, from AIP Publishing, plasma astrophysicists in France review recent developments within the field of studying the streaming instability triggered by cosmic rays within astrophysical and space plasma.

"Cosmic rays may help explain aspects of our galaxy from its smallest scales, such as protoplanetary disks and planets, to its largest scales, such as galactic winds," said Alexandre Marcowith, from the University of Montpellier.

Until now, cosmic rays were viewed as being a bit apart within galaxy "ecology." But because instability works well and is stronger than expected around cosmic ray sources, such as supernova remnants and pulsars, these particles likely have far more impacts on galactic dynamics and the star formation cycle than previously known.

"This is not really a surprise, but more of a paradigm shift," Marcowith said. "In science and astrophysics, everything is connected."

Supernova shock waves expanding the interstellar/intergalactic medium "are known to accelerate cosmic rays, and because cosmic rays are streaming away, they may have contributed to generating the magnetic field seeds necessary to explain the actual magnetic field strengths we observe around us," said Marcowith.

After the amplitude of a plasma wave is reduced or damped over time, much like those generated by a stone thrown into a lake, it heats the gas of the plasma. Meanwhile, it helps scatter cosmic rays.

For this to occur, the waves need wavelengths of the same order as the cosmic ray gyro radius. Cosmic rays possess a helical (spiral) motion around the magnetic field, and its radius is called the Larmor radius.

"Say you are driving a car on a winding road. If the wavelength is of the same order as your wheel size, it will be difficult to drive," said Marcowith.

Cosmic rays are strongly scattered by these waves, and the main instability at the origin of these perturbations (waves) is the streaming instability associated with the collective streaming motion of cosmic rays.

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Mystery of heavy elements in galactic cosmic rays

Scientists have used data from the Southwest Research Institute-led Magnetospheric Multiscale (MMS) mission to explain the presence of energetic heavy elements in galactic cosmic rays (GCRs). GCRs are composed of fast-moving energetic particles, mostly hydrogen ions called protons, the lightest and most abundant elements in the universe. Scientists have long debated how trace amounts of heavy ions in GCRs are accelerated.

The supernova explosion of a dying star creates massive shockwaves that propagate through the surrounding space, accelerating ions in their path to very high energies, creating GCRs. How heavy ions are energized and accelerated is important because they affect the redistribution of mass throughout the universe and are essential for the formation of even heavier and more chemically complex elements. They also influence how we perceive astrophysical structures.

"Heavy ions are thought to be insensitive to an incoming shockwave because they are less abundant, and the shock energy is overwhelmingly consumed by the preponderance of protons. Visualize standing on a beach as waves move the sand under your feet, while you remain in place," said SwRI's Dr. Hadi Madanian, the lead author of the paper about this research published in Astrophysical Journal Letters. "However, that classical view of how heavy ions behave under shock conditions is not always what we have seen in high-resolution MMS observations of the near-Earth space environment."

Shock phenomena also occur in the near-Earth environment. The Sun's magnetic field is carried through interplanetary space by the supersonic solar wind flow, which is obstructed and diverted by the Earth's magnetosphere, a bubble of protection around our home planet. This interaction region is called the bow shock due to its curved shape, comparable to the bow waves that occur as a boat travels through water. The Earth's bow shock forms at a much smaller scale than supernova shocks. However, at times, conditions of this small shock resemble those of supernova remnants. The team used high-resolution in-situ measurements from the MMS spacecraft at the bow shock to study how heavy ions are accelerated.

"We observed intense amplification of the magnetic field near the bow shock, a known property associated with strong shocks such as supernova remnants. We then analyzed how different ion species behaved as they encountered the bow shock," Madanian said. "We found that these enhanced fields significantly modify the trajectory of heavy ions, redirecting them into the acceleration zone of the shock."

While this behavior was not expected to occur for heavy ions, the team identified direct evidence for this process in alpha particles, helium ions that are four times more massive than protons and have twice the charge.

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Not so fast, supernova: Highest-energy cosmic rays detected in star clusters

 For decades, researchers assumed the cosmic rays that regularly bombard Earth from the far reaches of the galaxy are born when stars go supernova -- when they grow too massive to support the fusion occurring at their cores and explode.

Those gigantic explosions do indeed propel atomic particles at the speed of light great distances. However, new research suggests even supernovae -- capable of devouring entire solar systems -- are not strong enough to imbue particles with the sustained energies needed to reach petaelectronvolts (PeVs), the amount of kinetic energy attained by very high-energy cosmic rays.

And yet cosmic rays have been observed striking Earth's atmosphere at exactly those velocities, their passage marked, for example, by the detection tanks at the High-Altitude Water Cherenkov (HAWC) observatory near Puebla, Mexico. Instead of supernovae, the researchers posit that star clusters like the Cygnus Cocoon serve as PeVatrons -- PeV accelerators -- capable of moving particles across the galaxy at such high energy rates.

Their paradigm-shifting research provides compelling evidence for star forming regions to be PeVatrons and is published in two recent papers in Nature Astronomy and Astrophysical Journal Letters.

A characteristic of physics research is how collaborative it is. The research was conducted by Petra Huentemeyer, professor of physics at Michigan Technological University, along with recent graduate Binita Hona '20, doctoral student Dezhi Huang, former MTU postdoc Henrike Fleischhack (now at Catholic University/NASA GSFC/CRESST II), Sabrina Casanova at the Institute of Nuclear Physics Polish Academy of Sciences in Krakow, Ke Fang at the University of Wisconsin and Roger Blanford at Stanford, along with numerous other collaborators of the HAWC Observatory.

Huentemeyer noted that HAWC and physicists from other institutions have measured cosmic rays from all directions and across many decades of energy. It's in tracking the cosmic rays with the highest known energy, PeVs, that their origin becomes so important.

"Cosmic rays below PeV energy are believed to come from our galaxy, but the question is what are the accelerators that can produce them," Huentemeyer said.

Fleischhack said the paradigm shift the researchers have uncovered is that before, scientists thought supernova remnants were the main accelerators of cosmic rays.

"They do accelerate cosmic rays, but they are not able to get to highest energies," she said.

So, what is driving cosmic rays' acceleration to PeV energy?

"There have been several other hints that star clusters could be part of the story," Fleischhack said. "Now we are getting confirmation that they are able to go to highest energies."

Star clusters are formed from the remnants of a supernova event. Known as star cradles, they contain violent winds and clouds of swirling debris -- such as those noted by the researchers in Cygnus OB2 and cluster [BDS2003]8. Inside, several types of massive stars known as spectral type O and type B stars are gathered by the hundreds in an area about 30 parsecs (108 light-years) across.

"Spectral type O stars are the most massive," Hona said. "When their winds interact with each other, shock waves form, which is where acceleration happens."

The researchers' theoretical models suggest that the energetic gamma-ray photons seen by HAWC are more likely produced by protons than by electrons.

"We will use NASA telescopes to search for the counterpart emission by these relativistic particles at lower energies," Fang said.

The extremely high energy at which cosmic rays reach our planet is notable. Specific conditions are required to accelerate particles to such velocities.

The higher the energy, the more difficult it is to confine the particles -- knowledge gleaned from particle accelerators here on Earth in Chicago and Switzerland. To keep particles from whizzing away, magnetism is required.

Stellar clusters -- with their mixture of wind and nascent but powerful stars -- are turbulent regions with different magnetic fields that can provide the confinement necessary for particles to continue to accelerate.

"Supernova remnants have very fast shocks where the cosmic ray can be accelerated; however, they don't have the type of long confinement regions," Casanova said. "This is what star clusters are useful for. They're an association of stars that can create disturbances that confine the cosmic rays and make it possible for the shocks to accelerate them."

But how is it possible to measure atomic interactions on a galactic scale 5,000 light-years from Earth? The researchers used 1,343 days of measurements from HAWC detection tanks.

Huang explained how the physicists at HAWC trace cosmic rays by measuring the gamma rays these cosmic rays produce at galactic acceleration sites: "We didn't measure gamma rays directly; we measured the secondary rays generated. When gamma rays interact with the atmosphere, they generate secondary particles in particle showers."

"When particle showers are detected at HAWC, we can measure the shower and the charge of secondary particles," Huang said. "We use the particle charge and time information to reconstruct information from the primary gamma."

In addition to HAWC, the researchers plan to work with the Southern Wide-field Gamma-ray Observatory (SWGO), an observatory currently in the planning stages that will feature Cherenkov light detectors like HAWC but will be located in the southern hemisphere.

"It would be interesting to see what we can see in the southern hemisphere," Huentemeyer said. "We will have a good view of the galactic center that we don't have in the northern hemisphere. SWGO could give us many more candidates in terms of star clusters."

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Scientists link star-shredding event to origins of universe's highest-energy particles

 A team of scientists has detected the presence of a high-energy neutrino -- a particularly elusive particle -- in the wake of a star's destruction as it is consumed by a black hole. This discovery, reported in the journal Nature Astronomy, sheds new light on the origins of Ultrahigh Energy Cosmic Rays -- the highest energy particles in the Universe.

The work, which included researchers from more than two dozen institutions, including New York University and Germany's DESY research center, focused on neutrinos -- subatomic particles that are produced on Earth only in powerful accelerators.

Neutrinos -- as well as the process of their creation -- are hard to detect, making their discovery, along with that of Ultrahigh Energy Cosmic Rays (UHECRs), noteworthy.

"The origin of cosmic high-energy neutrinos is unknown, primarily because they are notoriously hard to pin down," explains Sjoert van Velzen, one of the paper's lead authors and a postdoctoral fellow in NYU's Department of Physics at the time of the discovery. "This result would be only the second time high-energy neutrinos have been traced back to their source."

Previous research by van Velzen, now at the Netherlands' Leiden University, and NYU physicist Glennys Farrar, a co-author of the new Nature Astronomy paper, found some of the earliest evidence of black holes destroying stars in what are now known as Tidal Disruption Events (TDEs). These findings set the stage for determining if TDEs could be responsible for producing UHECRs.

The research reported in Nature Astronomy offered support for this conclusion.

Previously, the IceCube Neutrino Observatory, a National Science Foundation-backed detector located in the South Pole, reported the detection of a neutrino, whose path was later traced by the Zwicky Transient Facility at Caltech's Palomar Observatory.

Specifically, its measurements showed a spatial coincidence of a high-energy neutrino and light emitted after a TDE -- a star consumed by a black hole.

"This suggests these star shredding events are powerful enough to accelerate high-energy particles," van Velzen explains.

"Discovering neutrinos associated with TDEs is a breakthrough in understanding the origin of the high-energy astrophysical neutrinos identified by the IceCube detector at the South Pole whose sources have so far been elusive," adds Farrar, who proposed in a 2009 paper that UHECRs could be accelerated in TDEs. "The neutrino-TDE coincidence also sheds light on a decades old problem: the origin of Ultrahigh Energy Cosmic Rays."

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