Tag: cosmic rays

• Tracking down energetic cosmic rays

  Once in a while, nature runs experiments that no human lab can match. Ultra-high-energy cosmic rays are a good example.

  Cosmic rays are fast, high energy particles from space that strike Earth’s atmosphere with energies far beyond what even the most powerful particle accelerators can routinely create. Most particles are the nuclei of atoms such as hydrogen or helium and a smaller fraction are electrons.

  When a cosmic-ray particle hits the atmosphere, it triggers a large ‘shower’ of particles spread over many square-kilometres. (One such shower may have caused a serious error in a computer onboard an Airbus A320 aircraft on October 30, causing it to suddenly drop 100 feet, injuring several people onboard.) By observing and measuring these showers, scientists hope to probe two big questions at once: what kinds of cosmic events accelerate matter to such extreme energies, and what happens to particle interactions at energies we can’t otherwise test.

  The Pierre Auger Observatory in Argentina is one of the world’s main instruments for this work. Its latest results, published in Physical Review Letters on December 9, focus on a curiously simple idea: does the energy spectrum of these cosmic rays — i.e. how many particles arrive at each energy — look the same from every direction of the sky?

  In the new study, the Pierre Auger Collaboration analysed data with three notable features. First, the Observatory recorded the spectrum above 2.5 EeV. One EeV is 10¹⁸ electron-volt (eV), a unit of energy applied to subatomic particles. Second, it recorded this spectrum across a wide declination range, from +44.8º to −90º. In this range, +90º is the celestial north pole and –90º is the celestial south pole. And third, the analysis included around 3.1 lakh events collected between 2004 and 2022.

  The direction in which a cosmic ray is coming from carries information about its origins. If, say, a handful of galaxies or starburst regions are responsible for the highest energy cosmic rays, then the spectrum should show a bump in that part from the sky, and nowhere else.

  Understanding the direction from which the most powerful cosmic rays come has become more important since the Collaboration found evidence before that these directions aren’t perfectly uniform. Above 8 EeV, for instance, the Observatory has reported a modest but clear imbalance across the sky. It has also sensed a similarly modest link between extremely energetic cosmic rays and specific parts of space.

  Against this backdrop, the new study is part of a larger effort to elevate ultra-high-energy cosmic rays from a curiosity into a way to map the location of ‘cosmic accelerators’ in the nearby universe.

  This is like the trajectory that neutrino astronomy has taken as well. For much of the 20th century, neutrinos frustrated physicists because they knew the particles were the universe’s second-most abundant, yet they remain extremely difficult to catch and study. Because neutrinos carry no electric charge and interact only weakly with matter, they pass through stars and magnetic fields almost untouched, making them unusually honest witnesses to violent places in the universe. However, physicists could take advantage of that only if they could build suitable detectors. And step by step that’s what happened. Today, experiments like IceCube in Antarctica realise neutrino astronomy: a way to study the universe using neutrinos. (Francis Halzen’s long push for this detector is why he’s been awarded the APS Medal for 2026.)

  Cosmic rays stand on the cusp of a similar opportunity. To this end, the Pierre Auger Collaboration had to find whether the spectrum is dependent or independent of direction. A direction-independent spectrum would push the field towards models in which different types of cosmic sources produce high-energy cosmic rays; a direction-dependent spectrum would do the opposite.

  The new result was firm: across declinations from −90° to +44.8°, the team didn’t find a meaningful change in the spectrum’s shape.

  Cosmic ray researchers read the energy spectrum as a sort of forensic record. Over many decades, experiments have shown that the spectrum doesn’t increase smoothly. If you plotted the energy on the graph, in other words, you wouldn’t see a smooth curve. Instead you’d see the curve bending and changing how steep it is in places (see image below). These bumps reflect changes in the sources of cosmic rays, in the chemical makeup of the particles (protons v. nuclei), and how cosmic rays lose energy as they travel intergalactic space before reaching Earth.

  The Collaboration’s new paper framed its findings in the context of two recent developments.

  First, above 8 EeV, cosmic rays aren’t arriving in perfectly random directions around Earth. Instead roughly one half of the sky supplies 6% more such cosmic rays. Physicists have interpreted this to mean the distribution is being shaped by large-scale structure in the nearby universe, by magnetic fields or by both.

  Second, the Collaboration previously identified evidence for an ‘instep’ feature near 10 EeV.

  

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  If you look at the energy curve (shown above), you’ll notice a shift in slope at three points. Top to bottom, they’re called the ‘knee’, the ‘ankle’, and the ‘instep’. At each of these points, physicists believe, the set of physical effects producing the cosmic rays changes.

  (LHAASO, a large detector array in China built to catch the particle showers made by very energetic gamma rays, recently reported signs that microquasar jets — where a stellar-mass black hole pulls in gas from a companion star and emits fast beams of radiation — could be accelerating cosmic rays to near the knee part of the spectrum.)

  A spectrum whose shape changes depending on the direction is a way to connect these two aspects. If the instep is due to a small number of nearby, unusually strong sources, you might expect it to show up more strongly in the part of the sky where those sources are located. If the instep is a generic feature produced by many broadly similar sources, it should appear in essentially the same way across the sky (after accounting for the modest unevenness of the dipole). In the new study, the Collaboration tried to  make that distinction sharper.

  The Pierre Auger Observatory detects showers of particles in the atmosphere with an array of water tanks spread over a large area. Showers arriving near-vertically and those arriving at an angle closer to the horizon behave differently because Earth’s magnetic field distorts their paths. So the analysis used different methods to reconstruct the cosmic rays based on their showers for angles up to 60º (vertical) and from 60º to 80º (inclined). Scientists inferred the energy in the rays based on the properties of the particles in the shower.

  In the declination range −90º and +44.8º, the Observatory found the spectrum didn’t vary significantly with declination, with or without the dipole. In other words, once the Collaboration accounted for the disuniform intensity in the sky, the spectrum’s shape didn’t change depending on the direction.

  The second major result was the ‘instep’, with the findings reinforcing previous evidence for this feature.

  Now, if the instep was mainly caused by a small number of nearby sources, it would be reasonable to expect the spectrum would change depending on the declination. But the study found the spectrum to be indistinguishable across declinations. This in turn, per the Collaboration, disfavours the possibility of a small number of nearby sources contributing ultra-high-energy cosmic rays. Instead, the spectrum’s key features could be set by many sources and/or effects acting together.

  The paper also suggested that the spectrum steepening near the instep could be due to a change in what the cosmic rays are made of: from lighter nuclei like helium to heavier nuclei like carbon and oxygen. If this bend in the curve is really due to a change in the cosmic rays’ composition, rather than in their sources, then cosmic rays coming from all directions should have this feature. And this is what the Pierre Auger Collaboration has reported: the spectrum’s shape doesn’t change by direction.

  According to the paper’s authors, because the spectrum looks the same in different parts of the sky, the next clues to cracking the mystery of cosmic rays’ origins need to come from tests that measure their composition more clearly, helping to explain the instep.

  Featured image: A surface detector tank of the Pierre Auger Observatory in 2007. Credit: Public domain.

  2025.12.13

• Challenging the neutrino signal anomaly

  A gentle reminder before we begin: you’re allowed to be interested in particle physics. 😉

  Neutrinos are among the most mysterious particles in physics. They are extremely light, electrically neutral, and interact so weakly with matter that trillions of them pass through your body each second without leaving a trace. They are produced in the Sun, nuclear reactors, the atmosphere, and by cosmic explosions. In fact neutrinos are everywhere — yet they’re almost invisible.

  Despite their elusiveness, they have already upended physics. In the late 20th century, scientists discovered that neutrinos can oscillate, changing from one type to another as they travel, which is something that the simplest version of the Standard Model of particle physics — the prevailing theory of elementary particles — doesn’t predict. Because oscillations require neutrinos to have mass, this discovery revealed new physics. Today, scientists study neutrinos for what they might tell us about the universe’s structure and for possible hints of particles or forces yet unknown.

  

  However, detecting neutrinos is very hard. Because they rarely interact with matter, experiments must build massive detectors filled with dense material in the hopes that a small fraction of neutrinos will collide inside with atoms. One way to detect such collisions uses Cherenkov radiation, a bluish glow emitted when a charged particle moves through a medium like water or mineral oil faster than light does in that medium.

  (This is allowed. The only speed limit is that of light in vacuum: 299,792,458 m/s.)

  The MiniBooNE experiment at Fermilab used a large mineral-oil Cherenkov detector. When neutrinos from the Booster Neutrino Beamline struck the atomic nuclei in the mineral oil, the interaction released charged particles, which sometimes produced rings of Cherenkov radiation (like ripples) that the detector recorded. In MiniBooNE’s data, the detection events were classified by the type of light ring produced. An “electron-like” event was one that looked like it had been caused by an electron. But because photons can also produce nearly identical rings when they strike the nuclei, the detector couldn’t always tell the difference. A “muon-like” event, on the other hand, had the distinctive ring pattern of a muon, which is a subatomic particle like the electron but 200-times heavier, and which travels in a straighter, longer track. To be clear, these labels described the detector’s view; they didn’t  guarantee which particle was actually present.

  MiniBooNE began operating in 2002 to test an anomaly that had been reported at the LSND experiment at Los Alamos. LSND had recorded more electron-like” events than predicted, especially at low energies below about 600 MeV. This came to be called the “low-energy excess” and has become one of the most puzzling results in particle physics. It raised the possibility that neutrinos might be oscillating into a hitherto unknown neutrino type, sometimes called the sterile neutrino — or it might have been a hint of unexpected processes that produced extra photons. Since MiniBooNE couldn’t reliably distinguish electrons from photons, the mystery remained unresolved.

  To address this, scientists built the MicroBooNE experiment at Fermilab. It uses a very different technology: the liquid argon time-projection chamber (LArTPC). In a LArTPC, charged particles streak through an ultra-pure mass of liquid argon, leaving a trail of ionised atoms in their wake. An applied electric field causes these trails to drift towards fine wires, where they are recorded. At the same time, the argon emits light that provides the timing of the interaction. This allows the detector to reconstruct interactions in three dimensions with millimetre precision. Crucially, it lets physicists see where the particle shower begins, so they can tell whether it started at the interaction point or some distance away. This capability prepared MicroBooNE to revisit the “low-energy excess” anomaly.

  MicroBooNE also had broader motivations. With an active mass of about 90 tonnes of liquid argon inside a 170-tonne cryostat, and 8,256 wires in its readout planes, it was the largest LArTPC in the US when it began operating. It served as a testbed for the much larger detectors that scientists are developing for the Deep Underground Neutrino Experiment (DUNE). And it was also designed to measure the rate at which neutrinos interacted with argon atoms, to study nuclear effects in neutrino scattering, and to contribute to searches for rare processes such as proton decay and supernova neutrino bursts.

  (When a star goes supernova, it releases waves upon waves of neutrinos before it releases photons. Scientists were able to confirm this when the star Sanduleak -69 202 exploded in 1987.)

  

  Initial MicroBooNE analyses using partial data already challenged the idea that MiniBooNE’s excess was due to the anomaly. However, the collaboration didn’t cover the full range of parameters until recently. On August 21, MicroBooNE published results from five years of operations, corresponding to 1.11 x 10²¹ protons on target, which was about a 70% increase over previous analyses. This complete dataset together with higher sensitivity and better modelling has provided the most decisive test so far of the anomaly.

  The MicroBooNE detector recorded neutrino interactions from the Booster Neutrino Beamline, a setup that produces neutrinos, using its LArTPC detector, which operated at about 87 K inside a cryostat. Charged particles from neutrino interactions produced ionisation electrons that drifted across the detector and were recorded by the wire. Simultaneous flashes of argon scintillation light, seen by photomultiplier tubes, gave the precise time of each interaction.

  In neutrino physics, a category of events grouped by what the detector sees in the final state is called a channel. Researchers call it a signal channel when it matches the kind of event they are specifically looking for, as opposed to background signals from other processes. With MicroBooNE, the team stayed on the lookout for two signal channels: (i) one electron and no visible protons or pions (abbreviated as 1e0p0π) and (ii) one electron and at least one proton above 40 MeV (1eNp0π). These categories reflect what MiniBooNE would’ve seen as electron-like events while exploiting MicroBooNE’s ability to identify protons.

  One important source of background noise the team had to cut from the data was cosmic rays — high-energy particles from outer space that strike Earth’s atmosphere, creating particle showers that can mimic neutrino signals. In 2017, MicroBooNE added a suite of panels around the detector. For the full dataset, the panels cut an additional 25.4% of background noise in the 1e0p0π channel while preserving 98.9% of signal events.

  

  In the final analysis, the MicroBooNE data showed no evidence of an anomalous excess of electron-like events. When both channels were combined, the observed events matched the expectations of the Standard Model of particle physics well. The agreement was especially strong in the 1e0p0π channel.

  In the 1eNp0π channel, MicroBooNE actually detected slightly fewer events than the Model predicted: 102 events v. 134. This shortfall, of about 24%, is however not enough to claim a new effect but enough to draw attention. But rather than confirming MiniBooNE’s excess, this result suggests there’s some tension in the models the scientists use to simulate how the neutrinos and argon atoms will interact. Argon has a large and complex nucleus, which makes accurate predictions challenging. The scientists have in fact stated in their paper that the deficit may reflect these uncertainties rather than new physics.

  The new MicroBooNE results have far-reaching consequences. Foremost, the results reshape the sterile-neutrino debate. For two decades, the LSND and MiniBooNE anomalies had been cited together as signs that the neutrino was oscillating into a previously undetected state. By showing that MiniBooNE’s excess was not due to extra electron-like interactions, MicroBooNE shows that the ‘extra’ events were not caused by excess electron neutrinos. This in turn casts doubt on the simplest explanation, of sterile neutrinos.

  As a result, theoretical models that once seemed straightforward now face strong tension. While more complex scenarios remain possible, the easy explanation is no longer viable.

  

  Second, they demonstrate the maturity of the LArTPC technology. The MicroBooNE team successfully operated a large detector for years, maintaining the argon’s purity and low-noise electronics required for high-resolution imaging. Its performance validates the design choices for larger detectors like DUNE, which use similar technology but at kilotonne scales. The experiment also showcases innovations such as cryogenic electronics, sophisticated purification systems, protection against cosmic rays, and calibration with ultraviolet lasers, proving that such systems can deliver reliable data over long periods of operation.

  Third, the modest deficit in the 1eNp0π channel points to the importance of better understanding neutrino-argon interactions. Argon’s heavy nucleus produces complicated final states where protons and neutrons may scatter or be absorbed, altering the visible event. These nuclear effects can lead to mismatches between simulation and data (possibly including the 24% deficit in the 1eNp0π signal channel). For DUNE, which will also use argon as its target, improving these models is critical. MicroBooNE’s detailed datasets and sideband constraints will continue to inform these refinements.

  Fourth, the story highlights the value of complementary detector technologies. MiniBooNE’s Cherenkov detector recorded more events but couldn’t tell electrons from photons; MicroBooNE’s LArTPC recorded fewer events but with much greater clarity. Together, they show how one experiment can identify a puzzle and another can test it with a different method. This multi-technology approach is likely to continue as experiments worldwide cross-check anomalies and precision measurements.

  Finally, the MicroBooNE results show how science advances. A puzzling anomaly inspired new theories, new technology, and a new experiment. After five years of data-taking and with the most complete analysis yet, MicroBooNE has said that the MiniBooNE anomaly was not due to electron-neutrino interactions. The anomaly itself remains unexplained, but the field now has a sharper focus. Whether the cause lies in photon production, detector effects or actually new physics, the next generation of experiments can start on firmer footing.

  2025.09.06

• A new source of cosmic rays?

  The International Space Station carries a suite of instruments conducting scientific experiments and measurements in low-Earth orbit. One of them is the Alpha Magnetic Spectrometer (AMS), which studies antimatter particles in cosmic rays to understand how the universe has evolved since its birth.

  Cosmic rays are particles or particle clumps flying through the universe at nearly the speed of light. Since the mid-20th century, scientists have found cosmic-ray particles are emitted during supernovae and in the centres of galaxies that host large black holes. Scientists installed the AMS in May 2011, and by April 2021, it had tracked more than 230 billion cosmic-ray particles.

  When scientists from the Massachusetts Institute of Technology (MIT) recently analysed these data — the results of which were published on June 25 — they found something odd. Roughly one in 10,000 of the cosmic ray particles were neutron-proton pairs, a.k.a. deuterons. The universe has a small number of these particles because they were only created in a 10-minute-long period a short time after the universe was born, around 0.002% of all atoms.

  Yet cosmic rays streaming past the AMS seemed to have around 5x greater concentration of deuterons. The implication is that something in the universe — some event or some process — is producing high-energy deuterons, according to the MIT team’s paper.

  Before coming to this conclusion, the researchers considered and eliminated some alternative explanations. Chief among them is the way scientists know how deuterons become cosmic rays. When primary cosmic rays produced by some process in outer space smash into matter, they produce a shower of energetic particles called secondary cosmic rays. Thus far, scientists have considered deuterons to be secondary cosmic rays, produced when helium-4 ions smash into atoms in the interstellar medium (the space between stars).

  This event also produces helium-3 ions. So if the deuteron flux in cosmic rays is high, and if we believe more helium-4 ions are smashing into the interstellar medium than expected, the AMS should have detected more helium-3 cosmic rays than expected as well. It didn’t.

  To make sure, the researchers also checked the AMS’s instruments and the shared properties of the cosmic-ray particles. Two in particular are time and rigidity. Time deals with how the flux of deuterons changes with respect to the flux of other cosmic ray particles, especially protons and helium-4 ions. Rigidity measures the likelihood a cosmic-ray particle will reach Earth and not be deflected away by the Sun. (Equally rigid particles behave the same way in a magnetic field.) When denoted in volts, rigidity indicates the extent of deflection the particle will experience.

  The researchers analysed deuterons with rigidity from 1.9 billion to 21 billion V and found that “over the entire rigidity range the deuteron flux exhibits nearly identical time variations with the proton, 3-He, and 4-He fluxes.” At rigidity greater than 4.5 billion V, the fluxes of deuterons and helium-4 ions varied together whereas those of helium-3 and helium-4 didn’t. At rigidity beyond 13 billion V, “the rigidity dependence of the D and p fluxes [was] nearly identical”.

  Similarly, they found the change in the deuteron flux was greater than the change in the helium-3 flux, both relative to the helium-4 flux. The statistical significance of this conclusion far exceeded the threshold particle physicists use to check whether an anomaly in the data is really real rather than the result of some fluke error. Finally, “independent analyses were performed on the same data sample by four independent study groups,” the paper added. “The results of these analyses are consistent with this Letter.”

  The MIT team ultimately couldn’t find a credible alternative explanation, leaving their conclusion: deuterons could be primary cosmic rays, and we don’t (yet) know the process that could be producing them.

  2024.06.28

• The search for a powerful natural particle accelerator

  Earth is almost constantly beset by a stream of particles from space called cosmic rays. These particles consist of protons, bundles of two protons and two neutrons each (alpha particles), a small number of heavier atomic nuclei and a smaller fraction of anti-electrons and anti-protons. Cosmic rays often have high energy – typically up to half of 1 GeV. One GeV is almost the amount of energy that a single proton has at rest. The Large Hadron Collider (LHC) itself can accelerate protons up to 7,000 GeV.

  But this doesn’t mean cosmic rays are feeble: historically, some detectors have recorded high-energy and very-high-energy cosmic rays. The most energetic cosmic ray – dubbed the “oh my god” particle – was a proton recorded over Utah in 1991 with an energy of around 3 x 10¹² GeV, which is around three-billion-times higher than the energy to which the LHC can accelerate protons today. This proton was travelling at 99.9% the speed of light in vacuum. This is a phenomenal amount of energy – about as much kinetic energy as a baseball moving at 95 km/hr but concentrated into the volume of a proton, which has 10⁴²-times less space in which to hold that energy.

  Detectors have also spotted some cosmic-ray events with energies exceeding 1,000,000 GeV – or 1 PeV. They’re uncommon compared to all cosmic-ray events but relatively more common than the likes of the “oh my god” particle. Physicists are interested in them because they indicate the presence of a natural particle accelerator somewhere in the universe that’s injecting protons with ginormous amounts of energy and sending them blasting off into space. One term for such natural accelerators seemingly capable of accelerating protons to 0.1-1 PeV is ‘PeVatron’. And the question is: where can we find a PeVatron?

  There are three broad sources of cosmic rays: from the Sun, from somewhere in the Milky Way galaxy and from somewhere beyond the galaxy. Most of the cosmic rays we have detected have been from the latter two sources. In fact, there’s a curious feature called the ‘knee’ that physicists believe could distinguish between these sources. If you plot the number of cosmic rays on the y-axis and the energies of the cosmic rays on the x-axis, you’ll find yourself looking at the famous Swordy plot:

  

  As you can see, the plot shows a peculiar bump, an almost imperceptible change in slope, when transitioning from the blue to the pink zones – this is the ‘knee’. Physicists have interpreted the cosmic rays above the knee to be from within the Milky Way and those below to be from outside the galaxy, although why this is so isn’t clear.

  Before cosmic rays interact with other particles in their way, they’re called primary cosmic rays. After their interaction, such as the atoms and molecules in Earth’s upper atmosphere, they produce a shower of secondary particles; these are the secondary cosmic rays. Physicists can get a tighter fit on the potential source of primary cosmic rays by analysing the direction at which they strike the atmosphere, the composition of the secondary cosmic rays, and the energies of both the primary and the secondary rays. This is why we suspect supernovae are one source of within-the-galaxy cosmic rays, with some possible mechanisms of action.

  One, for example, is shockfront acceleration: a proton could get trapped between two shockwaves from the same supernova. As the outer wave slows and the inner wave charges in, the proton could bounce rapidly between the two shockfronts and emerge greatly energised out of a gap. However, we don’t know what fraction of cosmic rays, at different energies, supernovae can account for.

  Potential extragalactic sources include active galactic nuclei – the centres of galaxies, including the neighbourhood of supermassive black holes – and the extremely powerful gamma-ray bursts. Physicists have associated them with cataclysmic events like neutron-star mergers and the formative events of black-holes.

  However, exercises to triangulate the sources of high-energy cosmic rays are complicated by galactic magnetic fields (which curve the paths of charged particles). A proton accelerated by the shockfront mechanism could also bump into some other particle as it emerges, producing a flash of gamma rays that physicists can look for – but only if they have a way to isolate it from other sources of gamma rays in a supernova’s vicinity. This is difficult work.

  Researchers from the US recently analysed gamma-ray data collected by the Fermi Gamma-ray Space Telescope (FGST), in low-Earth orbit, of the supernova remnant G106.3+2.7. Astrophysicists have suspected that this object could be a PeVatron for more than a decade, and the US research team used FGST data to check if they the suspicion could be true. The difficult bit? The data spanned 12 years.

  In 2008, physicists recorded very high energy (100-100,000 GeV) gamma rays from G106.3+2.7, located around 800 parsec (2,600 lightyears) away. The US research team figured that they could have been produced in two ways. Let’s call them Mechanism A and Mechanism B. Physicists already know Mechanism A is associated with cosmic rays while Mechanism B is not. The US team members used 12 years of data to characterise gamma-ray, X-ray and radio emissions around the remnant so they could determine which mechanism could have been responsible for all of them the way they have been observed, with the gamma rays as secondary cosmic rays.

  The team’s analysis found that the theory of Mechanism A almost exactly accounted for the energies of the gamma rays from the remnant while also accommodating the other radiation – whereas the theory of Mechanism B couldn’t explain the gamma rays and the remnant’s X-ray emissions together. In effect, the team had a way to justify the idea that G106.3+2.7 could be a PeVatron.

  Mechanism B is inverse Compton scattering by relativistic electrons. Inverse Compton scattering is when high-energy electrons collide with low-energy photons and the photons gain energy (in regular Compton scattering, the electrons gain energy). When this model couldn’t account for the gamma-ray emissions, the team invoked a modified version involving two sets of electrons, with each set accelerated to different energies by different mechanisms. But the team found that the FGST data continued to disfavour the involvement of leptons, and instead preferred the involvement of hadrons. Leptons – like electrons – are particles that don’t interact with other particles through the strong nuclear force. Hadrons, on the other hand, do, and they were implicated in Mechanism A: the decay of neutral pions.

  Pions are the lightest known hadrons and come in three types: π⁺, π⁰ and π^–. Neutral pions are π⁰. They have a very short lifetime, around 85 attoseconds – that’s 0.000000000000000085 seconds. And when they decay, they decay into gamma rays, i.e. high-energy photons.

  Some 380,000 years after the Big Bang, a series of events in the universe left behind some radiation that survives to this day. This relic radiation is called the cosmic microwave background, a sea of photons in the microwave frequency pervading the cosmos. When a cosmic-ray proton collides with one of these photons, a delta-plus baryon is formed that then decays into a proton and a neutral pion. The neutral pion then decays to gamma rays, which are detectable as secondary cosmic rays.

  

  Knowing the energy of the gamma rays allows physicists to work back to the energy of the cosmic ray. And according to the team’s calculations, the 2009 gamma-ray emission indicates G106.3+2.7 could be a PeVatron. As the team’s preprint paper concluded,

  “… only a handful, out of hundreds of radio-emitting supernova remnants, have been observed to emit very high energy radiation with a hard spectrum. The scarcity of PeVatron candidates and the rareness of remnants with very high energy emission make … G106.3+2.7 a unique source. Our study provides strong evidence for proton acceleration in this nearby remnant, and by extension, supports a potential role for G106.3+2.7-like supernova remnants in meeting the challenge of accounting for the observed cosmic-ray knee using galactic sources”.

  Featured image: An artist’s impression of supernova 1993J. Credit: NASA, ESA and G. Bacon (STScI).

  2022.08.29

• O Voyager, where art thou?

  On September 5, 1977, NASA launched the Voyager 1 space probe to study the Jovian planets Jupiter and Saturn, and their moons, and the interstellar medium, the gigantic chasm between various star-systems in the universe. It’s been 35 years and 9 months, and Voyager has kept on, recently entering the boundary between our System and the Milky Way.

  In 2012, however, when nine times farther from the Sun than is Neptune, the probe entered into a part of space completely unknown to astronomers.

  On June 27, three papers were published in Science discussing what Voyager 1 had encountered, a region at the outermost edge of the Solar System they’re calling the ‘heliosheath depletion region’. They think it’s a feature of the heliosphere, the imagined bubble in space beyond whose borders the Sun has no influence.

  “The principal result of the magnetic field observations made by our instrument on Voyager is that the heliosheath depletion region is a previously undetected part of the heliosphere,” said Dr. Leonard Burlaga, an astrophysicist at the NASA-Goddard Space Flight Centre, Maryland, and an author of one of the papers.

  “If it were the region beyond the heliosphere, the interstellar medium, we would have expected a change in the magnetic field direction when we crossed the boundary of the region. No change was observed.”

  More analysis of the magnetic field observations showed that the heliosheath depletion region has a weak magnetic field – of 0.1 nano-Tesla (nT), 0.6 million times weaker than Earth’s – oriented in such a direction that it could only have arisen because of the Sun. Even so, this weak field was twice as strong as what lay outside it in its vicinity. Astronomers would’ve known why, Burlaga clarifies, if it weren’t for the necessary instrument on the probe being long out of function.

  When the probe crossed over into the region, this spike in strength was recorded within a day. Moreover, Burlaga and others have found that the spike happened thrice and a drop in strength twice, leaving Voyager 1 within the region at the time of their analysis. In fact, after August 25, 2012, no drops have been recorded. The implication is that it is not a smooth region.

  “It is possible that the depletion region has a filamentary character, and we entered three different filaments. However, it is more likely that the boundary of the depletion region was moving toward and away from the sun,” Burlaga said.

  The magnetic field and its movement through space are not the only oddities characterising the heliosheath depletion region. Low-energy ions blown outward by the Sun constantly emerge out of the heliosphere, but they were markedly absent within the depletion region. Burlaga was plainly surprised: “It was not predicted or even suggested.”

  Analysis by Dr. Stamatios Krimigis, the NASA principal investigator for the Low-Energy Charged Particle (LECP) experiment aboard Voyager 1 and an author of the second paper, also found that cosmic rays, which are highly energised charged particles produced by various sources outside the System through unknown mechanisms, weren’t striking Voyager’s detectors equally from all directions. Instead, more hits were being recorded in certain directions inside the heliosheath depletion region.

  Burlaga commented, “The sharp increase in the cosmic rays indicate that cosmic rays were able to enter the heliosphere more readily along the magnetic fields of the depletion region.”

  Even though Voyager 1 was out there, Krimigis feels that humankind is blind: astronomers’ models were, are, clearly inadequate, and there is no roadmap of what lies ahead. “I feel like Columbus who thought he had gotten to West India, when in fact he had gone to America,” Krimigis contemplates. “We find that nature is much more imaginative than we are.”

  With no idea of how the strange region originated or whence, we’ll just have to wait and see what additional measurements tell us. Until then, the probe will continue approaching the gateway to the Galaxy.

  —

  This blog post, as written by me, first appeared in The Hindu‘s science blog on June 29, 2013.

  2013.06.29
• 

  On September 5, 1977, NASA launched the Voyager 1 space probe to study the Jovian planets Jupiter and Saturn, and their moons, and the interstellar medium, the gigantic chasm between various star-systems in the universe. It’s been 35 years and 9 months, and Voyager has kept on, recently entering the boundary between our System and the Milky Way.

  In 2012, however, when nine times farther from the Sun than is Neptune, the probe entered into a part of space completely unknown to astronomers.

  On June 27, three papers were published in Science discussing what Voyager 1 had encountered, a region at the outermost edge of the Solar System they’re calling the ‘heliosheath depletion region’. They think it’s a feature of the heliosphere, the imagined bubble in space beyond whose borders the Sun has no influence.

  “The principal result of the magnetic field observations made by our instrument on Voyager is that the heliosheath depletion region is a previously undetected part of the heliosphere,” said Dr. Leonard Burlaga, an astrophysicist at the NASA-Goddard Space Flight Centre, Maryland, and an author of one of the papers.

  “If it were the region beyond the heliosphere, the interstellar medium, we would have expected a change in the magnetic field direction when we crossed the boundary of the region. No change was observed.”

  More analysis of the magnetic field observations showed that the heliosheath depletion region has a weak magnetic field – of 0.1 nano-Tesla (nT), 0.6 million times weaker than Earth’s – oriented in such a direction that it could only have arisen because of the Sun. Even so, this weak field was twice as strong as what lay outside it in its vicinity. Astronomers would’ve known why, Burlaga clarifies, if it weren’t for the necessary instrument on the probe being long out of function.

  When the probe crossed over into the region, this spike in strength was recorded within a day. Moreover, Burlaga and others have found that the spike happened thrice and a drop in strength twice, leaving Voyager 1 within the region at the time of their analysis. In fact, after August 25, 2012, no drops have been recorded. The implication is that it is not a smooth region.

  “It is possible that the depletion region has a filamentary character, and we entered three different filaments. However, it is more likely that the boundary of the depletion region was moving toward and away from the sun,” Burlaga said.

  The magnetic field and its movement through space are not the only oddities characterising the heliosheath depletion region. Low-energy ions blown outward by the Sun constantly emerge out of the heliosphere, but they were markedly absent within the depletion region. Burlaga was plainly surprised: “It was not predicted or even suggested.”

  Analysis by Dr. Stamatios Krimigis, the NASA principal investigator for the Low-Energy Charged Particle (LECP) experiment aboard Voyager 1 and an author of the second paper, also found that cosmic rays, which are highly energised charged particles produced by various sources outside the System through unknown mechanisms, weren’t striking Voyager’s detectors equally from all directions. Instead, more hits were being recorded in certain directions inside the heliosheath depletion region.

  Burlaga commented, “The sharp increase in the cosmic rays indicate that cosmic rays were able to enter the heliosphere more readily along the magnetic fields of the depletion region.”

  Even though Voyager 1 was out there, Krimigis feels that humankind is blind: astronomers’ models were, are, clearly inadequate, and there is no roadmap of what lies ahead. “I feel like Columbus who thought he had gotten to West India, when in fact he had gone to America,” Krimigis contemplates. “We find that nature is much more imaginative than we are.”

  With no idea of how the strange region originated or whence, we’ll just have to wait and see what additional measurements tell us. Until then, the probe will continue approaching the gateway to the Galaxy.

  (This blog post first appeared on The Copernican on June 28, 2013.)

  2013.06.28