The Large-Sized Telescopes, or LSTs, do not observe gamma rays directly. Instead, they are built to catch the fleeting blue flash of Cherenkov light, the ephemeral glow emitted when a very high-energy gamma ray strikes Earth’s atmosphere and triggers a cascade of relativistic particles traveling faster than light moves through air. Each telescope’s 23-meter-diameter reflector funnels this nanosecond-scale flash onto an ultra-fast camera, allowing astronomers to reconstruct the direction and energy of the original gamma ray. The LSTs are the most sensitive instruments in the world for observing gamma rays at energies between approximately 20 gigaelectronvolts and several teraelectronvolts, a range that covers the transition between what space-based gamma-ray telescopes can see and what ground-based instruments excel at detecting.

That exceptional sensitivity at the lowest gamma-ray energies is what sets the LSTs apart. As very high-energy gamma rays travel across the Universe, they are progressively absorbed by the diffuse fog of optical and infrared photons that fills intergalactic space, the so-called extragalactic background light. The farther away a source lies, the more its gamma rays are attenuated before they reach Earth. The LSTs’ low energy threshold allows scientists to compensate for this absorption by detecting the faintest, lowest-energy gamma rays that still manage to survive the journey, opening a window onto sources at cosmological distances and letting researchers look further back into cosmic history than any previous ground-based gamma-ray facility.

The sub-array’s design also makes it uniquely suited to studying transient phenomena. Many of the most dramatic gamma-ray sources, including gamma-ray bursts, novae, and flaring active galactic nuclei, evolve on timescales of minutes or seconds. The LSTs’ sensitivity to short variability, combined with the statistical power gained from observing the same gamma-ray shower simultaneously from multiple telescopes, gives astronomers the temporal and spectral resolution needed to dissect these rapidly changing events with unprecedented accuracy.

The inauguration crowns a productive history for the project’s prototype. Even during its commissioning phase, the first Large-Sized Telescope, LST-1, has delivered groundbreaking scientific results since 2020. Its observations have included detailed studies of pulsars, the rapidly rotating stellar corpses whose beams of radiation sweep across Earth like cosmic lighthouses, as well as investigations of active galactic nuclei at cosmological distances and rapid follow-up observations of transient phenomena such as novae and gamma-ray bursts. These early results validated the telescope’s novel design and demonstrated that the instrument could achieve the lowest energy threshold ever reached by a ground-based gamma-ray observatory.

With LST-2, LST-3, and LST-4 now joining their pioneer sibling, the four telescopes will operate as a true array. When a gamma-ray shower strikes the atmosphere, each telescope records the Cherenkov flash from a slightly different vantage point. Combining these multiple perspectives substantially improves the array’s sensitivity, sharpens its angular resolution, and refines its energy reconstruction, opening a new level of precision in the study of the gamma-ray Universe. Stereoscopic observation also suppresses the background of cosmic-ray events that mimic gamma rays, a critical advantage when hunting for the faintest and most distant sources.

Professor Masahiro Teshima, Principal Investigator of the LST Collaboration and Spokesperson of the CTAO Consortium, emphasized the scientific promise of the completed sub-array. According to Teshima, the completion of the LST sub-array is the culmination of more than fifteen years of international collaboration and technological innovation, and the telescopes will allow scientists to explore some of the most extreme environments in the Universe, including supermassive black holes, neutron stars, the possible origins of cosmic rays, and the nature of dark matter. These targets represent some of the most fundamental open questions in modern astrophysics and particle physics, and the LSTs are designed to attack them from the low-energy frontier of the gamma-ray spectrum.

One of the most ambitious goals of the new sub-array is to reach back to the early Universe. The LSTs will provide a unique opportunity to probe the cosmos at an epoch roughly 12 billion years ago, when the Universe was less than two billion years old. Their sensitivity to the lowest-energy gamma rays will enable the detection and study of bright very high-energy gamma-ray bursts at high redshift, potentially extending gamma-ray observations to sources at redshifts of around z of approximately 4. Because gamma-ray bursts are among the most energetic explosions since the Big Bang, catching their high-energy emission from such vast distances would give cosmologists a new probe of star formation, black hole growth, and the physical conditions prevailing in the young Universe.

The LST sub-array is a key component of the broader Cherenkov Telescope Array Observatory, which will ultimately deploy more than 60 telescopes of three types across two hemispheres. In addition to the Large-Sized Telescopes, the observatory will operate Medium-Sized Telescopes and Small-Sized Telescopes, each optimized for a different portion of the enormous energy range from 20 GeV to 300 TeV. CTAO-North, at the Roque de los Muchachos Observatory on La Palma in Spain, and CTAO-South, at the Paranal Observatory in Chile’s Atacama Desert, together will give the observatory full-sky coverage. Over the coming years, the LSTs will undergo commissioning and integration into CTAO operations alongside the other telescopes planned for both arrays, addressing fundamental questions in astrophysics, particle physics, and cosmology, from the origin of cosmic rays to searches for deviations from Einstein’s theory of relativity.

The project’s scale extends beyond its optics. The CTAO is a Big Data undertaking that will generate hundreds of petabytes of data each year, roughly 12 petabytes after compression, and it is committed to operating as the first gamma-ray observatory of its kind to function as an open, proposal-driven facility providing public access to its high-level science data and software products. The LST project itself was made possible by a strong international partnership involving institutions from Brazil, Bulgaria, the Czech Republic, Croatia, France, Germany, Italy, Japan, Poland, Spain, and Switzerland, while the CTAO ERIC, the organization in charge of construction and operation, includes members from across Europe along with the European Southern Observatory, with further countries engaged as Strategic Partners or Third Parties. As the four gleaming reflectors on La Palma begin their coordinated watch on the night sky, astronomers anticipate a decade in which the most violent, distant, and enigmatic events in the cosmos will be observed with a clarity never before achievable from the ground.

Subject of Research: Inauguration of the Large-Sized Telescope sub-array at the Cherenkov Telescope Array Observatory's northern site for very high-energy gamma-ray astronomy

Article Title: A new era in exploring the extreme universe: The large-sized telescope sub-array is inaugurated on CTAO-north

Article References: A new era in exploring the extreme universe: The large-sized telescope sub-array is inaugurated on CTAO-north. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: CTAO, Large-Sized Telescope, gamma-ray astronomy, Cherenkov light, La Palma, Roque de los Muchachos Observatory, gamma-ray bursts, active galactic nuclei, dark matter, cosmic rays, high-energy astrophysics, early Universe

The mystery at the heart of TeV halos lies in how their parent pulsars inject energy into their surroundings. Pulsars are rapidly spinning neutron stars whose rotation gradually slows, releasing enormous quantities of rotational kinetic energy. According to the HAWC measurements, Geminga and Monogem each convert roughly ten percent of their total spin-down power into ultra-relativistic electron-positron pairs. These charged particles then race outward through the interstellar medium, colliding with ambient starlight, dust emission, and the cosmic microwave background. Each such collision, an inverse Compton scattering event, boosts the low-energy photon to teraelectronvolt energies, painting the halo in gamma rays that ground-based telescopes can detect. The intensity of the emission thus serves as a calorimeter, revealing how efficiently the pulsar converts its fading spin into exotic matter.

Yet the most startling revelation from the halos concerns not the pulsars but the medium around them. Cosmic rays normally diffuse through the Galaxy relatively efficiently, with a diffusion coefficient of about 4 x 10^28 square centimeters per second. But the observed size of the Geminga and Monogem halos demands a diffusion coefficient near these sources that is hundreds of times smaller, around 10^26 square centimeters per second. Were diffusion as rapid near Geminga as it is in the general interstellar medium, the inverse Compton glow produced by 35-teraelectronvolt electrons would extend roughly 200 parsecs, spanning some 60 degrees on the sky, far beyond the modest two-degree extension actually measured by HAWC and its predecessor Milagro. Something in the immediate vicinity of these pulsars tampers with the propagation of cosmic rays, and no one yet knows exactly what or how.

Several physical pictures have been proposed to explain the suppressed diffusion, ranging from self-generated turbulence, in which the escaping electrons themselves stir up magnetic fluctuations that trap them, to two-zone models in which a slow-diffusion bubble surrounds the pulsar and gives way to normal interstellar conditions at some boundary radius. To discriminate among these scenarios, astronomers need far more precise measurements of both the energy spectrum and the angular distribution of halo gamma rays. This is precisely where the Cherenkov Telescope Array enters the picture. CTA, the flagship next-generation observatory for very-high-energy gamma-ray astronomy, will span an energy range from 20 gigaelectronvolts to 300 teraelectronvolts with an energy resolution better than ten percent and angular resolution far exceeding existing instruments. Its two arrays, CTA North with 4 large and 9 medium telescopes and CTA South with 4 large, 14 medium, and 37 small telescopes, will together survey the sky with unprecedented sensitivity.

In their study, the researchers built a detailed theoretical framework describing how electrons injected by a pulsar propagate and radiate. They modeled the pulsar as a point source emitting electrons with a power-law spectrum characterized by an index alpha and an exponential cutoff energy, and they tied the injection rate to the pulsar’s spin-down luminosity, which decays over a characteristic timescale tau governed by the braking index n. As the electrons stream outward, they lose energy through synchrotron radiation in the ambient 3-microgauss magnetic field and through inverse Compton scattering off three radiation fields: the cosmic microwave background, infrared dust emission, and optical starlight. At the highest energies, the scattering enters the Klein-Nishina regime, where the cross section is suppressed relative to the classical Thomson limit, a subtlety the authors treated explicitly in their calculations.

The diffusion itself was modeled as a two-zone structure, with a suppressed coefficient D0 within a halo radius rh transitioning to the standard interstellar value beyond. The team adopted a default set of parameters: alpha equals 1.8, D0 equals 10^26 square centimeters per second, an energy dependence parameter delta of 0.33, a braking index of 3, a spin-down timescale of 12 kiloyears, a halo radius of 30 parsecs, an electron cutoff of 500 teraelectronvolts, and a conversion efficiency eta of 0.25, appropriate for Geminga at its measured distance of 250 parsecs and age of 340 kiloyears. Varying each parameter in turn, they computed how the resulting gamma-ray spectrum and surface brightness profile would change, comparing their predictions against the existing measurements from HAWC and HESS. A striking pattern emerged: while the braking index and spin-down timescale leave almost no imprint on the high-energy spectrum or the brightness profile, the diffusion parameters, the injected spectral index, the cutoff energy, and the halo radius all leave measurable signatures, and some affect the morphology differently at different photon energies.

To translate these theoretical distinctions into concrete observational forecasts, the team used the publicly available Gammapy software together with the prod5 instrument response function for the CTA-North array. They simulated 50 hours of observation of Geminga, dividing a 5-degree by 5-degree field of view into angular bins of 0.05 degrees on a side and twenty logarithmically spaced energy bins between 0.03 and 100 teraelectronvolts. Each mock dataset contained both the predicted gamma-ray signal and the residual cosmic-ray background. The extended nature of the halo inevitably degrades CTA’s sensitivity compared to a point source of the same flux, but the simulation showed that the observatory would nevertheless measure the halo’s spectrum with such precision that it could easily distinguish between an injected spectral index of 1.5, 1.8, or 2.0, and could identify a low cutoff energy of 50 teraelectronvolts or an energy-independent diffusion scenario with delta equal to zero.

The forecast grew even more powerful when the authors combined spectral and spatial information. By converting the quality of fit between competing models into statistical significances, they found that after only 50 hours, CTA could distinguish a wide swath of currently viable TeV halo models at the coveted five-sigma level or beyond. Among the discriminated scenarios are models with a small halo radius of 10 parsecs, an inflated diffusion coefficient of 10^27 square centimeters per second, and several combinations of spectral and diffusion parameters that the gamma-ray spectrum alone cannot separate. Only a few degeneracies survive, such as that between the default model and one with an energy dependence delta of 0.5, and the measurements remain relatively insensitive to the precise values of the spin-down timescale and braking index.

The stakes of these measurements extend well beyond pulsar astrophysics. The confirmation that Geminga and Monogem channel roughly a tenth of their spin-down power into high-energy electrons and positrons lends strong support to the idea that pulsars are responsible for the puzzling excess of cosmic-ray positrons reported by the PAMELA and AMS-02 experiments, a long-standing anomaly in cosmic-ray physics. Moreover, because a pulsar’s radio beam is visible only to observers aligned with its magnetic axis, while its TeV halo glows isotropically, halos offer a way to discover the many pulsars whose radio beams never sweep across the Earth. Of the roughly 3400 pulsars catalogued to date, the vast majority were found at radio wavelengths, suggesting that a large hidden population awaits detection through its halo emission.

The study also leaves the door open to further complications. Recent observations by HAWC and LHAASO have hinted that the emission around Geminga may be anisotropic, potentially reflecting directional particle transport along ordered magnetic fields. Although Hooper and his colleagues restricted their analysis to spherically symmetric halo models, they note that CTA’s superior angular resolution makes it well suited to testing such asymmetric features in the future. As the arrays in both hemispheres come online and begin their surveys, the humble gamma-ray glow surrounding dead stars may transform from a curiosity into one of the sharpest probes available for the physics of cosmic-ray transport, the evolution of neutron stars, and perhaps even the origin of some of the most energetic particles ever detected at Earth.

Subject of Research: Forecasting the ability of the Cherenkov Telescope Array to probe cosmic-ray diffusion and particle injection in pulsar TeV halos

Article Title: Unraveling TeV halos with the Cherenkov Telescope Array

Article References: Hooper, D., Pinetti, E., & Sokolenko, A. (2026). Unraveling TeV halos with the Cherenkov Telescope Array. The European Physical Journal C, 86(9), Article 1048. https://doi.org/10.1140/epjc/s10052-026-16259-x

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16259-x

Keywords: TeV halos, pulsars, Cherenkov Telescope Array, Geminga, cosmic rays, gamma-ray astronomy, inverse Compton scattering, diffusion, HAWC, HESS, LHAASO, neutron stars

The core problem the team addresses is kinematic. Conventional direct detection experiments, such as XENON, LUX-ZEPLIN (LZ) and PandaX, watch for the tiny nuclear recoils produced when a dark matter particle from the Galactic halo smashes into an atomic nucleus in a detector. But halo dark matter moves at only about 220 kilometres per second, roughly a thousandth the speed of light. A particle weighing less than a gigaelectronvolt (GeV), far lighter than a proton, simply cannot deliver enough momentum to produce a recoil above the keV-scale thresholds of these instruments. The result is a vast, theoretically motivated wilderness of parameter space, the entire sub-GeV regime, where ordinary direct detection is essentially blind.

The cosmic-ray boosted dark matter framework, or CRDM, offers a way through. Relativistic cosmic rays pervade the Galaxy, streaming at energies vastly exceeding those of the sluggish halo particles. When a cosmic ray elastically scatters off a dark matter particle, it can transfer an enormous fraction of its kinetic energy, kicking the dark matter up to velocities approaching a tenth the speed of light. Such a boosted particle carries enough punch to deposit a detectable keV-scale recoil even in a large liquid xenon detector buried more than a kilometre underground. The idea, first formalised by Bringmann and Pospelov and by Ema and colleagues in 2019, has since matured into a serious probe, and the LZ collaboration itself has applied it to derive model-independent bounds on the dark matter-nucleon scattering cross section.

What the new study adds is particle-physics structure. Rather than treating the interaction as a featureless constant, the authors embed the scattering in four benchmark simplified models, in which a Dirac fermion dark matter particle communicates with nucleons through a mediator: a scalar, a vector, a pseudoscalar or an axial-vector boson. Scalar and vector exchange produces spin-independent scattering, which benefits from coherent enhancement across all the nucleons in a nucleus, scaling with the square of the atomic mass. Pseudoscalar and axial-vector exchange produces spin-dependent scattering, which lacks that enhancement and is best probed by detectors containing hydrogen, such as the Borexino neutrino experiment in Italy. For each model the team wrote down the full interaction Lagrangians, the differential cross sections including hadronic and nuclear form factors, and the resulting flux of boosted dark matter arriving at Earth.

The calculation chains together several layers of astrophysics and particle physics. Starting from the local interstellar spectra of cosmic-ray nuclei up to nickel (atomic number 28), the authors integrate over the scattering kinematics to obtain the differential flux of boosted dark matter, weighted by the local dark matter density of about 0.3 GeV per cubic centimetre and an effective propagation distance that encodes the Galactic dark matter distribution under a Navarro-Frenk-White halo profile. They then track the boosted particles as they descend through the atmosphere and kilometres of rock, losing energy through further elastic scattering, before computing the event rate they would generate in the detector target. Crucially, the mediator propagator factor, one over the square of the mediator mass plus the momentum transfer squared, is kept in full rather than approximated, which turns out to matter enormously for light mediators.

Using published exclusion curves from LZ, XENON1T, XENONnT, MiniBooNE and Borexino, the team derived constraints on both the scattering cross section and the underlying coupling product, the combination of the dark matter and nucleon couplings normalised by four pi, for mediator masses spanning from a millionth of a GeV up to 1 GeV. In the model-independent, constant cross section case, the interplay between detector depth and sensitivity produces a distinctive band of exclusion. Cross sections that are too small yield no detectable excess, while cross sections that are too large cause the boosted dark matter to scatter out in the Earth’s crust before it ever reaches the detector. Because LZ and XENON sit roughly 1400 metres underground while MiniBooNE lies just below the surface, the experiments are complementary: the shallow detector catches the very large cross sections that deep experiments cannot see, stitching together a much wider excluded region.

The most striking theoretical result concerns the momentum dependence of light mediators. When the mediator mass falls below roughly 10 MeV, the propagator forces the differential cross section to scale approximately as the inverse fourth power of the momentum transfer. Scattering is then overwhelmingly dominated by small momentum transfers, pushing the recoil spectrum to energies right at the threshold of detection. In this regime LZ, with its exceptionally low energy threshold and the coherence boost of spin-independent interactions, delivers the strongest constraints. For heavier mediators the low-momentum enhancement disappears, sensitivity becomes statistics-driven, and high-exposure neutrino experiments such as Borexino become competitive. The authors also show that the common contact-interaction approximation, which assumes the amplitude scales as the inverse fourth power of the mediator mass, breaks down badly once the mediator mass drops below the characteristic momentum transfer of the recoil, rendering dashed-line approximations unreliable exactly where the new physics is most interesting.

Perhaps the most elegant feature of the results is a characteristic turnover in the exclusion limits for mediator masses around ten to the minus two to ten to the minus three GeV. This kink marks the transition between two regimes: above it, the mediator mass term dominates the propagator and the constraints track the mass; below it, the momentum transfer takes over and the limits flatten and shift. The team compared their dark matter bounds with independent constraints on light mediators from meson decays, stellar cooling, and the trapping and cooling limits from supernova SN1987A, noting that recasting those astrophysical and collider bounds onto the dark matter coupling product requires additional assumptions that they adopt consistently from earlier work.

The broader significance is twofold. Practically, the analysis extends the reach of existing detectors, machines built primarily to hunt for WIMPs, deep into the sub-GeV window without pouring a single extra litre of xenon. Conceptually, it demonstrates that momentum dependence is not a technical footnote but a decisive factor in interpreting any direct detection limit involving light mediators, and that the uncertainties in the Galactic dark matter distribution, which make the constraints scale with the square root of the effective propagation distance, must be handled honestly. As boosted dark matter searches mature, the crossover regions identified here, where detector thresholds, overburden attenuation and mediator dynamics all compete, are precisely where the next generation of experiments should focus if the universe’s most abundant matter particle is hiding below a GeV.

Subject of Research: Cosmic-ray boosted sub-GeV dark matter detection constraints via light mediator models

Article Title: New constraints on cosmic-ray boosted sub-Gev dark matter via light mediators

Article References: Yu, Y., Wang, G.-S., Zhang, B., Tang, T.-P., Su, B.-Y., & Feng, L. (2026). New constraints on cosmic-ray boosted sub-Gev dark matter via light mediators. The European Physical Journal C, 86(9), Article 1081. https://doi.org/10.1140/epjc/s10052-026-16368-7

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16368-7

Keywords: dark matter, cosmic rays, sub-GeV dark matter, light mediators, direct detection, LZ experiment, XENON, Borexino, spin-independent scattering, spin-dependent scattering, particle astrophysics, mediator propagator

The study, carried out by Liza Hovhannisyan of the A. I. Alikhanyan National Laboratory in Yerevan, Armenia, uses the CORSIKA Monte Carlo code, a well-established tool for simulating cosmic-ray air showers that includes the effects of atmospheric electric fields on particle transport. The simulations model a vertically oriented, uniform electric-field layer 2000 meters thick above the detector level, subdivided into twenty 100-meter slices so that the vertical growth of the electron population can be tracked in detail. Four high-altitude observational sites with distinct atmospheric densities were chosen, all of which have recorded thunderstorm-related particle enhancements: Aragats in Armenia at roughly 3000 meters above sea level, Nor Amberd at about 2000 meters, Lomnický Štít in the High Tatras at approximately 2600 meters, and the LHAASO site in China at around 4400 meters. Across the four configurations, the simulated field regions span altitudes of roughly 4000 to 6400 meters, where air densities range from 0.66 to 0.89 kilograms per cubic meter.

The electric fields applied in each configuration were tuned to the local runaway threshold, which scales with the normalized air density. Because thinner air at high altitude lowers the threshold field, the same absolute field strength means something quite different at LHAASO than it does at Nor Amberd. The simulations therefore used field ranges of 180 to 210 kilovolts per meter at LHAASO, 200 to 230 at Aragats, 210 to 240 at Lomnický Štít, and 230 to 260 at Nor Amberd, varied in steps of 10 kilovolts per meter. Each simulation injected a single seed electron with an initial energy spectrum drawn from the EXPACS model, following a power law between 1 and 300 megaelectronvolts, and between 500 and 1000 events were run for each field configuration to ensure statistical stability. Electrons and gamma rays were tracked until their energies fell below 0.05 megaelectronvolts, with particle numbers recorded at every layer boundary.

Within the supercritical region where the applied field exceeds the local runaway threshold, the number of runaway electrons grows approximately exponentially with distance, and the characteristic e-folding length of that growth is the central parameter of avalanche theory. A widely used empirical relation, introduced by J. R. Dwyer in 2003, links this avalanche length to the electric-field excess above the threshold through a single proportionality coefficient, with Dwyer reporting a value of about 7.2 times ten to the third kilovolts. When Hovhannisyan fitted this standard relation separately to the effective avalanche lengths extracted from the vertical electron profiles at each station, the results told a revealing story. The station-specific coefficients ranged from 8.6 to 11.2 times ten to the third kilovolts, all larger than the original value, and the spread between stations persisted even after fit-derived uncertainties were taken into account. A single global fit across all sixteen simulation points yielded a best-fit coefficient of about 9.58 times ten to the third kilovolts, but with a coefficient of determination of only about 0.90, leaving noticeable inter-station scatter.

The remedy was to let the coefficient itself depend on air density. By introducing a generalized relation with an additional density-dependent term, in which the avalanche length is multiplied by the normalized air density raised to a fitted exponent, the joint fit across all four stations produced a coefficient of about 6.39 times ten to the third kilovolts and an exponent of minus 0.923, with an uncertainty of plus or minus 0.083. That exponent, statistically indistinguishable from minus one, indicates an approximately inverse dependence on atmospheric density beyond what is already captured by the density-scaled runaway threshold. The improvement in descriptive power was striking: the coefficient of determination rose from roughly 0.90 to roughly 0.99, and plots comparing simulated and predicted avalanche lengths showed much better agreement, particularly at the larger avalanche lengths where the standard formulation deviated most from a one-to-one relation.

The study then turned to a second, complementary stage of avalanche evolution: what happens to the particles after they leave the accelerating field. Additional simulations for the Aragats configuration placed the detector level 25, 50, 100, and 200 meters below the lower boundary of the electric field, keeping all other parameters fixed. The results revealed sharply divergent transport behavior between the two particle species. The electron spectrum was progressively attenuated with increasing field-free distance, with a pronounced suppression of the high-energy component, and the electron population decreased rapidly and monotonically. Gamma rays, by contrast, persisted over larger distances, attenuated more gradually through stochastic interactions such as Compton scattering, photoelectric absorption, and pair production. At a separation of 100 meters, the electron component was already strongly depleted while a substantial gamma-ray component remained, making that distance a natural calibration point.

This asymmetry matters for a phenomenological quantity called the free path distance, introduced in earlier work to characterize how far energetic particles can propagate beyond the electric-field region and still reach ground-based detectors. Because the maximum electron energy at the field boundary cannot be measured directly, the formulation estimates it from the maximum gamma-ray energy at the detector through an empirical electron-gamma energy coefficient. For the 100-meter calibration separation, the simulated gamma-ray spectra changed little over the propagation distance, with characteristic maximum energies of about 29 to 31 megaelectronvolts at both levels, validating the use of the detector value as a proxy. Electron spectra, however, shifted markedly toward lower energies: at Aragats, the characteristic maximum dropped from roughly 38 megaelectronvolts at the field exit to around 16 megaelectronvolts after 100 meters of propagation through air.

To quantify the electron-gamma coefficient, the study employed a bootstrap-based statistical procedure with 2000 Poisson resampling realizations of the energy histograms, applying a criterion that defines the characteristic maximum energy as the highest energy with at least three consecutive bins containing more than five counts, which suppresses sensitivity to isolated outliers. The station-specific median coefficients clustered tightly, from about 1.31 at Aragats to 1.46 at Lomnický Štít, and pooling all stations yielded a median of about 1.38, in close agreement with an independent global least-squares optimization that gave 1.375. The adopted value of 1.37 replaces the earlier empirical coefficient of 1.2, which the bootstrap analysis showed systematically underestimates the free path distance at all four stations. The second parameter, the altitude-dependent electron energy-loss rate, was obtained from the NIST ESTAR stopping-power database scaled by local air density, yielding values between roughly 0.17 and 0.23 megaelectronvolts per meter across the sites. Applying the recalibrated formulation reproduced free path distances of 85 to 105 meters, consistent with the 100-meter calibration scale, and an independent test at a 50-meter separation reconstructed a median of 69.9 meters, correctly identifying the short-distance regime.

The practical payoff of this work lies in the interpretation of observations. Mountain observatories such as Aragats, Lomnický Štít, and LHAASO routinely detect particle bursts during thunderstorms, and inferring the structure and strength of the electric fields inside thunderclouds from those detections depends on having reliable models of both avalanche growth and post-field transport. By showing that a single density-independent coefficient leaves systematic residuals across sites spanning a substantial range of atmospheric densities, and by demonstrating that a simple power-law correction removes nearly all of that scatter, the study provides a unified scaling that can be applied across different altitudes and storm conditions. The recalibrated free path distance formulation likewise gives experimentalists a firmer bridge between the gamma-ray spectra they measure and the electron populations that produced them at the edge of the accelerating field.

The author notes that the results are derived for idealized, uniform electric-field configurations and should not be generalized to arbitrary thunderstorm field structures without further validation, a caveat that reflects the persistent difficulty of measuring the spatial and temporal structure of fields inside storm clouds. Even so, the near-perfect fit achieved by the density-dependent formulation, together with the stability of the recalibrated transport coefficients across four very different atmospheric environments, marks a meaningful step toward a common physical framework for the growing family of high-energy atmospheric phenomena. All simulation inputs, analysis scripts, and processed outputs are publicly archived, allowing other groups to reproduce the workflow and extend the calibration to new observational sites and more realistic field geometries.

Subject of Research: Density-dependent scaling and transport of relativistic runaway electron avalanches in thunderstorm electric fields

Article Title: Relativistic runaway electron avalanches: unified density-dependent scaling and transport

Article References: Hovhannisyan, L. (2026). Relativistic runaway electron avalanches: unified density-dependent scaling and transport. Geoscientific Model Development, 19(19), 9289-9300. https://doi.org/10.5194/gmd-19-9289-2026

Image Credits: AI Generated

DOI: 10.5194/gmd-19-9289-2026

Keywords: relativistic runaway electron avalanches, thunderstorm electric fields, cosmic rays, gamma-ray glows, terrestrial gamma-ray flashes, CORSIKA simulations, air density, avalanche length, free path distance, thunderstorm ground enhancements, Monte Carlo modeling, high-altitude observatories

The research, carried out by an international team of physicists and published in the journal Physical Review Letters, took an unprecedented approach to a long-standing problem. Rather than modeling how high-energy cosmic rays interact with the Earth’s atmosphere, the team recreated the process directly using CERN’s Large Hadron Collider, the world’s most powerful particle accelerator, located outside Geneva. The work was led by Jesse Liu, an assistant professor of physics at New York University, and included Cigdem Issever, a physicist at Humboldt-University of Berlin, along with colleagues from the DESY Research Center in Hamburg. All of the researchers are part of CERN’s ATLAS collaboration, the enormous international experiment devoted to exploring the fundamental nature of matter and the basic forces that shape the universe. Their goal was deceptively simple to state and extraordinarily difficult to achieve: make protons collide with oxygen under conditions that faithfully mimic a cosmic ray striking the sky, and then measure the debris with precision no simulation can match.

The choice of oxygen was not arbitrary. Oxygen is a foundational component of the Earth’s atmosphere, so any realistic account of how cosmic rays generate air showers must describe in detail what happens when a high-energy proton strikes an oxygen nucleus. In a landmark first for the Large Hadron Collider, the accelerator team introduced beams of oxygen nuclei into the machine for the first time and steered them into collisions with proton beams generated by the LHC itself. Those protons stand in for the cosmic rays that emanate from supernovae and other exotic astrophysical sources, arriving at the top of the atmosphere with energies far beyond anything the Sun can produce. By colliding protons and oxygen at an energy of 9.62 teraelectronvolts per nucleon pair, the experiment reproduced the collision conditions that occur when the most energetic cosmic rays plunge into the atmosphere, bringing a process that normally unfolds kilometers above our heads into a controlled laboratory setting.

Capturing what happens in those collisions demanded instrumentation of extraordinary speed and sensitivity. The collisions in the LHC occur up to 40 million times per second, and each one lasts a vanishingly small fraction of a moment before the resulting particles fly apart. To record the outcomes, the researchers used a sophisticated 100-million-pixel camera that is part of the ATLAS detector system. This device took millions of closeup photographs of the collision aftermath, registering the number of particles produced, their energies, and the angles at which they emerged from the proton-oxygen impacts. Those three quantities—the multiplicity, the energy spectrum, and the angular distribution of the secondary particles—are precisely the parameters that atmospheric air-shower models must get right. With millions of measured events in hand, the team assembled precise portraits of the birth of cosmic rainstorms, effectively turning the LHC into a machine for studying the sky rather than only the deepest structure of matter.

The measurements revealed something that the scientific community had long suspected but never before confirmed with direct data: the computer models that researchers have used for years to describe these interactions are, in many respects, significantly wrong. Issever emphasized the unusual value of the result, noting that the published data are unique and that the measurements were able to show that previous models for this type of interaction are actually very inaccurate. That finding matters far beyond the technical details of particle physics. Air-shower simulations underpin a wide range of astrophysical research, from interpreting the signals recorded by ground-based cosmic-ray observatories to estimating the energies and compositions of the original particles that struck the atmosphere. If the underlying collision physics is modeled inaccurately, every downstream inference inherits that error. By replacing modeled assumptions with measured reality, the new results give the entire field a firmer empirical foundation.

The significance of the work becomes clearer when one considers how different high-energy cosmic rays are from their solar counterparts. The particles streaming from the Sun carry energies low enough that their interactions with atmospheric gases, while still not fully understood, can be studied with a variety of direct and indirect methods, and their most famous consequence is the aurora that illuminates the Northern Hemisphere for millions of observers. High-energy cosmic rays, by contrast, are so energetic and so rare that no detector can simply sit and watch one collide with the atmosphere in a controlled way. Their collisions happen at altitudes and energies that are effectively inaccessible, which is why simulations became the standard tool. Those simulations extrapolate from particle-physics measurements made in other contexts, and different extrapolation schemes have yielded conflicting predictions about how many secondary particles an air shower should produce and how they should be distributed. The LHC experiment cuts through that uncertainty by measuring the relevant collisions directly.

Liu framed the achievement in terms of the bigger picture of cosmic understanding. We now have a better understanding of how particles from objects more exotic than the Sun interact with our atmosphere, Liu said, adding that by advancing knowledge of high-energy cosmic collisions in the Earth’s atmosphere, the team has taken another step toward unraveling the many mysteries of the universe. The phrase particles from objects more exotic than the Sun captures the astrophysical stakes of the measurement. Supernovae, and possibly other extreme cosmic accelerators, are capable of boosting protons and nuclei to energies millions of times higher than anything the Sun emits, and tracing those particles back to their sources is one of the great open challenges of high-energy astrophysics. Every improvement in the description of how they interact with air brings scientists closer to reading the sky as a record of the galaxy’s most violent events.

The technical details of the measurement, published under the title describing charged-particle production in proton-oxygen collisions at 9.62 teraelectronvolts as a probe of cosmic-ray air showers with the ATLAS detector, will now feed directly into the models used by cosmic-ray researchers worldwide. Air-shower simulations incorporate particle-production data as their physical input, so the new measurements can be inserted where previously only extrapolations existed. The effect is expected to sharpen predictions for the giant detector arrays that monitor extensive air showers across hundreds or thousands of square kilometers of the Earth’s surface, and to reduce the systematic uncertainties that have historically separated different experiments’ estimates of cosmic-ray composition and energy. In this sense, a particle-physics experiment designed to probe the fundamental nature of matter has delivered a gift to astrophysics, demonstrating once again how the boundaries between these disciplines blur at the highest energies.

The experiment also marks a milestone for the Large Hadron Collider itself. Oxygen beams had never before been circulated in the machine, and producing them required the accelerator complex to ionize, accelerate, and handle a new species of particle alongside its routine proton operations. That capability opens the door to future studies of other atmospheric and astrophysically relevant nuclei, potentially extending the laboratory reconstruction of cosmic collisions to an even broader range of elements that make up the air and the cosmic rays that strike it. For now, the team’s results stand as the world’s first measurements of cosmic rainstorms recreated in controlled laboratory conditions, a demonstration that some of the most energetic processes in the universe can be brought down to Earth, photographed millions of times over, and finally understood not through approximation but through direct observation. These new results, Liu concluded, significantly sharpen our knowledge of these subatomic interactions and will help scientists further explore the nature of cosmic particles raining from the sky.

Subject of Research: Laboratory recreation of high-energy cosmic ray collisions with atmospheric oxygen using proton-oxygen collisions at the Large Hadron Collider to improve air shower models.

Article Title: Physicists recreate particles from outer space to advance understanding of the universe’s mysteries

Article References: Physicists recreate particles from outer space to advance understanding of the universe’s mysteries. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cosmic rays, Large Hadron Collider, ATLAS, air showers, supernovae, oxygen beams, particle physics, CERN, Physical Review Letters, aurora, cosmic-ray simulations, astroparticle physics

The discovery emerged from a coordinated campaign between two of China’s flagship observatories: the Einstein Probe, a space-borne X-ray telescope, and the Large High Altitude Air Shower Observatory, known as LHAASO, which sits at high altitude in Sichuan province and detects the showers of secondary particles produced when ultrahigh-energy gamma rays strike Earth’s atmosphere. Together, the two instruments observed PSR J1740+1000, a middle-aged pulsar located roughly 4,600 light-years from Earth. What they found was an X-ray tail of unprecedented length, stretching southwest from the pulsar for about 32 arcminutes on the sky, which translates to approximately 42 light-years at the pulsar’s distance. This is the longest X-ray tail ever associated with a pulsar wind nebula.

The significance of the measurement lies in how it transforms our picture of particle transport. The prevailing assumption has long been that once high-energy particles escape their acceleration sites, magnetic turbulence in interstellar space scatters them rapidly, causing them to diffuse isotropically, meaning equally in all directions. Under that model, any directional structure should be erased within a relatively short distance of the source. The new observations defy that expectation. Over a scale of tens of light-years, the particles streaming away from PSR J1740+1000 maintain a coherent propagation direction, leaving behind an elongated trail of synchrotron X-ray emission that astronomers can read like a breadcrumb path.

The story began with earlier observations by the XMM-Newton satellite, which had detected an X-ray tail several light-years long in the vicinity of the pulsar. But XMM-Newton’s field of view and observing strategy limited how much of the structure it could capture. The Follow-up X-ray Telescope aboard the Einstein Probe, by contrast, was designed with a wide field of view and a low instrumental background, making it exceptionally well suited to searching large swaths of sky for faint, diffuse X-ray structures. In roughly 70,000 seconds of observations, the Einstein Probe revealed that the tail extends far beyond what XMM-Newton had seen. Remarkably, although XMM-Newton accumulated more than six times the observing time, it detected only a small fraction of the full structure, underscoring how much of the high-energy sky may still be hiding beyond the edges of conventional surveys.

The crucial connection came from LHAASO. In recent years, the observatory has catalogued a series of ultrahigh-energy gamma-ray sources, some of which reach energies exceeding one petaelectronvolt, a million billion electronvolts, produced by particles accelerated in the vicinity of supernova remnants, pulsars, and black holes. A persistent puzzle has been that several of these gamma-ray sources have no obvious astronomical counterpart nearby, no pulsar, remnant, or stellar cluster that could plausibly serve as their engine. These so-called orphan sources have been difficult to interpret. One possibility is that the parent particles simply traveled far from their acceleration site before producing detectable gamma rays, meaning the visible emission marks a waypoint on a long journey rather than the point of origin.

PSR J1740+1000 provides a striking confirmation of that idea. The ultrahigh-energy gamma-ray emission detected by LHAASO extends in the same direction as the newly mapped X-ray tail, and the two structures match closely in position on the sky. More importantly, the physical interpretation ties them together at the level of the particles themselves. The electrons responsible gain enormous energies within the pulsar wind nebula, the bubble of magnetized plasma that a rapidly spinning neutron star blows into its surroundings. As these electrons propagate along the tail, they spiral through magnetic fields and emit synchrotron radiation, a portion of which falls into the X-ray band and is recorded by the Einstein Probe. Simultaneously, the same electrons collide with low-energy photons that pervade interstellar space and boost them to extreme energies through inverse Compton scattering, producing the ultrahigh-energy gamma rays that LHAASO detects from the ground.

In effect, the X-rays and gamma rays are two footprints left by a single population of high-energy electrons, imprinted in different wavelength bands along the same path. The spatial alignment and the consistency of the energy spectra of the two emissions provide strong evidence that the extraordinarily long tail is genuinely the record of extended particle propagation, not a chance superposition of unrelated structures. For the first time, astronomers can follow a coherent trail of PeV-scale particles across a distance comparable to the spacing between neighboring stars in the galactic neighborhood, connecting the accelerator directly to the far-flung region where its particles finally reveal themselves.

Why the particles hold their direction over such vast distances remains an open question, and the researchers propose two candidate mechanisms. The first is that the interstellar magnetic field in this region is highly ordered, allowing the electrons to stream efficiently along the field lines while suppressing motion perpendicular to them, effectively creating a narrow magnetic track through the galaxy. The second is that a fast, collimated outflow, perhaps a jet-like wind, is continuously ejected from the pulsar wind nebula in that direction, carrying the particles outward the way water from a fire hose carries a jet far from its nozzle. The current data cannot yet distinguish between these scenarios, but both lead to the same consequential conclusion: high-energy particles do not always scatter into randomness within a short distance of their source, and anisotropic transport can persist across scales of tens of light-years.

The implications reach well beyond a single pulsar. As Gabriele Ponti and colleagues at the Italian National Institute for Astrophysics note in a commentary published in the same issue of the journal, the significance of the discovery extends past PSR J1740+1000 itself, because it demonstrates that a spatial offset between an ultrahigh-energy gamma-ray source and its candidate astronomical counterpart does not necessarily mean the association is false. Instead, such an offset may trace the propagation path of particles after they leave their acceleration site. In other words, the location where astronomers detect gamma rays may be merely the footprints of a long journey, not the birthplace of the particles. This reframing could resolve the identities of several of LHAASO’s orphan sources and reshape how scientists match gamma-ray detections to their astrophysical engines.

The study also stands as a demonstration of the power of coordinated, multiwavelength astronomy. By pairing a wide-field X-ray telescope in orbit with a ground-based gamma-ray observatory covering an entirely different energy regime, the research team stitched together a continuous physical narrative, following particles from the turbulent environment near a neutron star across 42 light-years of interstellar space. Each band captures a different stage of the electrons’ lives: synchrotron light marks their passage through magnetic fields close to the source, while inverse Compton gamma rays record their later interactions with ambient starlight far from it. As surveys by the Einstein Probe and LHAASO continue, astronomers expect this approach to uncover more such trails, opening a new window onto how the galaxy’s most extreme accelerators seed interstellar space with particles whose energies dwarf anything achievable in human laboratories, and gradually dismantling a century-old mystery one footprint at a time.

Subject of Research: Anisotropic propagation of high-energy cosmic-ray electrons from the pulsar wind nebula PSR J1740+1000, traced by joint X-ray and gamma-ray observations

Article Title: Einstein probe and LHAASO reveal a 42-light-year directed journey of cosmic rays

Article References: Einstein probe and LHAASO reveal a 42-light-year directed journey of cosmic rays. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cosmic rays, pulsar wind nebula, PSR J1740+1000, Einstein Probe, LHAASO, X-ray astronomy, ultrahigh-energy gamma rays, synchrotron radiation, inverse Compton scattering, interstellar magnetic fields, particle transport, high-energy astrophysics

The story begins with the physics of cosmic rays themselves. Most primary cosmic rays are protons, electrically charged particles whose trajectories are bent by Earth’s magnetic field before they ever reach the atmosphere. When they arrive, they collide with atomic nuclei in the air and generate cascades of secondary particles. Among these secondaries are muons, short-lived charged particles produced at high energies that travel at nearly the speed of light and penetrate deep into the atmosphere. As Sunil Gupta of the Tata Institute of Fundamental Research (TIFR) in Mumbai, one of the authors of the study, explains, the cosmic rays detected at ground level consist predominantly of these highly penetrating muons. The GRAPES-3 facility includes a 560-square-metre muon telescope that records around four billion muons every day, providing an extraordinarily dense stream of data.

Crucially, those muons are not evenly balanced in electric charge. Because the primary cosmic rays are mostly positively charged, positive muons are more abundant in the atmosphere than negative ones. Physicists quantify this imbalance with the muon charge ratio, the number of positive muons divided by the number of negative ones, which normally exceeds one. That small excess is what allows GRAPES-3 to do something it was never originally designed to do. Inside a thundercloud, electric charges become separated, producing intense electric fields and large potential differences. When muons cross these regions, the field acts on positive and negative particles in opposite ways, generally slowing positive muons while accelerating negative ones. If the two charges were present in equal numbers, these effects would cancel almost perfectly, leaving no detectable change in the total muon flux.

Hari Haran Balakrishnan of TIFR, first author of the new study, puts the point starkly: if nature provided equal numbers of positive and negative muons throughout the telescope’s field of view, the thunderstorm phenomenon simply could not be observed with the current setup. Because positive muons dominate, however, each thunderstorm leaves a small but measurable imprint on the total muon count, and from that imprint researchers can infer the electrical potential inside the cloud. Gupta describes muons as an ideal gift for these kinds of studies, likening them to an electric current flowing through the atmosphere. In an earlier analysis, the team reported a striking result: a potential of 1.3 gigavolts measured in a single thunderstorm, a figure that dwarfs anything obtained by conventional means.

The idea that thunderstorms might generate potentials on that scale is not new. As early as the 1920s, the physicist Charles Thomson Rees Wilson, who would later win the 1927 Nobel Prize in Physics, suggested that thunderclouds could reach such enormous voltages. Direct measurements, however, had previously topped out at only around 130 million volts, more than an order of magnitude lower. The difficulty is practical as much as conceptual: sampling the interior of a large, rapidly evolving storm generally requires instruments carried by aircraft or weather balloons into or near active weather, a hazardous and spatially limited approach. Muons invert the problem entirely. They pass through the atmosphere continuously, they can be monitored from the safety of the ground, and they sample the whole volume of the cloud along their paths.

But the earlier GRAPES-3 observations carried an unexpected puzzle alongside their headline result. Between April 2011 and December 2020, the telescope detected 487 thunderstorm events, and their distribution across the sky was strikingly lopsided. Fully 81.5 percent of the events appeared in the eastern part of the instrument’s field of view, while only 13.7 percent appeared in the west, a difference of almost six to one. The obvious suspicion was local weather: perhaps storms in this part of India genuinely favour the east. An independent array of instruments that monitors the atmospheric electric field said otherwise. Actual thunderstorms, measured directly, showed no comparable preference for the eastern sky, which meant the asymmetry had to originate in the detector rather than in the atmosphere.

The answer lies in an effect that atmospheric and cosmic-ray physicists have known about for nearly a century: the east-west effect. Because cosmic-ray protons are charged, Earth’s magnetic field filters their arrival directions. Lower-energy positive particles arriving from the east are screened out more strongly than those arriving from the west, so the mix of cosmic rays, and therefore of secondary muons, depends on direction. This directional threshold, known as the geomagnetic cutoff, varies significantly across GRAPES-3’s field of view. The consequence is that the muon charge ratio, the very quantity that makes thunderstorm sensing possible in the first place, is itself direction-dependent, and the telescope is therefore more sensitive to thunderstorms in some parts of the sky than in others.

To test whether this could account for the observed lopsidedness, the researchers turned to computer simulations that model the production and propagation of muons through the atmosphere. The simulations showed that the balance between positive and negative muons changes markedly with direction: the muon charge ratio is about 1.37 in the east but only 1.14 in the west. That difference, they found, is more than enough to explain the event counts. As Gupta emphasizes, it is not that the voltage is different; thunderstorms to the east are not necessarily more electrically powerful than those to the west. Rather, the larger imbalance between positive and negative muons arriving from that direction makes the telescope more responsive to their electrical effects. The simulations confirmed the mechanism in both directions: when the direction-dependent charge ratio was included, a strong east-west asymmetry emerged, closely resembling the observations, and when the same ratio was imposed in both directions the asymmetry disappeared entirely.

The resolution of the puzzle matters well beyond bookkeeping for one detector. Understanding this directional sensitivity is essential if GRAPES-3 is to serve as a reliable, calibrated probe of thunderstorm electricity, because any inferred potential must account for how efficiently the instrument responds along each line of sight. With that correction in hand, the technique offers something no balloon campaign can match: continuous, ground-based monitoring of the largest electric fields in nature, drawn from a particle flux that arrives whether or not anyone is watching. The broader significance is considerable, since thunderstorm electrification is linked to lightning initiation, to terrestrial gamma-ray flashes, and to the still-uncertain role of storms in the global atmospheric electrical circuit. What began as an instrument for studying particles from the cosmos has ended up illuminating one of the most familiar and, remarkably, still one of the most mysterious phenomena on our own planet. The same muons that carry messages about distant astrophysical sources now carry, quite literally, the voltage of the storm overhead.

Subject of Research: Using cosmic-ray muons to probe electric fields and potentials inside thunderclouds

Article Title: From the cosmos to thunderstorms

Article References: From the cosmos to thunderstorms. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cosmic rays, muons, thunderstorms, GRAPES-3, electric fields, geomagnetic field, muon charge ratio, atmospheric electricity, lightning, particle physics, TIFR, JCAP

The diffuse Galactic emission is the fog of high-energy astrophysics. It arises when cosmic rays, protons and electrons accelerated by sources such as supernova remnants, collide with interstellar gas, dust grains, magnetic fields, and the sea of starlight that permeates the Galaxy. Protons smashing into gas produce neutral pions that decay into gamma rays, while electrons scatter low-energy photons up to gamma-ray energies through inverse Compton processes and radiate synchrotron light in magnetic fields. Disentangling these components is notoriously difficult because their contributions overlap and depend on poorly constrained distributions of gas, radiation, and cosmic rays. Traditional models, such as the widely used GALPROP code, solve the cosmic-ray transport equation with detailed parameterizations, but they require extensive tuning and still struggle in the structurally complex inner Galaxy.

The research team, led by Xi Liu and colleagues at Sun Yat-sen University, took a deliberately different route. Rather than assuming a physical model from first principles, they trained two standard machine learning algorithms, Random Forest and K-Nearest Neighbors regression, to learn a direct mapping between Planck’s nine all-sky foreground maps, spanning 30 to 857 gigahertz, and the gamma-ray intensity predicted by the Fermi-LAT interstellar emission model across 28 energy bins from 50 megaelectronvolts to 814 gigaelectronvolts. Each Planck band traces a different physical regime: the lowest frequencies are dominated by synchrotron radiation from cosmic-ray electrons, intermediate bands mix free-free and spinning-dust emission, and the highest frequencies are dominated by thermal dust, a reliable tracer of interstellar gas.

The results are striking. In the 0.1 to 10 gigaelectronvolt range, where the diffuse gamma-ray sky is brightest, the models achieve coefficients of determination above 0.90, peaking at 0.96 at 687 megaelectronvolts. The predicted full-sky maps reproduce the major morphological features of the gamma-ray Galaxy, and the extracted spectral energy distribution of the inner Galactic plane matches the reference model closely. The team verified that the learned relationship is not a local accident: when a model trained on one Galactic hemisphere was tested on the opposite side, performance varied by no more than 6 percent regardless of how large a training region was used, indicating that the microwave-to-gamma-ray connection reflects a robust, large-scale property of the Milky Way rather than a statistical fluke of nearby structures.

Perhaps the most physically revealing result comes from asking which Planck frequencies matter most. When the high-frequency bands, dominated by thermal dust emission, were used alone, they achieved predictive power nearly identical to that of all nine bands in the 0.1 to 10 gigaelectronvolt range, while the low-frequency synchrotron channels performed markedly worse. This is exactly what one would expect if the diffuse gamma-ray emission at these energies is hadronically dominated: gamma-ray intensity is proportional to the product of gas column density and cosmic-ray density, and dust is an excellent proxy for the gas that serves as the target material. Above 10 gigaelectronvolts, however, the predictive power of the low-frequency bands rises, consistent with a growing leptonic contribution in which the synchrotron-emitting electron population also produces gamma rays through inverse Compton scattering. The analysis thus provides an independent, data-driven confirmation of the standard picture of Galactic gamma-ray production.

The residual maps, showing where the learned mapping breaks down, read like a tour of the Galaxy’s most enigmatic structures. Positive residuals appear toward the Magellanic Clouds and Centaurus A, where abundant gas and dust in nearby systems tempt the model to predict hadronic gamma-ray emission that the reference maps treat separately. Warm ionized gas complexes such as Barnard’s Loop in Orion and the Gum Nebula also show over-predictions, because free-free emission is subdominant across the Planck bands. On the other side, negative residuals trace Loop I and the North Polar Spur, hinting at inverse-Compton-dominated radiation fields or enhanced electron populations that submillimeter dust tracers cannot capture. Around the Galactic Center, the residuals mirror the shape of the Fermi Bubbles, the giant gamma-ray lobes whose faint, hard-spectrum microwave counterpart is too subtle for the model to disentangle without explicit priors.

The team also uncovered a curious hemispheric asymmetry: at high Galactic latitudes, the model systematically under-predicts the north while over-predicting the south by 10 to 30 percent on average. Because the discrepancy is spatially coherent rather than random, it may point to genuine physical differences between the two Galactic hemispheres, such as variations in cosmic-ray density or radiation fields. A segmented analysis of the inner Galaxy further showed that the asymmetry is concentrated within 30 degrees of the Galactic Center, likely linked to the Galactic bar and spiral-arm tangents, while the outer disk behaves with remarkable uniformity.

In a direct head-to-head comparison at roughly 4.3 gigaelectronvolts, the machine learning approach outperformed a GALPROP simulation tuned to the latest AMS-02 cosmic-ray data in the inner disk and Galactic Center region, achieving a coefficient of determination of 0.9465 and a mean absolute relative error of 14.7 percent, compared with 0.8624 and 22.3 percent for the physical model. GALPROP, by contrast, held a slight edge in the more homogeneous outer disk and in parts of the halo, confirming its reliability under simpler conditions. The authors emphasize that the two approaches are complementary: the data-driven model absorbs non-linear multi-frequency correlations that static gas distributions miss, while the physical simulation remains indispensable for interpretation and extrapolation.

An appendix to the study adds a compelling validation using raw Fermi-LAT photon counts rather than the smoothed emission model. As exposure accumulated over 40 weeks of mission data, the predictive performance climbed steadily and stabilized, and the negative residuals in the predicted maps turned out to coincide with 86 known gamma-ray point sources cataloged by Fermi, including pulsars, blazars, and a radio galaxy. Because the machine learning model learned the diffuse background purely from gas and dust maps without ever seeing point-source information, any compact gamma-ray emitter naturally surfaces as a localized deficit, offering an independent check on standard source-detection pipelines.

Beyond its immediate results, the work positions machine learning as a physically interpretable instrument rather than a black box. The learned mapping encodes real relationships between interstellar matter, radiation fields, and cosmic-ray processes, and its failures mark precisely the regions where conventional templates are incomplete or biased. The authors propose extending the framework with low-frequency radio surveys such as the Haslam 408-megahertz map, polarized microwave channels, and hydrogen-alpha data on ionized gas, and eventually integrating X-ray, ultra-high-energy gamma-ray, and neutrino observations. Such a multi-messenger, data-driven baseline could help separate standard emission from exotic components, sharpen constraints on cosmic-ray propagation, and, in an era when IceCube has detected Galactic neutrinos and LHAASO has measured PeV-scale diffuse emission, provide the empirical foundation that next-generation high-energy astrophysics will demand.

Subject of Research: Data-driven machine learning modeling of Galactic diffuse gamma-ray emission using multi-wavelength Planck observations

Article Title: Data-driven modeling of Galactic diffuse emission with multi-wavelength observations

Article References: Liu, X., Li, X.-D., Lin, S., Liu, Y., Shao, C., Yang, L., & Zhang, L. (2026). Data-driven modeling of Galactic diffuse emission with multi-wavelength observations. The European Physical Journal C, 86(9), Article 1135. https://doi.org/10.1140/epjc/s10052-026-16408-2

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16408-2

Keywords: Galactic diffuse emission, machine learning, Planck satellite, Fermi-LAT, cosmic rays, gamma-ray astronomy, interstellar medium, inverse Compton, hadronic emission, GALPROP, Fermi Bubbles, multi-messenger astrophysics

Cosmic rays are protons and atomic nuclei accelerated to energies far beyond anything achievable in human particle accelerators. They race through the Galaxy, deflected this way and that by tangled magnetic fields, so that by the time they arrive at Earth any clue about their point of origin has been scrambled beyond recognition. Yet when one of these particles collides with a proton in an interstellar gas cloud, the debris includes neutral pions that almost immediately decay into pairs of gamma rays. Those gamma rays, unlike their charged parents, travel in perfectly straight lines. By counting them, astronomers can effectively take an inventory of cosmic rays at the location of the cloud itself, no matter how far away it lies.

This idea is not new. The Fermi Gamma-ray Space Telescope pioneered the technique at gigaelectronvolt energies, measuring cosmic rays in nearby clouds and confirming that the flux in the local solar neighbourhood is broadly representative of the Galaxy on kiloparsec scales. Earlier this decade, the HAWC water Cherenkov observatory in Mexico found evidence for gamma-ray emission from passive molecular clouds at teraelectronvolt energies. What makes the new LHAASO measurement transformative is the energy reach. The collaboration reports spectral energy distributions consistent with gamma rays produced by cosmic rays interacting with the interstellar medium across the range from roughly 1 to 100 teraelectronvolts, meaning the clouds are revealing the population of cosmic-ray protons carrying energies up to several hundreds of teraelectronvolts and beyond, a regime previously accessible only through indirect and model-dependent methods.

The technical challenge is formidable. Individual giant molecular clouds are not bright gamma-ray sources at these energies; the emission is diffuse, extended across many square degrees, and must be extracted from backgrounds of charged cosmic-ray air showers that outnumber gamma-ray events by many thousands to one. LHAASO’s answer is a stacking analysis. Rather than trying to detect each cloud separately, the team aligned the data from five nearby clouds and combined their statistical weight, using the kilometre-squared detector array KM2A to reject the hadronic background and the water Cherenkov detector array WCDA to extend the reach toward lower energies. A likelihood-ratio test applied to the stacked maps produced a clear detection, with the test statistic profile showing a signal that could not plausibly arise from random fluctuations of the background.

The physical logic of the measurement rests on knowing how much target material each cloud contains. The gas mass is estimated from surveys of carbon monoxide line emission, converted to molecular hydrogen mass through a well-studied conversion factor, with corrections for dust opacity informed by Planck satellite maps that also account for so-called dark gas not traced by carbon monoxide. Combining the gamma-ray flux with the gas column density yields the cosmic-ray energy density and spectral shape in situ. The measured spectral energy distributions of the five clouds, when stacked, match what standard models of proton-proton interactions predict if the cosmic-ray spectrum at the clouds resembles the one measured near Earth. That consistency matters: it suggests that the cosmic-ray ‘sea’ is reasonably uniform, at least in the solar vicinity and at teraelectronvolt energies.

The really tantalizing part of the analysis is what it says about the cosmic-ray knee, the famous break in the all-particle cosmic-ray spectrum near a few petaelectronvolts, where the flux steepens dramatically and particles seem unable to reach higher energies. Discovered in 1958 and never fully explained, the knee is widely interpreted as the point where the Galaxy’s accelerators, most likely supernova remnants, run out of steam for protons. Direct measurements by balloon and space experiments such as DAMPE have mapped the proton spectrum up to a few hundred teraelectronvolts, and LHAASO’s own direct measurements have recently pushed precision into the knee region, but the exact break energy of the proton component remains contested. Gamma rays from the clouds offer an independent probe, because the knee in the proton spectrum must imprint itself as a softening in the gamma-ray spectrum at roughly one-tenth of the proton energy.

The collaboration therefore repeated its fit while introducing a deliberate break into the gamma-ray model, scanning over possible break energies and post-break spectral slopes. The result is a nuanced one. No clear knee signature is formally detected in the stacked gamma-ray data; the measured spectra remain compatible with an unbroken power law within the current uncertainties. However, the constraints derived from KM2A are broadly compatible with a proton cosmic-ray knee located at or above about 0.9 petaelectronvolts, although the team is careful to note that somewhat lower break energies cannot be excluded within the present error budget. This is broadly consistent with recent ground-based direct measurements of the proton spectrum in the knee region, adding an independent, calorimetric line of evidence to a decades-old puzzle.

Why does this matter beyond the specifics of one spectral feature? Because the method converts a class of ordinary astronomical objects into a distributed network of cosmic-ray detectors scattered around the Galaxy. Every giant molecular cloud within reach of an instrument like LHAASO becomes a point measurement of the local cosmic-ray density and spectrum, allowing astronomers to test whether the sea is truly uniform or whether it is modulated by nearby accelerators, by variations in the diffusion coefficient, or by the possibility that particle propagation changes character near the knee. Several theoretical works have argued that the diffusion of petaelectronvolt particles should be faster or differently structured than that of teraelectronvolt particles, and some models predict spatial gradients in the cosmic-ray density close to the Sun. The LHAASO clouds provide the empirical test bed for these ideas.

There are also systematic questions that the coming years of data will address. The gamma-ray flux from clouds is proportional to the assumed gas mass, so uncertainties in the carbon monoxide conversion factor propagate directly into the inferred cosmic-ray spectrum. The five clouds in the current sample were chosen for their proximity and high column density, and each additional year of exposure sharpens the stacked signal. With longer baselines, the collaboration could potentially resolve individual clouds rather than relying on stacking, break the sample into subsets at different distances from known supernova remnants, and tighten the limits on where the proton knee truly falls. The detectors themselves continue to operate at 4,400 metres above sea level, where staff maintain the water recycling and power systems year round to keep the arrays running.

For now, the result stands as proof of concept for one of the most elegant ideas in high-energy astrophysics: that the Galaxy’s most mundane structures, cold clouds of molecular hydrogen and carbon monoxide, can be read as detectors for its most exotic particles. In the same way that astronomers learned to treat interstellar gas as a tracer of magnetic fields and star formation, they can now treat it as a calorimeter for petaelectronvolt physics. If cosmic rays hold the secret of the Galaxy’s most powerful accelerators, the clouds, it turns out, have been recording that secret all along. LHAASO has just learned how to read the record, one gamma ray at a time, and the map of cosmic rays across the Milky Way is only beginning to take shape.

Subject of Research: Detection of ultrahigh-energy gamma-ray emission from giant molecular clouds to measure galactic cosmic-ray density and spectra

Article Title: LHAASO detection of ultrahigh-energy γ-ray emission towards giant molecular clouds

Article References: The LHAASO Collaboration, Cao, Z., Aharonian, F., Bai, Y. X., Bao, Y. W., Bastieri, D., Bi, X. J., Bi, Y. J., Bian, W., Blunier, J., Butkevich, A. V., Cai, C. M., Cai, Y. Y., Cao, W. Y., Cao, Z., Chang, J., Chang, J. F., Chen, E. S., Chen, G. H., … Zuo, X. (2026). LHAASO detection of ultrahigh-energy γ-ray emission towards giant molecular clouds. Nature Astronomy. https://doi.org/10.1038/s41550-026-02975-7

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02975-7

Keywords: LHAASO, gamma-ray astronomy, cosmic rays, giant molecular clouds, cosmic-ray knee, PeV, particle astrophysics, KM2A, Nature Astronomy, cosmic-ray propagation, very-high-energy gamma rays, Milky Way

The heart of the system is remarkably familiar hardware. Kosmiko is built around a Raspberry Pi 5 single-board computer that simultaneously controls two Sony IMX477 camera modules, the same class of sensor found in the popular Raspberry Pi High Quality Camera. The researchers operate these complementary metal-oxide-semiconductor, or CMOS, active-pixel sensors in raw acquisition mode with the lenses capped, meaning the cameras work in complete darkness. In that darkness, the only signals that should appear are electronic noise and, crucially, the faint electrical fingerprints left behind when an ionizing particle passes through the silicon. Each crossing particle generates a shower of electron-hole pairs along its track, and the sensor readout electronics record this deposited charge as an unusually bright or saturated pixel, or as an elongated cluster of pixels tracing the particle’s path.

What separates Kosmiko from a simple curiosity is the rigor of its calibration strategy. CMOS sensors are imperfect devices: individual pixels differ in their dark response, some run persistently hot, others are noisy, and a few fire spuriously regardless of conditions. Rather than treating these defects as a nuisance, the Montpellier team places each pixel on its own footing. Every measurement run begins with a calibration step in which the intrinsic threshold of each pixel is derived from its statistical behavior in darkness. During the subsequent continuous image acquisition, the intensity of each pixel is compared online against its own personalized threshold. Only pixels whose intensities exceed their individual thresholds are stored; everything else is discarded immediately. This per-pixel discrimination drastically reduces the data volume, which is what makes long-duration acquisitions practical on modest, low-power hardware.

The methodology also confronts the problem of bad actors among the pixels. Hot pixels, noisy pixels, and recurrent pixels that repeatedly exceed their thresholds for mundane reasons are identified and filtered out, so that genuine particle candidates are not drowned in a sea of sensor pathology. The authors complement this with a suite of camera-health indicators based on dark-level statistics, noise distributions, and percentiles, allowing an operator to verify at any time that the sensor is behaving as expected and that any change in event rates reflects the radiation environment rather than a drifting instrument. This kind of self-monitoring is essential for any detector intended to run unattended for days or weeks.

Once the raw frames have been reduced to candidate events, the offline analysis layer of Kosmiko extracts physically meaningful observables. These include the frame-of-interest rate, which counts how many images contain at least one candidate, and the cluster rate, which counts localized groups of bright pixels. The software also characterizes the intensity distributions of events, indicators of track-like topology that suggest a particle crossed the sensor rather than merely triggering a single defective pixel, and the occurrence of saturated pixels, whose maximum-registering response is typical of heavily ionizing particles depositing large amounts of energy. Together these observables allow researchers to compare ionizing-event rates under different conditions without pretending to know more than the data can support.

Indeed, the team is explicit about the scope of the instrument. Kosmiko is not designed to identify individual particle species. A muon streaking down from the upper atmosphere, a gamma ray from a radioactive decay, and an alpha particle from radon progeny all leave charge in silicon, and disentangling them requires more than intensity histograms alone. Instead, the goal is a documented and deployable methodology for comparing ionizing-event rates under different conditions in the natural radiation environment. That comparative capability is scientifically valuable in its own right, because variations in event rate with altitude, shielding, underground depth, or proximity to weak radioactive sources all carry information about the composition and behavior of ambient radiation.

The experimental evidence presented in the paper spans an impressive range of radiation conditions. The authors performed tests with multi-alpha emitters and with a cobalt-60 gamma source using the PRESERVE platform, a facility funded by the French Alternative Energies and Atomic Energy Commission, the Occitanie region, and the European Union through European Regional Development Funds. They conducted ground-level measurements of the natural background, and, in collaboration with the Laboratoire Souterrain à Bas Bruit in Rustrel, France, a National Underground, Low-Noise Laboratory operated with CNRS, they acquired data deep underground, where the cosmic-ray flux is heavily suppressed by hundreds of meters of rock. Comparing event rates across these radically different environments demonstrates exactly the kind of condition-to-condition sensitivity the instrument was built to provide.

The Kosmiko project sits within a growing movement to democratize particle physics. Previous efforts have shown that smartphone cameras can detect muons, and distributed networks of amateur detectors have been proposed as a complement to large observatories. Studies using CMOS sensors in cell phones for gamma detection and classification, along with dedicated cosmic-ray camera experiments using Raspberry Pi hardware, have established the basic feasibility of the idea. Kosmiko advances this tradition by combining it with the radiation-hardening and reliability expertise of the Montpellier group, whose prior work has focused on how atmospheric neutrons and natural radiation cause single-event upsets in advanced microelectronic memories. The same physics that threatens avionics and satellites, in other words, becomes a measurable signal in a hobby-grade camera.

Reproducibility is the watchword throughout the publication. The complete hardware configuration, the software workflow, and the resulting data products are described in detail so that other laboratories, classrooms, and citizen scientists can rebuild the platform and compare results meaningfully. The analysis pipeline is built on widely used open scientific tools, and the authors discuss practical observations about the camera components and the local measurement environment that would otherwise have to be rediscovered by trial and error. The team notes that no specific funding was received for the work, underscoring how inexpensive the platform is; the write-up credits Frédéric Wrobel with designing the experimental concept and methodology, with colleagues at the Institut d’Électronique et des Systèmes and the underground laboratory contributing to the alpha, cobalt-60, ground-level, and underground campaigns.

The implications reach beyond fundamental physics. A network of calibrated, low-cost pixel detectors could monitor natural radiation environments across wide geographic areas, support education by letting students see particle tracks with their own equipment, and provide empirical grounding for models of the terrestrial cosmic-ray flux such as those used to estimate radiation doses and predict electronic failure rates. Because the entire chain, from consumer sensor to per-pixel calibration to event-rate observables, is documented and deployable, Kosmiko lowers the barrier to participation in radiation measurement to the price of a small computer and two camera modules. In an era when frontier physics often demands billion-euro machines, there is something quietly revolutionary about silicon meant for photography quietly recording the passage of particles born light-years away, one calibrated pixel at a time.

Subject of Research: Low-cost CMOS image sensors used as pixel-calibrated detectors of natural ionizing radiation

Article Title: Kosmiko: from CMOS imaging sensors to pixel-calibrated particle instruments

Article References: Wrobel, F., Wrobel, J., Maraine, T., Boch, J., Risso, C., Dilillo, L., & Saigné, F. (2026). Kosmiko: from CMOS imaging sensors to pixel-calibrated particle instruments. Experimental Astronomy, 62(2), Article 21. https://doi.org/10.1007/s10686-026-10081-9

Image Credits: AI Generated

DOI: 10.1007/s10686-026-10081-9

Keywords: cosmic rays, CMOS sensor, active-pixel sensor, Raspberry Pi, ionizing radiation, Sony IMX477, particle detection, calibration, radiation environment, Experimental Astronomy, citizen science, underground laboratory