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1. The TeV Diffuse Cosmic Neutrino Spectrum and the Nature of Astrophysical Neutrino Sources

   https://doi.org/10.3847/1538-4357/ac7649  

   Fang, Ke; Gallagher, John S.; Halzen, Francis (July 2022, The Astrophysical Journal)

   Abstract The diffuse flux of cosmic neutrinos has been measured by the IceCube Observatory from TeV to PeV energies. We show that an improved characterization of this flux at lower energies, TeV and sub-TeV, reveals important information on the nature of the astrophysical neutrino sources in a model-independent way. Most significantly, it could confirm the present indications that neutrinos originate in cosmic environments that are optically thick to GeV–TeV γ -rays. This conclusion will become inevitable if an uninterrupted or even steeper neutrino power law is observed in the TeV region. In such γ -ray-obscured sources, the γ -rays that inevitably accompany cosmic neutrinos will cascade down to MeV–GeV energies. The requirement that the cascaded γ -ray flux accompanying cosmic neutrinos should not exceed the observed diffuse γ -ray background puts constraints on the peak energy and density of the radiation fields in the sources. Our calculations inspired by the existing data suggest that a fraction of the observed diffuse MeV–GeV γ -ray background may be contributed by neutrino sources with intense radiation fields that obscure the high-energy γ -ray emission accompanying the neutrinos. 

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2. Cascaded Gamma-Ray Emission Associated with the KM3NeT Ultrahigh-energy Event KM3-230213A

   https://doi.org/10.3847/2041-8213/adbbec  

   Fang, Ke; Halzen, Francis; Hooper, Dan (March 2025, The Astrophysical Journal Letters)

   Abstract A neutrino-like event with an energy of ∼220 PeV was recently detected by the KM3NeT/ARCA telescope. If this neutrino comes from an astrophysical source or from the interaction of an ultrahigh-energy cosmic ray in the intergalactic medium, the ultrahigh-energy gamma rays that are coproduced with the neutrinos will scatter with the extragalactic background light, producing an electromagnetic cascade and resulting in emission at GeV-to-TeV energies. In this Letter, we compute the gamma-ray flux from this neutrino source considering various source distances and strengths of the intergalactic magnetic field (IGMF). We find that the associated gamma-ray emission could be observed by existing imaging air Cherenkov telescopes and air shower gamma-ray observatories, unless the strength of the IGMF isB ≳ 3 × 10⁻¹³G or the ultrahigh-energy gamma rays are attenuated inside of the source itself. In the latter case, this source is expected to be radio-loud. 

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3. Search for a diffuse flux of photons with energies above tens of PeV at the Pierre Auger Observatory

   https://doi.org/10.1088/1475-7516/2025/05/061  

   Abdul_Halim, A; Abreu, P; Aglietta, M; Allekotte, I; Almeida_Cheminant, K; Almela, A; Aloisio, R; Alvarez-Muñiz, J; Ambrosone, A; Ammerman_Yebra, J; et al (May 2025, Journal of Cosmology and Astroparticle Physics)

   Abstract Diffuse photons of energy above 0.1 PeV, produced through the interactions between cosmic rays and either interstellar matter or background radiation fields, are powerful tracers of the distribution of cosmic rays in the Galaxy. Furthermore, the measurement of a diffuse photon flux would be an important probe to test models of super-heavy dark matter decaying into gamma-rays. In this work, we search for a diffuse photon flux in the energy range between 50 PeV and 200 PeV using data from the Pierre Auger Observatory. For the first time, we combine the air-shower measurements from a 2 km²surface array consisting of 19 water-Cherenkov surface detectors, spaced at 433 m, with the muon measurements from an array of buried scintillators placed in the same area. Using 15 months of data, collected while the array was still under construction, we derive upper limits to the integral photon flux ranging from 13.3 to 13.8 km^-2sr^-1yr^-1above tens of PeV. We extend the Pierre Auger Observatory photon search program towards lower energies, covering more than three decades of cosmic-ray energy. This work lays the foundation for future diffuse photon searches: with the data from the next 10 years of operation of the Observatory, this limit is expected to improve by a factor of ∼20. 

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4. Performance of the HAWC Observatory and TeV Gamma-Ray Measurements of the Crab Nebula with Improved Extensive Air Shower Reconstruction Algorithms

   https://doi.org/10.3847/1538-4357/ad5f2d  

   Albert, A; Alfaro, R; Alvarez, C; Andrés, A; Arteaga-Velázquez, J C; Avila_Rojas, D; Ayala_Solares, H A; Babu, R; Belmont-Moreno, E; Bernal, A; et al (September 2024, The Astrophysical Journal)

   Abstract The High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, located on the side of the Sierra Negra volcano in Mexico, has been fully operational since 2015. The HAWC collaboration has recently significantly improved their extensive air shower reconstruction algorithms, which has notably advanced the observatory performance. The energy resolution for primary gamma rays with energies below 1 TeV was improved by including a noise-suppression algorithm. Corrections have also been made to systematic errors in direction fitting related to the detector and shower plane inclinations, $\mathcal{O}(0.°1)$biases in highly inclined showers, and enhancements to the core reconstruction. The angular resolution for gamma rays approaching the HAWC array from large zenith angles (>37°) has improved by a factor of 4 at the highest energies (>70 TeV) as compared to previous reconstructions. The inclusion of a lateral distribution function fit to the extensive air shower footprint on the array to separate gamma-ray primaries from cosmic-ray ones based on the resultingχ²values improved the background rejection performance at all inclinations. At large zenith angles, the improvement in significance is a factor of 4 compared to previous HAWC publications. These enhancements have been verified by observing the Crab Nebula, which is an overhead source for the HAWC Observatory. We show that the sensitivity to Crab-like point sources (E^−2.63) with locations overhead to 30° zenith is comparable to or less than 10% of the Crab Nebula’s flux between 2 and 50 TeV. Thanks to these improvements, HAWC can now detect more sources, including the Galactic center. 

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5. Microquasar jet-cocoon systems as PeVatrons

   https://doi.org/10.1103/p6r1-qg5q  

   Zhang, B Theodore; Kimura, Shigeo S; Murase, Kohta (December 2025, Physical Review D)

   The origin of Galactic cosmic rays (CRs), particularly around the knee region (∼3 PeV), remains a major unanswered question. Recent observations by LHAASO suggest that the knee is shaped mainly by protons, with a transition to heavier elements at higher energies. Microquasars—compact jet-emitting sources—have emerged as possible PeV CR accelerators, especially after detections of ultrahigh-energy gamma rays from these systems. We propose that the observed proton spectrum (hard below a few PeV, steep beyond) arises from the reacceleration of sub-TeV Galactic CRs via shear acceleration in large-scale microquasar jet-cocoon structures. Our model also naturally explains the observed spectrum of energies around a few tens of PeV by summing up heavier nuclei contributions. Additionally, similar reacceleration processes in radio galaxies can contribute to ultrahigh-energy CRs, bridging Galactic and extragalactic origins. Combined with low-energy CRs from supernova remnants and galaxy clusters around the second knee region, this scenario could provide a unified explanation for CRs across the entire energy spectrum. 

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