Abstracts of Interest
Selected by:
Imogen Barnsley
Abstract: 2504.10043
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Title:Instrumenting a Lake as a Wide-Field Gamma-ray Detector
View PDF HTML (experimental)Abstract:Ground-level particle detection has recently emerged as an extremely powerful approach to TeV-PeV gamma-ray astronomy. The most successful observatories of this type, HAWC and LHAASO, utilise water-Cherenkov based detector units, housed in tanks or buildings. Here we explore the possibility of deploying water-Cherenkov detector units directly in to a natural or artificial lake. Possible advantages include reduced cost and improved performance due to better shielding. The lake concept has been developed as an option for the future Southern Wide-view Gamma-ray Observatory, and is now under consideration for a possible future extension of the observatory, beyond its recently selected land site. We present results from prototypes operated in a custom built facility, and concepts for full-scale array deployment and long-term operation.
Abstract: 2504.10308
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Title:Search for Kink Events in Variable Fermi-LAT Blazars
View PDF HTML (experimental)Abstract:This study explores the detection of Quasi-Periodic Oscillations (QPOs) in blazars as a method to identify kink events within their jets, utilizing both $\gamma$-ray and polarized light observations. Focusing on a sample of 9 blazars, we analyze $\gamma$-ray light curves to identify significant QPOs. In addition to $\gamma$-ray data, we incorporated polarized light data corresponding to the same temporal segments to cross-validate the presence of QPOs. However, the limited availability of comprehensive polarized data restricted our ability to perform a thorough analysis across all datasets. Despite these limitations, our analysis reveals a segment where QPOs in polarized light coincided with those observed in $\gamma$-rays, providing preliminary evidence supporting the kink origin of these oscillations.
Abstract: 2504.10333
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Title:The Pierre Auger Observatory: Results and Prospects
View PDF HTML (experimental)Abstract:The Pierre Auger Observatory advances the study of ultra-high-energy cosmic rays through a hybrid system of surface and fluorescence detectors. This paper presents recent results, including refined spectrum measurements, anisotropy evidence, and new insights into cosmic-ray composition. Studies at energies beyond terrestrial accelerators reveal implications for particle physics. The AugerPrime upgrade will further enhance particle identification and extend the sensitivity to photons and neutrinos, broadening the Observatory's capability to explore cosmic-ray sources and propagation, paving the way for new discoveries.
Abstract: 2504.10576
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Title:Cosmogenic neutrinos as probes of new physics
View PDF HTML (experimental)Abstract:The scattering of extremely energetic cosmic rays with both cosmic microwave background and extragalactic background light, can produce $\mathcal{O}(10^{18} \,{\rm eV})$ neutrinos, known as cosmogenic neutrinos. These neutrinos are the only messengers from the extreme cosmic accelerators that can reveal the origin of the most energetic cosmic rays. Consequently, much effort is being devoted to achieving their detection. In particular, the GRAND project aims to observe the $\nu_\tau$ and $\bar \nu_\tau$ components of the cosmogenic neutrino flux in the near future using radio antennas. In this work, we investigate how the detection of cosmogenic neutrinos by GRAND can be used to probe beyond the Standard Model physics. We identify three well-motivated scenarios which induce distinct features in the cosmogenic neutrino spectrum at Earth: neutrino self-interactions mediated by a light scalar ($\nu$SI), pseudo-Dirac neutrinos (PD$\nu$) and neutrinos scattering on ultra-light Dark Matter ($\nu$DM). We show these scenarios can be tested by GRAND, using 10 years of cosmogenic neutrino data, in a region of parameter space complementary to current experiments. For the $\nu$SI model,, we find that GRAND can constrain the coupling to $\nu_\tau$ in the range [$10^{-2},10^{-1}$] for a scalar with mass in the range 0.1 to 1 GeV. For PD$\nu$, we find that GRAND is sensitive to sterile-active mass squared splitting in the range [$10^{-15},10^{-13}$] ${\rm eV}^2$. Finally, for the $\nu$DM model, assuming a heavy mediator, GRAND can do substantially better than the current limits from other available data. These results rely on the fact that the actual cosmogenic flux is around the corner, not far from the current IceCube limit.
Abstract: 2504.10742
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Title:Unified Solar Modulation Potential for Same-Charge Cosmic Rays and Implications for Local Interstellar Spectra
View PDFAbstract:The energy spectra of cosmic rays (CRs) below tens of GeV are significantly modulated by solar activity within the heliosphere. To investigate the properties of Galactic CRs, it is important to determine the unmodulated local interstellar spectrum (LIS). Recent high-precision temporal measurements of CR energy spectra, released by the AMS-02 collaboration, provide a crucial observational foundation for this endeavor. In this study, we employ the widely used force-field approximation (FFA) model to analyze the AMS-02 data, and attempt to derive the LIS for CR protons and positrons. By applying a non-LIS method, we derive temporal variations of the relative solar modulation potential, $\Delta\phi$, for individual particle species. Our analysis demonstrates that the FFA provides sufficient accuracy in explaining the AMS-02 spectral measurements of all particles during the low solar activity period. Notably, the derived $\Delta\phi(t)$ for protons and positrons, as well as for electrons and antiprotons, exhibit excellent consistency, indicating that particles with the same charge sign can be effectively described within a unified FFA framework during the low solar activity period. Having established a well-constrained proton LIS and its associated modulation potential, we apply the common modulation behavior between positrons and protons to demodulate the AMS-02 positron measurements, and derive the positron LIS without necessitating prior knowledge of its characteristics. This LIS is useful for quantitative investigations into potential exotic origins of CR positrons.
Abstract: 2504.10756
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Title:Subspace Approximations to the Focused Transport Equation of Energetic Particles, I. The Standard Form
View PDF HTML (experimental)Abstract:The Fokker-Planck equation describing the transport of energetic particles interacting with turbulence is difficult to solve analytically. Numerical solutions are of course possible but they are not always useful for applications. In the past a subspace approximation was proposed which allows to compute important quantities such as the characteristic function as well as certain expectation values. This previous approach was applied to solve the one-dimensional Fokker-Planck equation which contains only a pitch-angle scattering term. In the current paper we extend this approach in order to solve the Fokker-Planck equation with a focusing term. We employ two- and three-dimensional subspace approximations to achieve a pure analytical description of particle transport. Additionally, we show that with higher dimensions, the subspace method can be used as a hybrid analytical-numerical method which produces an accurate approximation. Although the latter approach does not lead to analytical results, it is much faster compared to pure numerical solutions of the considered transport equation.
Abstract: 2504.10847
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Title:Cosmic-Ray Constraints on the Flux of Ultra-High-Energy Neutrino Event KM3-230213A
View PDF HTML (experimental)Abstract:The detection of a $\simeq220$~PeV muon neutrino by the KM3NeT neutrino telescope offers an unprecedented opportunity to probe the Universe at extreme energies. We analyze the origin of this event under three scenarios, viz., a transient point source, a diffuse astrophysical emission, and line-of-sight interaction of ultrahigh-energy cosmic rays (UHECR; $E \gtrsim 0.1$~EeV). Our analysis includes the flux from both a KM3NeT-only fit and a joint fit, incorporating data from KM3NeT, IceCube, and Pierre Auger Observatory. If the neutrino event originates from transients, it requires a new population of transient that is energetic, gamma-ray dark, and more abundant than known ones. In the framework of diffuse astrophysical emission, we compare the required local UHECR energy injection rate at $\gtrsim4$ EeV, assuming a proton primary, with the rate derived from the flux measurements by Auger. This disfavors the KM3NeT-only fit at all redshifts, while the joint fit remains viable for $z\gtrsim 1$, based on redshift evolution models of known source populations. For cosmogenic origin from point sources, our results suggest that the luminosity obtained at redshifts $z \lesssim 1$ from the joint fit is compatible with the Eddington luminosity of supermassive black holes in active galactic nuclei.
Abstract: 2504.10907
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Title:Evidence for GeV Gamma-Ray Emission from Intense GRB 240529A During the Afterglow's Shallow Decay Phase
View PDF HTML (experimental)Abstract:X-ray light curves of gamma-ray burst (GRB) afterglows exhibit various features, with the shallow decay phase being particularly puzzling. While some studies report absence of the X-ray shallow decay for hyper-energetic GRBs, recently discovered GRB 240529A shows a clear shallow decay phase with an isotropic gamma-ray energy of \SI{2.2e54}{erg}, making it a highly unusual case compared to typical GRBs. In order to investigate the physical mechanism of the shallow decay, we perform the \textit{Fermi}-LAT analysis of GRB 240529A along with \textit{Swift}-XRT analysis. We find no jet break feature in the X-ray light curve and then give the lower bound of the collimation-corrected jet energy of $>10^{52}$~erg, which is close to the maximum rotational energy of a magnetar. Our LAT data analysis reveals evidence of GeV emission with a statistical significance of $4.5\sigma$ during the shallow decay phase, which can be interpreted as the first case for hyper-energetic GRBs with a typical shallow decay phase. The GeV to keV flux ratio is calculated to be $4.2\pm2.3$. Together with X-ray spectral index, this indicates an inverse Compton origin of the GeV emission. Multiwavelength modeling based on time-dependent simulations tested two promising models, the energy injection and wind models. Both models can explain the X-ray and gamma-ray data, while our modeling demonstrates that gamma-ray observations, along with future GeV--TeV observations by CTAO, will distinguish between them.
Abstract: 2504.11542
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Title:Scaler rates from the Pierre Auger Observatory: a new proxy of solar activity
View PDF HTML (experimental)Abstract:The modulation of low-energy galactic cosmic rays reflects interplanetary magnetic field variations and can provide useful information on solar activity. An array of ground-surface detectors can reveal the secondary particles, which originate from the interaction of cosmic rays with the atmosphere. In this work, we present an investigation of the low-threshold rate (scaler) time series recorded in 16 years of operation by the Pierre Auger Observatory surface detectors in Malargue, Argentina. Through an advanced spectral analysis, we detected highly statistically significant variations in the time series with periods ranging from the decadal to the daily scale. We investigate their origin, revealing a direct connection with solar variability. Thanks to their intrinsic very low noise level, the Auger scalers allow a thorough and detailed investigation of the galactic cosmic-ray flux variations in the heliosphere at different timescales and can, therefore, be considered a new proxy of solar variability.
Abstract: 2504.11712
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Title:Ultra-high energy cosmic rays with UFA-15 source model in Bumblebee gravity theory
View PDF HTML (experimental)Abstract:We explore the effects of Bumblebee gravity on the propagation of ultra-high energy cosmic rays (UHECRs) using astrophysical sources modeled in the Unger-Farrar-Anchordoqui (UFA) framework (2015), which includes star formation rate (SFR), gamma-ray bursts (GRBs), and active galactic nuclei (AGN). We compute the density enhancement factor for various source separation distances ($d_\text{s}$s) up to 100 Mpc within the Bumblebee gravity scenario. Additionally, we calculate the CRs flux and their suppression, comparing the results with observational data from the Pierre Auger Observatory (PAO) and the Telescope Array through $\chi^2$ and $\chi_\text{red}^2$ analysis for the flux and Levenberg-Marquardt algorithm for suppression. The anisotropy in CRs arrival directions is examined, with corresponding $\chi^2$ and $\chi_\text{red}^2$ values obtained from the PAO surface detector data (SD 750 and SD 1500). Finally, we present skymaps of flux and anisotropy under different model assumptions, providing insights into the observational signatures of UHECRs in Bumblebee gravity. Our results show that increasing the Bumblebee gravity parameter $l$ enhances the density factor $\xi$, particularly at low energies, highlighting Lorentz violation's impact on CRs' propagation. Larger $d_\text{s}$ values amplify deviations from the $\Lambda$CDM model, with AGN sources dominating at high energies and GRB/SFR sources at lower energies. The skymaps indicate the structured flux patterns at large $d_\text{s}$ and structured anisotropy at higher energies.
Abstract: 2504.11985
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Title:A Heavy-Metal Scenario of Ultra-High-Energy Cosmic Rays
View PDF HTML (experimental)Abstract:The mass composition of ultra-high-energy cosmic rays is an open problem in astroparticle physics. It is usually inferred from the depth of the shower maximum (Xmax) of cosmic-ray showers, which is only ambiguously determined by modern hadronic interaction models. We examine a data-driven scenario, in which we consider the expectation value of Xmax as a free parameter. We test the novel hypothesis whether the cosmic-ray data from the Pierre Auger Observatory can be interpreted in a consistent picture, under the assumption that the mass composition of cosmic rays at the highest energies is dominated by high metallicity, resulting in pure iron nuclei at energies above ~40 EeV. We investigate the implications on astrophysical observations and hadronic interactions, and we discuss the global consistency of the data assuming this heavy-metal scenario. We conclude that the data from the Pierre Auger Observatory can be interpreted consistently if the expectation values for Xmax from modern hadronic interaction models are shifted to larger values.
Abstract: 2504.13271
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Title:Sensitivity of BEACON to Ultra-High Energy Diffuse and Transient Neutrinos
View PDF HTML (experimental)Abstract:Ultra-high energy neutrinos ($E>10^{17}$ eV) can provide insight into the most powerful accelerators in the universe, however their flux is extremely low. The Beamforming Elevated Array for COsmic Neutrinos (BEACON) is a detector concept which efficiently achieves sensitivity to this flux by employing phased radio arrays on mountains, which search for the radio emission of up-going extensive air showers created by Earth-skimming tau neutrinos. Here, we calculate the point-source effective area of BEACON and characterize its sensitivity to transient neutrino fluences with both short ($<15$ min) and long ($> 1$ day) durations. Additionally, by integrating the effective area, we provide an updated estimate of the diffuse flux sensitivity. With just 100 stations, BEACON achieves sensitivity to short-duration transients such as nearby short gamma-ray bursts. With 1000 stations, BEACON achieves a sensitivity to long-duration transients, as well as the cosmogenic flux, ten times greater than existing experiments at 1 EeV. With an efficient design optimized for ultrahigh energy neutrinos, BEACON is capable of discovering the sources of neutrinos at the highest energies.
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