CRPropa high statistics simulations for UHECR anisotropy studies

The detection of high-energy astrophysical multimessengers establishes a connection between ultra-high-energy cosmic rays (UHECRs) and powerful cosmic accelerators. Interactions of UHECRs with radiation fields and interstellar matter generate very-high-energy (VHE) gamma rays and neutrinos, making them key components in the multimessenger framework. This study examines the cosmogenic gamma-ray and neutrino fluxes resulting from UHECR propagation in starburst galaxies with supernova remnants, with a particular focus on NGC 1068, a well-established high-energy neutrino source. Using extragalactic simulations, we calculate the upper limit on cosmic-ray luminosity, applying upper limits on gamma-ray fluxes derived from observations by H.E.S.S. and MAGIC observatories. Our analysis incorporates the effects of both extragalactic and galactic magnetic fields on particle propagation, constraining the maximum extragalactic magnetic field (EGMF) intensity to 10 −14 G to ensure that at least 90% of injected UHECRs successfully reach Earth. The results provide upper limits on gamma-ray and neutrino fluxes, estimates of UHECR luminosity for individual sources, and predictions for the detection capabilities of the Cherenkov Telescope Array Observatory regarding gamma-ray emission from NGC 1068. Combining gamma-ray, neutrino, and UHECR observations reinforces the importance of multimessenger approaches in understanding the nature of high-energy astrophysical sources and their role in cosmic-ray acceleration.

We study how the properties of transient sources of ultra-high-energy cosmic rays (UHECRs) can be accessed by exploiting UHECR experiments, taking into account the propagation of UHECRs in magnetic structures which the sources are embedded in, i.e., clusters of galaxies and filamentary structures. Adopting simplified analytical models, we demonstrate that the structured extragalactic magnetic fields (EGMFs) play crucial roles in unveiling the properties of the transient sources. These EGMFs unavoidably cause significant delay in the arrival time of UHECRs as well as the Galactic magnetic field, even if the strength of magnetic fields in voids is zero. Then, we show that, given good knowledge on the structured EGMFs, UHECR observations with high statistics above $10^{20}$ eV allow us to constrain the generation rate of transient UHECR sources and their energy input per burst, which can be compared with the rates and energy release of known astrophysical phenomena. We also demonstrate that identifying the energy dependence of the apparent number density of UHECR sources at the highest energies is crucial as a clue to such transient sources. Future UHECR experiments with extremely large exposure are required to reveal the nature of transient UHECR sources.

We discuss how lensing by magnetic fields in galaxy clusters affects ultrahigh energy cosmic ray (UHECR) observations. As specific example,we use Virgo together with the cluster magnetic fields obtained earlier in a constrained simulation of structure formation including MHD processes.We find that, if M87 is the single source of UHECRsfrom Virgo, the emitted flux is strongly anisotropic in the most interesting energy range, (50–100) EeV, and differs from the average value by a factor five or more for a significant fraction of observers. Since magnetic lensing is energy dependent, the external energy spectrum as seenby different observers varies strongly too. These anisotropies areaveraged out in the case that all active galactic nucleiin Virgo emit UHECRs. In both cases, the anisotropies of the emitted UHECR flux may introduce an important bias in theinterpretation of UHECR data like, e.g., the determination of the sourcedensity ns and the source energy spectrum of UHECRs.

During their propagation through intergalactic space, ultrahigh energy cosmic rays (UHECRs) interact with the background radiation fields. These interactions give rise to energetic electron/positron pairs and photons which in turn feed electromagnetic cascades, contributing to the isotropic gamma-ray background (IGRB). The gamma-ray flux level generated in this way highly depends upon the UHECR propagation distance, as well as the evolution of their sources with redshift. Recently, the Fermi-LAT collaboration reported that the majority of the total extragalactic gamma-ray flux originates from extragalactic point sources. This posits a stringent upper limit on the IGRB generated via UHECR propagation, and subsequently constrains their abundance in the distant Universe. Focusing on the contribution of UHECR at energies below the ankle within a narrow energy band ($(1-4)\times 10^{18}$eV), we calculate the diffuse gamma-ray flux generated through UHECR propagation, normalizing the total cosmic ray energy budget in this band to that measured. We find that in order to not over-produce the new IGRB limit, a local "fog" of UHECR produced by nearby sources may exist, with a possible non-negligible contribution from our Galaxy. Following the assumption that a given fraction of the observed IGRB at 820GeV originates from UHECR, we obtain a constraint on the maximum distance for the majority of their sources. With other unresolved source populations still contaminating the new IGRB limit, and UHECR above the ankle invariably contributing also to this background, the results presented here are rather conservative.

We present a review and analysis of the experimental data on cosmic rays with energies above 1014-1015 eV and up to 1020 eV. At the beginning a brief survey of the direct experimental data on the cosmic-ray energy spectrum and mass composition is given. We then consider the choice of the hadron interaction model at energies above 1015 eV that is essential for the interpretation of the experimental data on extensive air showers (EAS). Much attention is given to the well known knee in the primary energy spectrum at approximately 3*1015 eV and also to an analysis of this phenomenon in the framework of the recently developed Hall diffusion model. In doing so we use as yet scarce data on cosmic-ray anisotropy and mass composition at superhigh energies 1014-1017 eV. A considerable part of our review is concerned with cosmic rays of ultrahigh energies 1017-1020 eV. Using the galactic origin model for cosmic rays with energies 1014-1020 eV the possibility of explaining all the relevant data at energies above 3*1018-1019 eV is discussed: evidence of the 'ankle' in the primary energy spectrum, the upper bound of cosmic-ray anisotropy, and evidence for the increasing proton abundance. The prospects for future cosmic-ray investigations at superhigh energies are outlined and the great importance of development in the neutron and proton astronomy for energies above 1018-1019 eV is pointed out.

The origin of ultra-high energy cosmic rays (UHECRs) is one of the most mystifying issues in astroparticle physics. It has been suggested that gamma-ray bursts (GRBs) are excellent acceleration sites for cosmic rays. The propagation of UHECRs from the GRB host galaxy to the Earth should generate delayed secondary photons and neutrinos. Here we present a dedicated search for delayed UHECR and neutrino emission centered around the position of nearby GRB 980425/SN 1998bw. Located at a distance of 36.9 Mpc, GRB 980425/SN 1998bw is well within the Greisen-Zatsepin-Kuzmin (GZK) distance horizon. We find no evidence for UHECR or neutrino clustering around the GRB 980425/SN 1998bw position between 2004 and 2020. Under ideal propagation conditions, we propose that it might be possible to detect an excess from delayed UHECRs around GRB 980425/SN 1998bw within the next 100 years if the intergalactic magnetic field (IGMF) strength is B ≤ 3 × 10-13 G.

Journal of Physics G: Nuclear and Particle Physics is committed to bringing its readers timely reviews in topics of interest to the nuclear and particle physics community, and an active programme of Topical Reviews is scheduled for future issues. Appropriate topics and authors are identified by the Editorial Board prior to invitation and commissioning of reviews, but any suggestions of suitable topics or offers of review contributions will be welcomed by the Board.

We propose that a hierarchical shock model—including supernova remnant shocks, galactic wind termination shocks, and accretion shocks around cosmic filaments and galaxy clusters—can naturally explain the cosmic-ray spectrum from ∼1 GeV up to ∼ 200 EeV. While this framework applies to the entire cosmic-ray spectrum, in this work we focus on its implications for ultra-high-energy cosmic rays (UHECRs). We perform a hydrodynamic cosmological simulation to investigate the power processed at shocks around clusters and filaments. The downstream flux from nearby shocks around the local filament accounts for the softer, lower-energy extragalactic component around the ankle, and the upstream escaping flux from nearby clusters accounts for the transition to a hard spectral component at the highest energies. This interpretation is in agreement with UHECR observations. We suggest that a combination of early Universe galactic outflows, cosmic-ray streaming instabilities, and a small-scale turbulent dynamo can increase magnetic fields enough to attain the required rigidities. Our simulation suggests that the available volume-averaged power density of accretion shocks exceeds the required UHECR luminosity density by 3 orders of magnitude. We show that microgauss magnetic fields at these shocks could explain both the origin of UHECRs and potentially contribute to the diffuse radio synchrotron background below 10 GHz. The shock-accelerated electrons produce a hard radio background without overproducing diffuse inverse Compton emission. These results motivate further observational tests with upcoming facilities to help distinguish accretion shocks from other UHECR sources.

Telescope Array (TA) and Auger experiments reported anisotropies in the arrival direction of ultra-high-energy cosmic rays (UHECRs). In particular, Auger Collaboration reported a correlation between UHECR events and the flux model of assumed sources and suggested a contribution of starburst galaxies (SBGs) to the anisotropy of UHECRs. However, in their study, the effect of coherent deflections by the galactic magnetic field (GMF) is not taken into account. In this study, we investigated the effect of the GMF on the arrival directions of UHECRs using the cosmic ray propagation code CRPropa3. We used a backtracking technique which consists of propagating antiparticles to map the flux outside the galaxy to at the earth. We estimate the systematic effects caused by GMF in the reported likelihood analysis. We conduct likelihood analysis for mock UHECR datasets based on the flux pattern through the GMF model. We found systematic decrease of $(f_{\rm ani}, \:\theta)$ due to GMF. As prospects for the TAx4 experiment and joint analysis of Auger and TA collaborations, we develop the likelihood analysis method with the convolution of the rigidity spectrum.

The origin of ultra high energy cosmic rays (UHECR) is a long-standing mystery. The aim of the Telescope Array (TA) experiment is to reveal this by observing the spectrum, anisotropy, and mass composition by utilizing an array of surface detectors (SD) and fluorescence detectors. One of the uncertainties in UHECR observation derives from the hadronic interaction model used for air shower Monte Carlo (MC) simulations. The number of muons observed at the ground from UHECR induced air showers is expected to depend upon the composition of primary cosmic rays. The MC prediction depends also on hadronic interaction models. One may test the hadronic interaction models by comparing the measured number of muons with the MC prediction. We studied muon densities in the UHE extensive air showers by analyzing the signal of surface detector stations for highly inclined showers which should have high muon purity. A high muon purity condition is imposed that requires the geometry of the shower and relative position of the given station. The condition implies that the muons dominate the signal, i.e. contribute about $\sim$65% of the total signal. The number of particles from air showers observed on the condition of muon purity is typically 1.88 $\pm$ 0.08 (stat.) $\pm$ 0.43 (syst.) times larger than the MC prediction value using the QGSJET I$\hspace{-.1em}$I-03 model for protons. The lateral distribution of the muons also falls more slowly in the data than the MC, causing larger discrepancy at larger lateral distance. The same feature was also obtained for other hadronic models, such as QGSJET I$\hspace{-.1em}$I-04.

The year 2007 has furnished us with outstanding results about the origin of the most energetic cosmic rays: a flux suppression as expected from the GZK-effect has been observed in the data of the HiRes and Auger experiments and correlations between the positions of nearby AGN and the arrival directions of trans-GZK events have been observed by the Pierre Auger Observatory. The latter finding marks the beginning of ultra high-energy cosmic ray astronomy and is considered a major breakthrough starting to shed first light onto the sources of the most extreme particles in nature. This report summarizes those observations and includes other major advances of the field, mostly presented at the 30th International Cosmic Ray Conference held in Mérida, Mexico, in July 2007. With increasing statistics becoming available from current and even terminated experiments, systematic differences amongst different experiments and techniques can be studied in detail which is hoped to improve our understanding of experimental techniques and their limitations.

Ultra high energy cosmic rays: The disappointing model

A posteriori anisotropy study of ultra-high energy cosmic rays (UHECRs) with the Pierre Auger Observatory (PAO) has shown evidence of excess of cosmic ray particles above 55 EeV within $18^{\circ}$ of the direction of the radio galaxy Centaurus A. However, the origin of the excess remains elusive. We simulate the propagation of different species of particles coming from the direction of Centaurus A in the Galactic magnetic fields, and find that only particles of nuclear charge $Z\la 10$ can avoid being deflected outside of the $18^{\circ}$ window of Centaurus A. On the other hand, considering the increasingly heavy composition of UHECRs at the highest energies measured by PAO, a plausible scenario for cosmic rays from the direction of Centaurus A can be found if they consist of intermediate-mass nuclei. The chemical composition of cosmic rays can be further constrained by lower-energy cosmic rays of the same rigidity. We find that cosmic ray acceleration in the lobes of Centaurus A is not favored, while acceleration in the stellar winds that are rich in intermediate-mass nuclei, could meet the requirement. This suggests that the observed excess may originate from cosmic ray accelerators induced by stellar explosions in the star-forming regions of Centaurus A and/or the Centaurus cluster located behind Centaurus A.

A cosmic acceleration mechanism is introduced which is based on the wakefields excited by the Alfven shocks in a relativistically flowing plasma. We show that there exists a threshold condition for transparency below which the accelerating particle is collision‐free and suffers little energy loss in the plasma medium. The stochastic encounters of the random accelerating‐decelerating phases results in a power‐law energy spectrum: f(e) ∝ 1/e2. As an example, we discuss the possible production of super‐GZK ultra high energy cosmic rays (UHECR) in the atmosphere of gamma ray bursts. The estimated event rate in our model agrees with that from UHECR observations.

The study of the propagation of ultra-high-energy cosmic rays (UHECRs) is a key step to unveiling the secret of their origin. Up to now only the influence of the galactic and extragalactic magnetic fields was considered. In this article we focus our analysis on the influence of the magnetic field of the galaxies standing between possible UHECR sources and us. Our main approach is to start from the well-known galaxy distribution up to 120 Mpc. We use the most complete galaxy catalog: the LEDA catalog. Inside a sphere of 120 Mpc, we extract 60,130 galaxies with known positions. In our simulations we assign a halo dipole magnetic field (HDMF) to each galaxy. The code developed is able to retro-propagate a charged particle from the arrival points of UHECR data across our galaxy sample. We present simulations in the case of the Virgo Cluster and show that there is a nonnegligible deviation in the case of protons of 7 × 1019 eV, even if the B value is conservative. Then special attention is devoted to the AGASA triplet, where we find that NGC 3998 and NGC 3992 could be possible source candidates.