Propagation Of Ultra-high Energy Cosmic Rays Research Articles

Indication of a local fog of subankle ultrahigh energy cosmic rays

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.

A Bayesian analysis of the 69 highest energy cosmic rays detected by the Pierre Auger Observatory

The origins of ultra-high energy cosmic rays (UHECRs) remain an open question. Several attempts have been made to cross-correlate the arrival directions of the UHECRs with catalogs of potential sources, but no definite conclusion has been reached. We report a Bayesian analysis of the 69 events from the Pierre Auger Observatory (PAO), that aims to determine the fraction of the UHECRs that originate from known AGNs in the Veron-Cety & Veron (VCV) catalog, as well as AGNs detected with the Swift Burst Alert Telescope (Swift-BAT), galaxies from the 2MASS Redshift Survey (2MRS), and an additional volume-limited sample of 17 nearby AGNs. The study makes use of a multi-level Bayesian model of UHECR injection, propagation and detection. We find that for reasonable ranges of prior parameters, the Bayes factors disfavour a purely isotropic model. For fiducial values of the model parameters, we report 68% credible intervals for the fraction of source originating UHECRs of 0.09+0.05-0.04, 0.25+0.09-0.08, 0.24+0.12-0.10, and 0.08+0.04-0.03 for the VCV, Swift-BAT and 2MRS catalogs, and the sample of 17 AGNs, respectively.

Prospects for ultrahigh-energy particle observation based on the lunar orbital LORD space experiment

The problem of searching for highest-energy cosmic rays and neutrinos in the Universe is reviewed. Possibilities for using the radio method for detecting particles of energies above the GZK cut-off are analyzed. The method is based on the registration of coherent radio emission produced by cascades of most energetic particles in radio-transparent lunar regolith. The Luna-Resurs Orbiter space mission to be launched in the near future (2020) involves the Lunar Orbital Radio Detector (LORD). The design of the LORD space instrument and its scientific potentialities for registration of low-intense cosmic ray particle fluxes of energies above the GZK cut-off up to 1024eV are discussed. The designed LORD module (including the antenna, amplification, and data-acquisition systems) now is under construction. Exposure and capabilities of the LORD space experiment for detection of ultrahigh-energy cosmic rays and neutrinos have been compared with those for well-known current and proposed experiments. The LORD space experiment will make it possible to obtain important information on the highest-energy particles in the Universe, to verify modern models for the origin and the propagation of ultrahigh-energy cosmic rays and neutrinos.

Effects of uncertainties in simulations of extragalactic UHECR propagation, using CRPropa and SimProp

The results of simulations of extragalactic propagation of ultra-high energy cosmic rays (UHECRs) have intrinsic uncertainties due to poorly known physical quantities and approximations used in the codes. We quantify the uncertainties in the simulated UHECR spectrum and composition due to different models of extragalactic background light (EBL), different photodisintegration setups, approximations concerning photopion production and the use of different simulation codes. We discuss the results for several representative source scenarios with proton, nitrogen or iron at injection. For this purpose we used SimProp and CRPropa, two publicly available codes for Monte Carlo simulations of UHECR propagation. CRPropa is a detailed and extensive simulation code, while SimProp aims to achieve acceptable results using a simpler code. We show that especially the choices for the EBL model and the photodisintegration setup can have a considerable impact on the simulated UHECR spectrum and composition.

CRPropa: A public framework to propagate UHECRs in the universe

To answer the fundamental questions concerning the origin and nature of ultra-high energy cosmic rays (UHECRs), it is important to confront data with simulated astrophysical scenarios. These scenarios should include detailed information on particle interactions and astrophysical environments. To achieve this goal one should make use of computational tools to simulate the propagation of these particles. For this reason the CRPropa framework was developed. It allows the propagation of UHECRs with energies $\gtrsim$10$^{17}$ eV and secondary gamma rays and neutrinos. The newest version, CRPropa 3, reflects an efficient redesign of the code as well as several new features such as time dependent propagation in three dimensions, galactic magnetic field effects and improved treatment of interactions, among other enhancements.

THE ISOTROPY PROBLEM OF SUB-ANKLE ULTRA HIGH ENERGY COSMIC RAYS

We study the time dependent propagation of sub-ankle ultra-high energy cosmic rays (UHECRs) originating from point-like Galactic sources. We show that drift in the Galactic magnetic field (GMF) may play an important role in the propagation of UHECRs and their measured anisotropy, particularly when the transport is anisotropic. To fully account for the discreteness of UHECR sources in space and time, a Monte Carlo method is used to randomly place sources in the Galaxy. The low anisotropy measured by Auger is not generally characteristic of the theoretical models, given that the sources are distributed in proportion to the star formation rate, but it can possibly be understood as a) intermittency effects due to the discrete nature of the sources or, with extreme parameters, b) a cancellation of drift current along a current sheet with the outward radial diffusive flux. We conclude that it is possible to interpret the Galactic sub-ankle CR flux as being due entirely to intermittent discrete Galactic sources distributed in proportion to star formation, but only with probability of roughly 35 percent, of which the spectrum is in accord with observations about 30 percent of the time. An alternative explanation for the low anisotropy may be that they are mostly extragalactic and/or heavy.

Propagation of UHECRs in the universe

The origin, propagation, and mechanisms of acceleration of the ultra-high energy cosmic rays (UHECRs) are not yet well understood. Aiming for a better interpretation of the available experimental data, these data have to be confronted with theoretical models. A realistic simulation of the propagation of UHECRs in the universe should take into account all the relevant energy loss processes due to the interaction with astrophysical backgrounds, as well as the intervening cosmic magnetic fields. Cosmological effects, such as the redshift dependence of the photon backgrounds and the adiabatic expansion of the universe, can play an important role in the aforementioned processes. Here we present results of simulations of the propagation of UHECR through the large scale structure of the universe considering cosmological and magnetic field effects simultaneously.

HERMES: Simulating the propagation of ultra-high energy cosmic rays

The study of ultra-high energy cosmic rays (UHECR) at Earth cannot prescind from the study of their propagation in the Universe. In this paper, we present HERMES, the ad hoc Monte Carlo code we have developed for the realistic simulation of UHECR propagation. We discuss the modeling adopted to simulate the cosmology, the magnetic fields, the interactions with relic photons and the production of secondary particles. In order to show the potential applications of HERMES for astroparticle studies, we provide an estimation of the surviving probability of UHE protons, the GZK horizons of nuclei and the all-particle spectrum observed at Earth in different astrophysical scenarios. Finally, we show the expected arrival direction distribution of UHECR produced from nearby candidate sources. A stable version of HERMES will be released in the next future for public use together with libraries of already propagated nuclei to allow the community to perform mass composition and energy spectrum analysis with our simulator.

Very high energy photons and neutrinos: Implications for UHECR

Depending on the environment of the acceleration processes for ultra-high energy cosmic rays, photons and neutrinos may be produced by interactions in the sources. If so, observations of gamma-rays or neutrinos could pinpoint the sources. This paper reviews some aspects of the search for photons and neutrinos from possible sources of ultra-high energy cosmic rays. Both photons and neutrinos are potential tracers of sites of cosmic-ray acceleration because, being neutral, they are not deviated by intervening magnetic fields. Photons are plentiful, but their interpretation is complicated by interactions in the sources and spectral modifications due to cascading in transit. In addition, their origin can be from radiation by electrons as well as from decay of neutral pions. In contrast, high-energy neutrinos would directly imply acceleration of hadrons. Because they interact only weakly, neutrinos are not distorted by interactions in the sources, but for the same reason they are hard to detect. Photons and neutrinos provide complementary types of information about cosmic-ray sources, but they both indicate underlying acceleration of charged particles. It is therefore natural to select known sources of TeV gamma-rays as potential sources of neutrinos. It is useful to distinguish two ways of producing gamma-rays and neutrinos: at the sources of ultra-high energy cosmic rays (UHECR) and during propagation through the cosmos. In the first case, the observed spectra of the neutral messengers ( and ν) depend on the source spectrum, on cosmic evolution of the sources and on conditions inside or near the acceleration region. Cosmogenic neutrinos and photons produced during propagation of UHECR depend on the injected spectrum as a function of energy and red shift, but not on conditions at the sources. It is generally assumed that the highest energy cosmic rays are accelerated in extra-galactic sources, and that the transition from the lower energy population of galactic cosmic rays occurs somewhere between 0.3 and 3EeV. Protons and nuclei with energies above 30EeV begin to suffer significant energy losses during propagation from sources more distant than a few hundred Mpc. Ten EeV, being below the suppression energy, but above the transition region is a suitable energy around which to estimate the energy content of the cosmic rays from extra-galactic sources. The observed energy flux at 10EeV by HiRes (1) and Telescope Array (2 )i s

THE ROLE OF STRUCTURED MAGNETIC FIELDS ON CONSTRAINING PROPERTIES OF TRANSIENT SOURCES OF ULTRA-HIGH-ENERGY COSMIC RAYS

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.

The Magnetic Field of the Milky Way from Faraday Rotation of Pulsars and Extragalactic Sources

Faraday rotation towards polarised pulsars and extragalactic sources is the best observable for determining the configuration of the magnetic field of the Galaxy in its plane and also at high latitudes. The Galactic magnetic field plays an important role in numerous astrophysical processes, including star formation and propagation of ultrahigh-energy cosmic rays; it is also an important component in measurements of the cosmological microwave background. This review article provides a brief overview of the latest advancements in the field, from an observer’s point of view. The most recent results based on pulsar rotation measures are discussed, which show that we have begun to confidently resolve the main features of the Galactic magnetic field on kiloparsec scales, both in the Solar neighbourhood and at larger distances. As we are currently in great anticipation of polarisation observations with new, state-of-the-art telescopes and hardware, a brief overview of how much this field of research will benefit from the upcoming pulsar surveys is also given.

Need for a local source of ultrahigh-energy cosmic-ray nuclei

Recent results of the Pierre Auger (Auger) fluorescence detectors indicate an increasingly heavy composition of ultra-high energy (UHE) cosmic rays (CRs). Assuming that this trend continues up to the highest energies observed by the Auger surface detectors we derive the constraints this places on the local source distribution of UHE CR nuclei. Utilizing an analytic description of UHE CR propagation we derive the expected spectra and composition for a wide range of source emission spectra. We find that sources of intermediate-to-heavy nuclei are consistent with the observed spectra and composition data above the ankle. This consistency requires the presence of nearby sources within 60 Mpc and 80 Mpc for silicon and iron only sources, respectively. The necessity of these local sources becomes even more compelling in the presence nano-Gauss local extragalactic magnetic fields.

Propagation of ultrahigh-energy cosmic rays in an expanding Universe

A numerical code is described that simulates the propagation of protons and ultrahigh-energy nuclei in an expanding Universe, taking into account the interaction with the background electromagnetic radiation. Using this code, we calculate the intensity of extragalactic cosmic rays, sources of which are jets of galaxies with active nuclei.

Studies of UHECR propagation in the galactic magnetic field

We present results of simulations of ultra-high energy cosmic ray (UHECR) propagation in the Galactic magnetic field (GMF). Different assumptions on the large-scale GMF structure and/or primary particle lead to distinctly different deflection patterns of the highest energy cosmic rays (CR). The GMF modifies the exposure of an UHECR experiment to the extragalactic sky. We estimated these effects for the Pierre Auger experiment. Further forward-tracking studies under plausible UHECR sources scenarios will allow for direct comparison with the observed correlation between the nearby active galactic nuclei (AGN) and the highest energy Auger events.

Propagation of ultrahigh-energy cosmic rays

We briefly describe the energy loss processes of ultrahigh-energy protons, heavier nuclei and γ-rays in interactions with the universal photon fields of the Universe. We then discuss the modification of the accelerated cosmic-ray energy spectrum in propagation by the energy loss processes and the charged cosmic-ray scattering in the extragalactic magnetic fields. The energy lost by the ultrahigh-energy cosmic rays goes into γ-rays and neutrinos that carry additional information about the sources of highest energy particles. The new experimental results of the HiRes and the Auger collaborations are discussed in view of the predictions from propagation calculations.

On the acceleration of ultra-high-energy cosmic rays

Ultra-high-energy cosmic rays (UHECRs) hit the Earth's atmosphere with energies exceeding 10(18)eV. This is the same energy as carried by a tennis ball moving at 100 km h-1, but concentrated on a subatomic particle. UHECRs are so rare (the flux of particles with E>10(20)eV is 0.5 km -2 per century) that only a few such particles have been detected over the past 50 years. Recently, the HiRes and Auger experiments have reported the discovery of a high-energy cut-off in the UHECR spectrum, and Auger has found an apparent clustering of the highest energy events towards nearby active galactic nuclei. Consensus is building that the highest energy particles are accelerated within the radio-bright lobes of these objects, but it remains unclear how this actually happens, and whether the cut-off is due to propagation effects or reflects an intrinsically physical limitation of the acceleration process. The low event statistics presently allows for many different plausible models; nevertheless observations are beginning to impose strong constraints on them. These observations have also motivated suggestions that new physics may be implicated. We present a review of the key theoretical and observational issues related to the processes of propagation and acceleration of UHECRs and proposed solutions.

High energy gamma ray counterparts of astrophysical sources of ultra-high energy cosmic rays

If ultra-high energy cosmic rays (UHECRs) are accelerated at astrophysical point sources, the identification of such sources can be achieved if there is some kind of radiation at observable wavelengths that may be associated with the acceleration and/or propagation processes. No radiation of this type has so far been detected or at least no such connection has been claimed. The process of photopion production during the propagation of UHECRs from the sources to the Earth results in the generation of charged and neutral pions. The neutral (charged) pions in turn decay to gamma quanta and electrons that initiate an electromagnetic cascade in the universal photon background. We calculate the flux of this gamma radiation in the GeV–TeV energy range and find that for source luminosities compatible with those expected from small scale anisotropies in the directions of arrival of UHECRs, the fluxes can be detectable by future Cerenkov gamma ray telescopes, such as VERITAS and HESS, provided the intergalactic magnetic field is not larger than ∼10 −10 Gauss and for source distances comparable with the loss length for photopion production.

Constrained simulations of the magnetic field in the local Universe and the propagation ofultrahigh energy cosmic rays

We use simulations of large-scale structure formation to study the build-up ofmagnetic fields (MFs) in the intergalactic medium. Our basic assumption is thatcosmological MFs grow in a magnetohydrodynamical (MHD) amplificationprocess driven by formation of structure from a magnetic seed field present athigh red-shift. This approach is motivated by previous simulations of the MFs ingalaxy clusters which, under the same hypothesis as we adopt here, succeeded inreproducing Faraday rotation measurements (RMs) in clusters of galaxies. OurΛCDM initial conditions for the dark matter density fluctuations have been statisticallyconstrained by the observed large-scale density field within a sphere of 110 Mpcaround the Milky Way, based on the IRAS 1.2-Jy all-sky red-shift survey. As aresult, the positions and masses of prominent galaxy clusters in our simulationcoincide closely with their real counterparts in the local Universe. We find excellentagreement between RMs of our simulated galaxy clusters and observational data. Theimproved numerical resolution of our simulations compared to previous workalso allows us to study the MFs in large-scale filaments, sheets and voids. Bytracing the propagation of ultrahigh energy (UHE) protons in the simulated MFs weconstruct full-sky maps of expected deflection angles of protons with arrival energiesE = 1020 eVand 4 × 1019 eV, respectively. Accounting only for the structures within 110 Mpc, we find that strongdeflections are only produced if UHE protons cross galaxy clusters. The total area on thesky covered by these structures is however very small. Over still larger distances,multiple crossings of sheets and filaments may give rise to noticeable deflectionsover a significant fraction of the sky; the exact amount and angular distributiondepends on the model adopted for the magnetic seed field. On the basis of ourresults we argue that over a large fraction of the sky the deflections are likelyto remain smaller than the present experimental angular sensitivity. Therefore,we conclude that forthcoming air shower experiments should be able to locatesources of UHE protons and shed more light on the nature of cosmological MFs.

Cosmic Magnetic Fields and Their Influence on Ultra-High Energy Cosmic Ray Propagation

We discuss the influence of large scale cosmic magnetic fields on the propagation of hadronic cosmic rays above 1019 eV based on large scale structure simulations. Our simulations suggest that rather substantial deflection up to several tens of degrees at 1020 eV are possible for nucleon primaries. Further, spectra and composition of cosmic rays from individual sources can depend on magnetic fields surrounding these sources in intrinsically unpredictable ways. This is true even if deflection from such individual sources is small. We conclude that the influence of large scale cosmic magnetic fields on ultra-high energy cosmic ray propagation is currently hard to quantify. We discuss possible reasons for discrepant results of simulations by Dolag et al. which predict deflections of at most a few degrees for nucleons. We finally point out that even in these latter simulations a possible heavy component would in general suffer substantial deflection.

High energy gamma ray counterparts of astrophysical sources of ultra-high energy cosmic rays

If ultra-high energy cosmic rays (UHECRs) are accelerated at astrophysical point sources, the identification of such sources can be achieved if there is some kind of radiation at observable wavelengths that may be associated with the acceleration and/or propagation processes. No radiation of this type has so far been detected or at least no such connection has been claimed. The process of photopion production during the propagation of UHECRs from the sources to the Earth results in the generation of charged and neutral pions. The neutral (charged) pions in turn decay to gamma quanta and electrons that initiate an electromagnetic cascade in the universal photon background. The ultra high energy protons interaction produces also electron positron pairs with typical energy of 700 TeV which initiate a faster electromagnetic cascade. We calculate the flux of the overall gamma radiation in the GeV-TeV energy range and find that for source luminosities compatible with those expected from small scale anisotropies in the directions of arrival of UHECRs, the fluxes can be detectable by future Cerenkov gamma ray telescopes, such as VERITAS and HESS for source distances comparable with the loss length for photopion production. The effects of a widespread extra-galactic magnetic field on the cascade flux are discussed for various intensities and simple configurations.