Composition Of Ultra-high Energy Cosmic Rays Research Articles

EXPLORING THE ULTRA-HIGH ENERGY SKY: STATUS AND FIRST RESULTS OF THE PIERRE AUGER OBSERVATORY

The Pierre Auger Southern Observatory, a hybrid detector for the study of ultra-high energy cosmic rays (UHECRs), is now operating for more than three years and approaching completion. This paper describes the present status and the performance of the Observatory, with an emphasis on the advantages provided by the combined use of two different detection techniques. A panorama of the most recent results related to the spectrum, composition and arrival directions of the UHECR is then presented, along with a brief discussion of their impact on our knowledge of the UHECRs' origin and composition.

Towards event-by-event studies of the ultrahigh-energy cosmic-ray composition

We suggest a method which improves the precision of studies of the primary composition of ultra-high-energy cosmic rays. Two principal ingredients of the method are: (1) comparison of the observed and simulated parameters for individual showers, without averaging over arrival directions and (2) event-by-event selection of simulated showers by the physical observables and not by the reconstructed primary parameters. A detailed description of the algorithm is presented and illustrated by several examples.

Cosmogenic neutrinos from the propagation of ultrahigh energy nuclei

We calculate the flux of neutrinos generated by the propagation of ultrahigh energy nucleiover cosmological distances. The propagation takes into account the interactions withcosmic background radiations including the CMB and the most recent estimates of higherenergy (infrared, optical, and ultraviolet) backgrounds. We assume that the composition ofultrahigh energy cosmic rays (UHECRs) at the source is the same as the oneobserved at low energies. This assumption fits the present data well at the highestenergies. We compare the cosmogenic neutrino flux from mixed composition sourcesto that from pure proton sources. We find that the neutrino flux in the mixedcomposition case has a high energy peak, mainly due to photopion production offCMB photons, of similar shape and amplitude to those in the proton case. At lowenergies both composition cases have significant neutrino flux with a peak around1014.5 eV due tothe higher energy backgrounds. The mixed composition case induces a higher flux of neutrinos at energiesbelow 1013 eV due to the neutron decay component that extends down to low energies. Detection ofdiffuse neutrino fluxes at ultrahigh energies can strongly constrain the source distributionof UHECRs whereas fluxes at lower energies could be used to constrain confinement ofVHE and UHE cosmic rays if combined with composition analysis from cosmic rayexperiments.

A likelihood method for measuring the ultrahigh energy cosmic ray composition

Air fluorescence detectors traditionally determine the dominant chemical composition of the ultrahigh energy cosmic ray flux by comparing the averaged slant depth of the shower maximum, X max, as a function of energy to the slant depths expected for various hypothesized primaries. In this paper, we present a method to make a direct measurement of the expected mean number of protons and iron by comparing the shapes of the expected X max distributions to the distribution for data. The advantages of this method includes the use of information of the full distribution and its ability to calculate a flux for various cosmic ray compositions. The same method can be expanded to marginalize uncertainties due to choice of spectra, hadronic models and atmospheric parameters. We demonstrate the technique with independent simulated data samples from a parent sample of protons and iron. We accurately predict the number of protons and iron in the parent sample and show that the uncertainties are meaningful.

Ultra high energy cosmic rays: Identification of possible sources, energy spectra, and propagation

This review is devoted to the problem of the origin of cosmic rays (CR) with energy E > 4 × 10 19 eV. The review mainly deals with the hypothesis of CR acceleration in active galactic nuclei (AGN). The review consists of the following five sections: Introduction, three chapters, and Conclusions. The following points are considered in Introduction: shower arrays for detecting ultra high energy cosmic rays (UHECR), the Greisen- Zatsepin-Kuzmin (GZK) cutoff, and the hypotheses of the UHECR origin. Since CRs are of extraterrestrial ori- gin, some astrophysical data used in CR physics are presented in a special section. The identification of UHECR sources with energies E > 4 × 10 19 eV is analyzed in Chapter 2. Chapter 3 deals with CR acceleration in sources and CR escape from sources. Chapter 4 is devoted to particle propagation in intergalactic space with allowance for CR interaction with the microwave background radiation and energy losses (blackbody or GZK cutoff). The propagation of UHECR protons is analyzed and their spectra at the Earth are calculated numerically. In Conclu- sions, we formulate the results and list the nuclear data needed to develop the model in more detail, namely, to study the propagation of CR nuclei in intergalactic space and to analyze the UHECR composition at the Earth.

Detection and imaging of atmospheric radio flashes from cosmic ray air showers

The nature of ultrahigh-energy cosmic rays (UHECRs) at energies >10(20) eV remains a mystery. They are likely to be of extragalactic origin, but should be absorbed within approximately 50 Mpc through interactions with the cosmic microwave background. As there are no sufficiently powerful accelerators within this distance from the Galaxy, explanations for UHECRs range from unusual astrophysical sources to exotic string physics. Also unclear is whether UHECRs consist of protons, heavy nuclei, neutrinos or gamma-rays. To resolve these questions, larger detectors with higher duty cycles and which combine multiple detection techniques are needed. Radio emission from UHECRs, on the other hand, is unaffected by attenuation, has a high duty cycle, gives calorimetric measurements and provides high directional accuracy. Here we report the detection of radio flashes from cosmic-ray air showers using low-cost digital radio receivers. We show that the radiation can be understood in terms of the geosynchrotron effect. Our results show that it should be possible to determine the nature and composition of UHECRs with combined radio and particle detectors, and to detect the ultrahigh-energy neutrinos expected from flavour mixing.

Estimate of the mass composition of cosmic rays above 4×1019 eV

The arrival directions of extensive air showers are analyzed on the basis of world data. It is found that the zenith-angle distributions for energies E>1019 eV and E>4×1019 eV differ from each other. According to our estimates, the SUGAR array detects no showers above 1020 eV. The mass composition of ultrahigh-energy cosmic rays is estimated. Cosmic rays with E>4×1019 eV most likely consist of superheavy nuclei.

A New Method for Calculating Arrival Distribution of Ultra–High‐Energy Cosmic Rays above 1019eV with Modifications by the Galactic Magnetic Field

We present a new method for calculating arrival distribution of ultra-high-energy cosmic rays (UHECRs), including modifications by the Galactic magnetic field. We perform numerical simulations of UHE antiprotons, which are injected isotropically at the Earth, in the Galaxy and record the directions of velocities at the Earth and outside the Galaxy for all of the trajectories. We then select some of them so that the resultant mapping of the velocity directions outside the Galaxy of the selected trajectories corresponds to a given source location scenario, applying Liouville's theorem. We also consider energy-loss processes of UHE protons in the intergalactic space. Applying this method to the source location scenario that is adopted in our recent study and can explain the Akeno Giant Air Shower Array (AGASA) observation above 4 × 1019 eV, we calculate the arrival distribution of UHECRs, including lower energy (E > 1019 eV) ones. We find that our source model can reproduce the large-scale isotropy and the small-scale anisotropy on UHECR arrival distribution above 1019 eV observed by the AGASA. We also demonstrate the UHECR arrival distribution above 1019 eV, with the event number expected by future experiments in the next few years. The interesting feature of the resultant arrival distribution is the arrangement of the clustered events in the order of their energies, reflecting the directions of the Galactic magnetic field. This is also pointed out by Alvarez-Muniz, Engel, & Stanev. This feature will allow us to obtain some kind of information about the composition of UHECRs and the magnetic field with increasing amount of data.

Signatures of ultra-high energy cosmic ray composition from propagation of nuclei in intergalactic photon fields

We present a calculation of nuclei propagation with energies above 10 18 eV in the intergalactic photon field. The calculation is based on a Monte-Carlo approach for the nucleus–photon interaction as well as the intergalactic magnetic field. We then assume that the ultra-high energy cosmic rays (UHECR) are nuclei which are emitted from extra-galactic point sources. Four bumps are found in the energy spectrum of the UHECR which form clusters in the distribution of their arrival directions. Based on this calculation, the energy distribution of the clustered events is discussed.

Arrival directions and chemical composition of ultrahigh-energy cosmic rays

Chemical composition of ultrahigh-energy cosmic rays is estimated through the reliably determined (both experimentally and theoretically) distribution of the number of showers in the galactic latitude. Experimental data at energies of ∼1019 eV agree with the theoretical calculations, provided that cosmic rays involve predominantly heavy nuclei. An enhanced flux of cosmic rays from the galactic plane is detected at energies of ∼1019 eV.

First results from the High Resolution Fly's Eye experiment

The High Resolution Fly's Eye Experiment (HiRes) is the only operational experiment of it's kind in the world. HiRes is the direct descendent of the original Fly's Eye experiment. HiRes is designed to make stereo observations of Ultra High Energy Cosmic Rays (UHECR) via atmospheric fluorescence. HiRes will measure the energy spectrum, composition, and anisotropy of UHECR. The first of two sites, HiRes-1, has been in operation since June, 1997. Over 1000 hours of good weather data has been taken, monocularly, and analyzed. A preliminary energy spectrum is presented based on this data set and compared to the results from the original Fly's Eye experiment. The second site, HiRes-2, began limited data-taking in April, 1999. The installation of the HiRes-2 detector is now complete and commissioning is underway. Some high energy events have already been observed in stereo.

Potential of TACTIC for cosmic ray mass composition investigations

A feasibility-study is in progress presently for investigating the prospects of studying elemental composition of ultra-high energy cosmic-rays using the Čerenkov telescope array TACTIC. We discuss here the proposed methodology and some preliminary results.

Ultrahigh energy cosmic ray composition from surface air shower and underground muon measurements at Soudan 2.

The Soudan 2 experiment has performed time-coincident cosmic ray air shower and underground muon measurements. Comparisons to Monte Carlo predictions show that such measurements can make statistically significant tests of the primary composition in the knee region of the cosmic ray spectrum. The results do not support any significant increase in the average primary mass with energy in the range of ∼104 TeV per nucleus. Some systematic uncertainties remain, however, particularly in the Monte Carlo modeling of the cosmic ray shower.Received 19 July 1994DOI:https://doi.org/10.1103/PhysRevD.52.2760©1995 American Physical Society