Ultrahigh energy cosmic rays

This is a review of the most resent results from the investigation of the ultrahigh energy cosmic rays, particles of energy exceeding ${10}^{18}\mathrm{eV}$. After a general introduction to the topic and a brief review of the lower energy cosmic rays and the detection methods, the two most recent experiments, the High Resolution Fly’s Eye and the Southern Auger Observatory, are described. Results from these two experiments on the cosmic ray energy spectrum, the chemical composition of these cosmic rays, and searches for their sources are presented. An analysis of the controversies in these results and the projects in development and construction that can help solve the remaining problems with these particles is also presented.

Figure 1

Differential energy spectrum of cosmic rays of energy above ${10}^{11}\mathrm{eV}$ multiplied by $E^2$. The positions of the cosmic rays knee and ankle are indicated with gray arrows. The experiments that contribute data to this graph are shown. The equivalent laboratory energy of the Large Hadron Collider is also shown.

Figure 2

Heitler’s schematic evolution of an electromagnetic cascade. At each stage of the cascade the number of particle is multiplied by two, through either pair creation or single photon bremsstrahlung. The evolution stops when individual particle energy fall below the critical energy, about 80 MeV in air.

Figure 3

Schematic evolution of an hadronic cascade. At each step roughly $1/3$ of the energy is transferred from the hadronic cascade to the electromagnetic one.

Figure 4

Evolution of the position of $X_{\max }$ as a function of energy (elongation rate) for iron and proton induced air showers. Elongation rate of different nuclear species are with nearly constant slope and almost parallel to each other. Shown are the results of detailed simulation performed by the Auger Collaboration using various interaction models.

Figure 5

Example of detection using a surface array. The upper right inset shows the whole Auger surface array and the footprint of the shower, each dot represents a detector and the spacing between them is 1.5 km. The lower inset shows details of this footprint with the estimated contours of the particle density levels. The curve represents the adjusted LDF (lateral distribution function) and the center point represents the measured densities as a function of the distance to the shower core. From the Auger Collaboration.

Figure 6

Sketch of the detection principles of a fluorescent detector. The fluorescent light emitted by the air shower is collected on a large mirror and focussed onto a camera composed of photomultipliers (Auger Collaboration).

Figure 7

Geometry of the detection of an air shower by a fluorescent telescope. From 138.

Figure 8

Comparison of the positions of supernova remnants (x’s) with GeV (Fermi/LAT: triangles) and TeV (circles) gamma-ray sources.

Figure 9

A simple model of the knee, which extends the low energy spectrum and composition to high energy with an exponential cutoff at ${10}^7\mathrm{GeV}$. The model is normalized to the all-particle flux measured by the Kascade experiment (50), which is shown with full squares. The measurements of the Tibet III experiment (49) is shown with empty squares.

Figure 10

Results from studies of the cosmic ray composition in the region of the knee compared to the predictions of the simple model presented in Fig. 9.

Figure 11

Hillas plot for candidate acceleration sites, relating their size and magnetic field strength. To accelerate a given particle species above 100 EeV objects must lie above the corresponding lines.

Figure 12

Sketch of the HiRes fluorescent experiment. Each rectangle represents a fluorescent telescope including a mirror and a camera. Each site of the HiRes detectors has a nearly full azimuth coverage and site 2, which consists of two rings of mirrors covers elevation from 3° to 30° while site 1, with a single ring, covers elevation from 3° to 16°.

Figure 13

The Pierre Auger observatory at the end of March 2009. Individual white dots represent Cherenkov tanks, while gray ones are unequiped positions. A denser (infill) area is visible in the upper left. Big white dots at the periphery of the array are fluorescent detector sites with the field of view of individual telescope given by the radial white line. Also shown is the Central Laser Facility (CLF) used for FD calibration and atmospheric monitoring purpose.

Figure 14

Cosmic ray spectrum as presented by AGASA, HiRes, and Auger.

Figure 15

Cosmic ray spectrum as measured by HiRes and Auger.

Figure 16

Energy loss length for protons, nuclei, and gamma rays. The heavier shading points at the proton and gamma ray ${\mathrm{L}}_{\mathrm{loss}}$ and the light one shows the contribution of the $e^{+}{e}^{-}$ pair production. The adiabatic energy loss is not included.

Figure 17

Contribution of different redshifts to the arrival spectrum for $E^{-2}$ injection spectrum with no cosmological evolution. The thick gray line shows the sum of the contributions from these five redshifts while the black line is a result of a full integration.

Figure 18

Fraction of photon showers above certain energy determined by the hybrid analysis (labeled h) and the surface detector (sd). The shaded area shows limits set by Haverah Park (56), AGASA (182), and Yakutsk (111).

Figure 19

Depth of maximum measurements of UHECR by the HiRes Collaboration (2005) analysis shown with empty circles and 2010 analysis with full circles and the Auger Collaboration; full squares are compared with the predictions of three different interaction models for H and Fe.

Figure 20

Width of the $X_{\max }$ distributions as measured by HiRes (full circles) and Auger (full squares). Note that the width is presented in different ways (see text) and the points cannot be compared directly.

Figure 21

The models of (a) 59 and (b) 68 compared to the more recent data of Auger (full squares) and HiRes (circles).

Figure 22

Correlation of the arrival directions of UHECR with AGN from the VCV catalog. The shaded part of the sky is not visible by Auger. The gray squares are the AGN within $z$ less than 0.018. The Auger events are shown as circles. The first 27 events are half-filled. The 13 HiRes events are shown with black dots. The thin lines show the six regions of the sky to which Auger has equal exposure. The wide gray line is the supergalactic plane.

Figure 23

Number of events as a function of the angular distance to Cen A. The thick gray line shows the expectation from isotropic distribution of the cosmic ray sources.

Figure 24

The upper limits on the ultrahigh energy neutrino fluxes derived by 210 ( labeled) and 151 are compared to a calculation of cosmogenic neutrinos produced by UHE protons by 103. Electron neutrinos and antineutrino fluxes are plotted with a dashed line. One can see the contribution of neutron decay at lower energy.

Figure 25

Neutrino fluxes calculated from the Auger energy spectrum interpretation that produces the larger amount of cosmogenic neutrinos. The solid line shows the sum of ${\nu }_{\mu }+{\overline{\nu }}_{\mu }+{\nu }_e$. The dash-dottted line shows the flux of ${\overline{\nu }}_e$. Any heavier nuclei contribution to the UHECR flux can only raise the dash-dotted line and decrease the solid one. The thick gray line shows the median (halfway between optimistic and pessimistic) limit set by Auger on ${\nu }_{\tau }$.

Figure 26

Longitudinal profile of a $0.2\pm 0.02\mathrm{EeV}$ shower recoded in coincidence by the HEAT and Coihueco fluorescent telescope of the Auger observatory.

Figure 27

Sketch of the telescope array geometry. The surface detectors are indicated with full diamonds and the telescope stations with arcs.