P. Sokolsky

Measurement of the flux of ultrahigh energy cosmic rays from monocular observations by the high resolution fly's eye experiment

We have measured the cosmic ray spectrum above 10^17.2 eV using the two air fluorescence detectors of the High Resolution Flys Eye observatory operating in monocular mode. We describe the detector, photo-tube and atmospheric calibrations, as well as the analysis techniques for the two detectors. We fit the spectrum to a model consisting of galactic and extra-galactic sources.

The Utah Fly's Eye detector

Abstract We report the details of the design, operation and performance of the University of Utah Flys Eye detector which was built to record the passage of ultra-high energy cosmic rays through the atmosphere via atmospheric fluorescence. Emphasized in the presentation are (1) light production by charged particles in the atmosphere, (2) kinematics of an EAS as seen by the Flys Eye, (3) signal to noise considerations and its impact on detector design, (4) details of detector hardware and software, (5) detector calibration, (6) techniques employed in measurement of shower longitudinal development profiles and primary particle energy, and (7) assessment of detector performance by a comparison of Monte Carlo and real data distributions.

Indications of Proton-Dominated Cosmic-Ray Composition above 1.6 EeV

We report studies of ultrahigh-energy cosmic-ray composition via analysis of depth of air shower maximum (X(max)), for air shower events collected by the High-Resolution Flys Eye (HiRes) observatory. The HiRes data are consistent with a constant elongation rate d/d[log(E)] of 47.9+/-6.0(stat)+/-3.2(syst) g/cm2/decade for energies between 1.6 and 63 EeV, and are consistent with a predominantly protonic composition of cosmic rays when interpreted via the QGSJET01 and QGSJET-II high-energy hadronic interaction models. These measurements constrain models in which the galactic-to-extragalactic transition is the cause of the energy spectrum ankle at 4x10(18) eV.

A Study of the Composition of Ultra-High-Energy Cosmic Rays Using the High-Resolution Fly’s Eye

The composition of Ultra High Energy Cosmic Rays (UHECR) is measured with the High Resolution Flys Eye cosmic ray observatory (HiRes) data using the Xmax technique. Data were collected in stereo between 1999 November and 2001 September. The data are reconstructed with well-determined geometry. Measurements of the atmospheric transmission are incorporated in the reconstruction. The detector resolution is found to be 30 g cm^-2 in Xmax and 13% in Energy. The Xmax elongation rate between 10^18.0 eV and 10^19.4 eV is measured to be 54.5 +/- 6.5 (stat) +/- 4.5 (sys) g cm^-2 per decade. This is compared to predictions using the QGSJet01 and SIBYLL 2.1 hadronic interaction models for both protons and iron nuclei. CORSIKA-generated Extensive Air Showers (EAS) are incorporated directly into a detailed detector Monte Carlo program. The elongation rate and the Xmax distribution widths are consistent with a constant or slowly changing and predominantly light composition. A simple model containing only protons and iron nuclei is compared to QGSJet and SIBYLL. The best agreement between the model and the data is at 80% protons for QGSJet and 60% protons for SIBYLL.The composition of ultra-high-energy cosmic rays is measured with the High Resolution Flys Eye cosmic-ray observatory data using the Xmax technique. Data were collected in stereo between 1999 November and 2001 September. The data are reconstructed with well-determined geometry. Measurements of the atmospheric transmission are incorporated in the reconstruction. The detector resolution is found to be 30 g cm-2 in Xmax and 13% in energy. The Xmax elongation rate between 1018.0 and 1019.4 eV is measured to be 54.5 ± 6.5 ± 4.5 g cm-2 per decade. This is compared with predictions using the QGSJet01 and SIBYLL 2.1 hadronic interaction models for both protons and iron nuclei. CORSIKA-generated extensive air showers are incorporated directly into a detailed detector Monte Carlo program. The elongation rate and the Xmax distribution widths are consistent with a constant or slowly changing and predominantly light composition. A simple model containing only protons and iron nuclei is compared with QGSJet and SIBYLL. The best agreement between the model and the data is for 80% protons for QGSJet and 60% protons for SIBYLL.

Measurement of the cosmic-ray energy spectrum and composition from 1017 to 1018.3 eV using a hybrid technique

We study the spectrum and average mass composition of cosmic rays with primary energies between 10^{17} eV and 10^{18} eV using a hybrid detector consisting of the High Resolution Flys Eye (HiRes) prototype and the MIA muon array. Measurements have been made of the change in the depth of shower maximum as a function of energy. A complete Monte Carlo simulation of the detector response and comparisons with shower simulations leads to the conclusion that the cosmic ray intensity is changing f rom a heavier to a lighter composition in this energy range. The spectrum is consistent with earlier Flys Eye measurements and supports the previously found steepening near 4 \times 10^{17} eV .

Search for correlations between HiRes stereo events and active galactic nuclei

Abstract We have searched for correlations between the pointing directions of ultrahigh energy cosmic rays observed by the High Resolution Fly’s Eye experiment and active galactic nuclei (AGN) visible from its northern hemisphere location. No correlations, other than random correlations, have been found. We report our results using search parameters prescribed by the Pierre Auger collaboration. Using these parameters, the Auger collaboration concludes that a positive correlation exists for sources visible to their southern hemisphere location. We also describe results using two methods for determining the chance probability of correlations: one in which a hypothesis is formed from scanning one half of the data and tested on the second half, and another which involves a scan over the entire data set. The most significant correlation found occurred with a chance probability of 24%.

Evidence for Changing of Cosmic Ray Composition between 10**17-eV and 10**18-eV from Multicomponent Measurements

The average mass composition of cosmic rays with primary energies between

Measurement of the Cosmic Ray Energy Spectrum and Composition from 10^{17} to 10^{18.3} eV Using a Hybrid Fluorescence Technique

10^{17}

Search for cross-correlations of ultrahigh-energy cosmic rays with BL Lacertae objects

eV and

The prototype high-resolution Fly's Eye cosmic ray detector

10^{18}