B. R. Dawson

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

Study of Broad-Scale Anisotropy of Cosmic-Ray Arrival Directions from 2 x 1017 to 1020 Electron Volts from Fly's Eye Data

We report results on the broad-scale anisotropy of arrival directions of cosmic rays in the energy range from 2×1017 to 1020 eV. The data were taken by the Flys Eye detector in both monocular and stereo modes of operation. We look for dependence on galactic latitude or supergalactic latitude by fitting the data to a Wdowczyk and Wolfendale plane enhancement function and a north-south gradient functional form. We report a small but statistically significant galactic plane enhancement in the energy range between 2×1017 and 3.2×1018 eV. The probability that this anisotropy is due to fluctuations of an isotropic distribution is less than 0.06%. The most significant galactic plane enhancement factor fE=0.104±0.036 is in the energy range (0.4-1.0)×1018 eV. No statistically significant evidence for a north-south gradient is found. There is no sign of significant deviation from isotropic background when the data are analyzed in terms of supergalactic latitude distributions.

Search for diffuse cosmic gamma rays above 200 TeV

A search for γ-rays in the cosmic radiation above 200 TeV was realised using a two-level array of scintillators. Surface counters measure the size and direction of extensive air showers while counters buried 3 m below the ground are used to measure their muon content in detail. No evidence for an excess number of muon-poor showers are found and it is concluded that γ-rays comprise less than 0.4% of all cosmic rays above 200 TeV and less than 0.05% above 1000 TeV (90% CL)

Measurements of cosmic-ray air shower development at energies above 10 to the 17th eV

Measurements are presented of the depth of maximum for extensive air showers (EAS) produced by cosmic-ray nuclei with energies above 10 to the 17th eV. The air showers were observed using the University of Utahs Flys Eye detectors operating in stereo mode. Measurements of the atmospheric nitrogen fluorescence light generated by EAS have been used to obtain longitudinal development curves of individual showers and, in particular, the depth of maximum of each shower. The data imply an elongation rate of 69.4 + or - 5.0 g/sq cm per decade above 10 to the 17th eV without correction for triggering and resolution effects. These effects contribute approximately +5 g/sq cm per decade to the apparent elongation rate. The distribution of depths of shower maximum has a mean value of 690 + or - 3 g/sq cm (with an estimated systematic uncertainty of + or - 20 g/sq cm) and a width (standard deviation) of 85 + or - 2 g/sq cm for showers with energies above 3 x 10 to the 17th eV. The tail of the distribution has a logarithmic decrement of lambda = 70 + or - 14 g/sq cm. Comparison with predictions for a pure proton and amorexa0» pure iron flux indicates that the data are not consistent with either and are instead consistent with a mixture. 14 refs.«xa0less

The Fly's Eye detector: Present and future

Abstract The operation and recent results of the Flys Eye cosmic ray observatory are described. This detector observes ⩾ 10 17 eV cosmic ray extensive air showers via air fluorescence. Results on the cosmic ray spectrum, composition and the ultrahigh energy neutrino flux, are presented. Plans for a next generation high resolution eye are described.

Mapping the U.H.E. sky in search of point sources

Abstract Techniques are presented for mapping the sky to search for density excesses attributable to point sources. By incorporating event-by-event angular resolution information, a scatter plot of arrival directions becomes the map of a smooth density function. Methods are discussed for quantitatively comparing that observed density function with the expected density function and evaluating the statistical significance of density excesses. Analysis of Flys Eye data provides examples of the techniques.

A coarse-grain search for anisotropy in the arrival directions of cosmic rays above 10 to the 17th eV

The cosmic-ray intensity measured by the Flys Eye reveals no statistically significant anisotropy when averaged over large solid angles. The sky is divided into six lobes of equal solid angle which are centered on the directions of the Galactic coordinate axes. For each of nine logarithmic energy bands, the number of detected cosmic rays in each lobe is compared with the number expected from an isotropic intensity. The excesses and deficits are not large compared to statistical uncertainties in the expected numbers. Harmonic analysis of the right-ascension distribution of cosmic rays is also reported for the different energy bands. 15 refs.

Sporadic and periodic 10-1000 TeV gamma rays from Cygnus X-3

During 1985 June, July, August, and October, small air showers from the direction of Cygnus X-3 were observed using the University of Utah Flys Eye. Useful spectral information was obtained from these showers. The combined data from 1985 June, July, and August show a 3.9 ó excess at 4.8 hr phase 0.65-0.70 for showers with energies above 100 TeV. The excess flux, averaged over all phases, is 4.5 ± 1.2 x 10 cms. During a short run in 1985 October, following a Cyg X-3 radio outburst, only upper limits for the fluxes were obtained. Evidence was obtained for a sporadic outburst in 1985 June 17 UT. The outburst occurred at various phases of the 4.8 hr Cygnus X-3 period. Part of the signal above 250 TeV was near phase 0, when the companion star eclipses the neutron star in some models of Cygnus X-3. The energy flux during the outburst was about 10 ergs cms, which is quite large compared with long-term periodic fluxes. Averaged over all the observing time, however, the sporadic flux is roughly comparable to the periodic flux. Subject headings: gamma rays: general stars: individual (Cyg X-3) X-rays: binaries

Measurement of cosmic ray air shower development at energies above 1017eV

Abstract We present measurements of the depth of maximum for extensive air showers (EAS) produced by cosmic ray nuclei with energies above 10 17 eV. The air showers were observed using the University of Utahs Flys Eye detectors operating in stereo mode. Measurements of the atmospheric nitrogen fluorescence light generated by EAS have been used to obtain longitudinal development curves of individual showers and, in particular, the depth of maximum of each shower. The data imply an elongation rate of 89 ± 6 g cm −2 per decade above 10 17 eV. The distribution of depths of shower maximum has a mean value of 690 ± 3 g cm −2 (with an estimated systematic uncertainty of ± 20 g cm −2 ) and a width (standard deviation) of 85 ± 2 g cm −2 for showers with energies above 3 × 10 17 eV. The tail of the distribution has a logarithmic decrement of λ = 70 ± 14 g cm −2 . Comparisons with predictions for a pure proton and a pure iron flux indicate that the data are not consistent with either.

Measurement of the angular distribution of Cerenkov light in ultra-high-energy extensive air showers

The authors report on a direct measurement of the angular distribution of Cerenkov light in extensive air showers with energies greater than 1017 eV. The measurement is performed by viewing sections of showers simultaneously at different emission angles with two Flys Eye detectors. The data imply a value of the multiple scattering parameter theta 0=4.0+or-1.2 degrees for threshold energies between 20 and 60 MeV.