K. D. Green

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 .

A large air shower array to search for astrophysical sources emitting γ-rays with energies ≥1014 eV

We describe the technical details and the performance of a large array which detects both the electron and muon components in extensive air showers with energies ≥ 1014 eV. The array was designed to search for γ-rays from astrophysical sources. The background of cosmic rays is reduced by the selection of muon poor events. The array consists of 1089 scintillation detectors on the surface covering an area of 0.23 km2 and 1024 scintillation counters of 2.5 m2 each, buried 3 m below the surface for muon detection. Each of the surface detectors has its own local electronics and local data storage controlled by a microprocessor. The array is located at Dugway, Utah USA (40.2°N, 112.8°W) where the average atmospheric depth is 870 g/cm2.

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)

A northern sky survey for astrophysical point sources of 100 TeV gamma radiation

The CASA-MIA experiment is a very large extensive air shower detector with good angular resolution. A part of this instrument has been used to search the visible sky for astrophysical point sources of gamma rays with energies ≥100 TeV. Approximately 90% of the charged cosmic-ray background is rejected through measurement of the muon content of the showers. Stringent limits are placed on the flux of 100 TeV gamma rays from previously unknown point sources across a large fraction of the northern sky. The complete instrument, with significantly greater sensitivity, has been in operation for more than a year

A search for ultra{endash}high-energy gamma-ray emission from the Crab Nebula and Pulsar

We have examined 2.4 × 109 events recorded by the Chicago Air Shower Array-Michigan Muon Array (CASA-MIA) experiment for evidence of ultra-high-energy (100 TeV) neutral particle and gamma-ray emissions from the Crab Nebula and Pulsar between 1990 March 4 and 1995 October 9. No such emissions have been detected. Over this interval, the 90% confidence level integral flux limits for continuous emission above 141 TeV are 2.08 × 10-14 cm-2 s-1 for any neutral particle (which can produce a detectable air shower) and 9.21 × 10-15 cm-2 s-1 for gamma rays. There is no evidence of transient emission on timescales of 1 day or longer. The 90% confidence level integral flux limit for emission on a single day is conservatively estimated to be 4.40 × 10-12 cm-2 s-1 for any neutral particle and 5.09 × 10-12 cm-2 s-1 for gamma rays. There is no evidence for emission from the pulsar at the radio period on intervals of 1 day or longer. The 2 σ (97.7% confidence) limit on the pulsed flux on the most significant day is 3.29 × 10-12 cm-2 s-1 for any neutral particle with mass less than 21 MeV, and 8.31 × 10-12 cm-2 s-1 for gamma rays. For an interval of 162 sidereal days beginning 1992 March 1, the pulsed flux limit was 2.39 × 10-13 cm-2 s-1 for any neutral particle with mass less than 21 MeV, again above 141 TeV.

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

A search for diffuse gamma rays with energies above 1014 eV from molecular clouds in the galaxy

Diffuse gamma‐rays from molecular clouds are excellent tracers of cosmic rays in the galaxy over a wide range of energies. For example, diffuse emission detected by EGRET already places significant constraints on the spectrum and origin of galactic cosmic rays at GeV energies. Likewise, by measuring diffuse gamma rays with ground‐based air shower experiments, we can probe the galactic distribution of cosmic rays in the energy regime above 100 TeV.The Chicago Air Shower Array (CASA) which operates in coincidence with the Michigan muon array (MIA) is the world’s most sensitive experiment to gamma‐rays with energies ≳100 TeV, and is well‐suited for studies of diffuse sources based upon the muon content of air showers. We describe a search for diffuse gamma‐array emission from molecular cloud regions observed by CASA‐MIA. If we assume that the flux of cosmic rays is uniform in the galaxy, then we predict that diffuse emission will probably be detectable by CASA‐MIA within the lifetime of the experiment. Furth...

A search for astrophysical point sources of 100 TeV gamma rays by the UMC collaboration

The CASA‐MIA experiment is a very large extensive air shower detector with good angular resolution. This instrument has been used to search the entire visible sky for astrophysical point sources of 100 TeV gamma rays. Approximately 90% of the isotropic charged cosmic ray background is rejected by measuring the muon content of the showers. Stringent limits are placed on the possible flux of 100 TeV sources across a large part of the Northern sky.

Preliminary results from the Chicago air shower array and the Michigan muon array

The Chicago Air Shower Array (CASA) is a large area surface array designed to detect extensive air showers (EAS) produced by primaries with energy ∼100 TeV. It operates in coincidence with the underground Michigan Muon Array (MIA). Preliminary results are presented from a search for steady emission and daily emission from three astrophysical sources: Cygnus X‐3, Hercules X‐1, and the Crab nebula and pulsar. There is no evidence for a significant signal from any of these sources in the 1989 data.

A search for diffuse sources of ultra high energy gamma-rays

We describe a search for diffuse emission from several candidate sources of gamma-rays with energieauthor00 TeV, using the Chicago Air Shower Array / Michigan Muon Array (CASA-MIA) experiment. Since March 1990, CASA-MIA has recorded over 2.0 × 10 9 air shower events. Searches for diffuse emission are based upon the muon content of air showers. Candidate source regions include molecular clouds in the galaxy and the galactic plane. Results from CASA-MIA are compared to predicted diffuse emission.