J. Matthews

The cosmic ray energy spectrum between 1014 and 1016 eV

The energy spectrum of cosmic rays with primary energies between 1014 eV and 1016 eV has been studied with the CASA-MIA air shower array. The measured differential energy spectrum is a power law (djdE ∝ E−y) with spectral indices γ of 2.66±0.02 below approximately 1015 eV and 3.00±0.05 above. A new method is used for measuring primary energy derived from ground-based data in a compositionally insensitive way. In contrast with some previous reports, the “knee” of the energy spectrum does not appear sharp, but rather a smooth transition over energies from 1015 eV to 3.0 × 1015 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.

Limits on the Isotropic Diffuse Flux of Ultrahigh Energy {gamma} Radiation

Diffuse ultrahigh energy gamma-radiation can arise from a variety of astrophysical sources, including the interaction of extremely high energy cosmic rays with the 3K microwave background radiation or the collapse of topological defects created in the early Universe. We describe a sensitive search for diffuse gamma-rays at ultrahigh energies using the CASA-MIA experiment. An isotropic flux of radiation is not detected, and we place stringent upper limits on the fraction of the gamma-ray component relative to cosmic rays (less than one part in 10,000) at energies from 570 TeV to 55,000 TeV. This result represents the first comprehensive constraint on the gamma-ray flux at these energies.

The cosmic ray composition between 1014 and 1016 eV

Abstract The mass composition of cosmic rays with primary energies between 1014 eV and 1016 eV has been studied using the surface and buried scintillators of the CASA-MIA air shower array. Near 1014 eV, the composition of cosmic rays is in agreement with direct measurements, roughly half light elements (protons and helium) and half heavier elements. The average mass increases with energy, becoming heavier above 1015 eV. The mass changes coincide with the spectral steepening of the energy spectrum known as the knee. There is evidence for rigidity dependence in the spectral change. A method of calculating the primary cosmic ray energy which is insensitive to the composition is employed to achieve these results.

Constraints on Gamma-ray Emission from the Galactic Plane at 300 TeV

We describe a new search for diffuse ultra-high-energy gamma-ray emission associated with molecular clouds in the Galactic disk. The Chicago Air Shower Array (CASA), operating in coincidence with the Michigan muon array (MIA), has recorded over 2.2 × 109 air showers from 1990 April 4 to 1995 October 7. We search for gamma rays based upon the muon content of air showers arriving from the direction of the Galactic plane. We find no significant evidence for diffuse gamma-ray emission, and we set an upper limit on the ratio of gamma rays to normal hadronic cosmic rays at less than 2.4 × 10-5 at 310 TeV (90% confidence limit) from the Galactic plane region: (50° < l < 200°; -5° < b < 5°). This limit places a strong constraint on models for emission from molecular clouds in the Galaxy. We rule out significant spectral hardening in the outer Galaxy, and conclude that emission from the plane at these energies is likely to be dominated by the decay of neutral pions resulting from cosmic-ray interactions with passive target gas molecules.

Limits on the flux of energetic neutrinos from the sun

Abstract A number of authors have proposed mechanisms by which the sun could be a strong source of energetic neutrinos. We search for an excess signal of penetrating neutral particles from the direction of the sun. We employ two data samples. One sample studies energies from 400 MeV to 2 GeV. The other studies v μ interactions above 2 GeV where the atmospheric background is lower. Our results are compared with the general background of atmospheric neutrinos from other directions. No significant excess has been found. These results can be used to set limits on possible dark matter candidates.

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)

High statistics search for ultrahigh energy γ-ray emission from Cygnus X-3 and Hercules X-1

We have carried out a high statistics (2 Billion events) search for ultra-high energy gamma-ray emission from the X-ray binary sources Cygnus X-3 and Hercules X-1. Using data taken with the CASA-MIA detector over a five year period (1990-1995), we find no evidence for steady emission from either source at energies above 115 TeV. The derived upper limits on such emission are more than two orders of magnitude lower than earlier claimed detections. We also find no evidence for neutral particle or gamma-ray emission from either source on time scales of one day and 0.5 hr. For Cygnus X-3, there is no evidence for emission correlated with the 4.8 hr X-ray periodicity or with the occurrence of large radio flares. Unless one postulates that these sources were very active earlier and are now dormant, the limits presented here put into question the earlier results, and highlight the difficulties that possible future experiments will have in detecting gamma-ray signals at ultra-high energies.

A search for ultra-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.

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