S. Oser

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.

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.

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.

The STACEE-32 ground based gamma-ray detector

We describe the design and performance of the Solar Tower Atmospheric Cherenkov Effect Experiment detector in its initial configuration (STACEE-32). STACEE is a new ground-based gamma-ray detector using the atmospheric Cherenkov technique. In STACEE, the heliostats of a solar energy research array are used to collect and focus the Cherenkov photons produced in gamma-ray induced air showers. The large Cherenkov photon collection area of STACEE results in a gamma-ray energy threshold below that of previous detectors.

Prototype test results of the solar tower atmospheric Cherenkov effect experiment (STACEE)

STACEE is a proposed atmospheric Cherenkov telescope for ground-based gamma-ray astrophysics between 25 and 500 GeV. The telescope will make use of the large solar mirrors (heliostats) available at a solar research facility to achieve an energy threshold lower than any existing ground-based instrument. This paper describes the development of STACEE, including an overview of the complete instrument design and a discussion of results from recent prototype tests at the large solar heliostat field of Sandia National Laboratories.

Performance of the STACEE Atmospheric Cherenkov Telescope

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is located at the National Solar Thermal Test Facility of Sandia National Laboratories in Albuquerque, New Mexico, USA. The field of solar tracking mirrors (heliostats) around a central receiver tower is used to direct Cherenkov light from atmospheric showers onto secondary mirrors on the tower, which in turn image the light onto cameras of photomultiplier tubes. The STACEE Collaboration has previously reported a detection of the Crab Nebula with approximately 7 standard deviation significance, using 32 heliostats (STACEE-32). This result demonstrates both the viability of the technique and the suitability of the site. We are in the process of completing an upgrade to 48 heliostats (STACEE-48) en route to an eventual configuration using 64 heliostats (STACEE-64) in early 2001. In this paper, we summarize the results obtained on the sensitivity of STACEE-32 and our expectations for STACEE-48 and STACEE-64.

Solar tower atmospheric Cherenkov effect experiment (STACEE) for ground based gamma ray astronomy

The STACEE experiment is being developed to study very high energy astrophysical gamma rays between 50 and 500 GeV. During the last few years this previously unexplored region has received much attention due to the detection of sources up to about 10 GeV by the EGRET instrument on board the CGRO. However, the paucity of detected sources at ∼1 TeV indicates that fundamental processes working within these sources and/or in the intergalactic space are responsible for the cutoff in the photon spectra of the EGRET sources. The cutoff or the spectral change of these sources can be observed with ground-based Cherenkov detectors with a very low threshold. The use of large arrays of mirrors at solar power facilities is a promising way of lowering the threshold. Using this concept a series of tests were conducted at the National Solar Thermal Test Facility (NSTTF) at Sandia National Laboratories (Albuquerque, NM) with a full size prototype of the STACEE telescope system. The tests show that STACEE will be capable o...

GeV γ-ray astronomy with STACEE-64

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a new low-threshold atmospheric Cherenkov detector, using heliostat mirrors at a solar research facility to achieve a large collection area for Cherenkov light. The newest version of this detector, STACEE-64, should run at a threshold of 70 GeV or lower. Possible science for STACEE-64 in this energy range includes the study of AGN, supernova remnants, the extragalactic UV background, and exotic dark matter.