The gamma ray content of ultrahigh energy cosmic radiation.

Abstract

Data from the Utah-Michigan (UM) extensive air shower (EAS) array at Dugway, Utah are examined for evidence of a gamma ray component in UHE cosmic rays. The UM array consists of a 3 $\times$ 10$\sp4$ m$\sp2$ surface array with 50 m$\sp2$ of charged particle detectors, and a 1200 m$\sp2$ buried array for detecting muons. Data from the large muon array are used to search for a gamma ray signature, since gamma rays are expected to produce far fewer muons in EAS than hadronic cosmic ray primaries. When no evidence for gamma rays is found, limits are set based on expected gamma ray behavior. For a photon energy threshold of 200 TeV, results include: a limit on the omnidirectional gamma ray fraction of cosmic rays at 0.2% (90% c.l.), a limit on the galactic gamma ray fraction of cosmic rays at 7 $\times$ 10$\sp{-5}$, no strong evidence for enduring gamma ray point sources at fluxes $\approx$4 $\times 10\sp{-14}\ \rm cm\sp{-2}s\sp{-1}$ in the declination range 20$\sp\circ$-60$\sp\circ$, and no strong evidence for bursts on a 1 day time scale at a flux sensitivity $\approx$2 $\times 10\sp{-12}\ \rm cm\sp{-2}s\sp{-1}$ in the same declination range. An interesting excess above the expected background from the direction of Hercules X-1 was observed for a 19 week interval (April-August 1989) with possible flux (8.0 $\pm$ 2.5) $\times$ 10$\sp{-14}$ cm$\sp{-2}$s$\sp{-1}$. These limits and measurements are based on muon-poor data. While no strong signal of UHE gamma rays is found, the methods show promise for future application to the large Chicago Air Shower Array-Michigan Array (CASA-MIA).