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

Abstract

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. Furthermore, if there are sources of cosmic rays in close proximity to certain molecular clouds, then the spectrum of gamma‐rays from these clouds will be stronger and harder. By searching for such enhancements in the diffuse emission, and by correlating the CASA‐MIA results with emission detected at lower energies by EGRET, we may identify or constrain the nature of cosmic rays sources in both energy regimes.