DIFFUSE HIGH-ENERGY GAMMA-RAY EMISSION BEYOND THE SOLAR CIRCLE - THE CEPHEUS AND POLARIS FLARES AND THE PERSEUS ARM

We present a study of the diffuse, high-energy gamma-ray emission dete cted in the outer Galaxy toward the Cepheus flare by the EGRET instrum ent on the Compton Gamma Ray Observatory. The region studied (l = 100 degrees to 130 degrees, b = -5 degrees to 32 degrees) contains the pro minent local clouds of the Cepheus and Polaris flares and the molecula r cloud complex associated with Cas A and NGC 7538, the largest in the Perseus arm. Gas along the line of sight is partitioned into three di stance ranges. The gamma-ray emissivity and N(H-2)/W-CO ratio in the l ocal range are found to be (1.84 +/- 0.10) x 10(-26) s(-1) sr(-1) and (0.92 +/- 0.14) x 10(20) cm(-2) (K km s(-1))(-1), respectively, both d erived for the energy range E > 100 MeV. In the Perseus arm, these qua ntities are (1.11 +/- 0.09) x 10(-26) s(-1) sr(-1) and (2.5 +/- 0.9) x 10(20) cm(-2) (K km s(-1))(-1). The decrease of emissivity from the s olar circle to the Perseus arm is significant, although the variation of N(H-2)/W-CO, is not. No significant variations of these quantities within each distance range are found. The gamma-ray emissivity in the Perseus arm is much less than predicted by models that assume a simple coupling between cosmic-ray density and surface density of interstell ar gas. The derived emissivity in the Perseus arm constrains the amoun t of molecular gas not detected in CO to be at most one-tenth the amou nt of atomic hydrogen. A bright, soft spectrum gamma-ray source in thi s held reported from analysis of COS B data is not seen by EGRET. The spectrum of the extragalactic isotropic emission toward Cepheus is con sistent with findings from other regions that have been studied with E GRET data.