Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...463..609d&link_type=abstract
Astrophysical Journal v.463, p.609
Astronomy and Astrophysics
Astronomy
43
Cosmic Rays: General, Cosmology: Diffuse Radiation, Gamma Rays: Observations, Ism: Clouds, Radio Lines: Ism
Scientific paper
We present a study of the diffuse, high-energy gamma-ray emission detected in the outer Galaxy toward the Cepheus flare by the EGRET instrument on the Compton Gamma Ray Observatory. The region studied (l = 100° to 130°, b = -5° to +32°) contains the prominent local clouds of the Cepheus and Polaris flares and the molecular 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 distance ranges. The gamma-ray emissivity and N(H2)/WCO ratio in the local range are found to be (1.84±0.10) x 10-26 s-1sr-1 and (0.92±0.14) x 1020 cm-2 (K km s-1)-1, respectively, both derived for the energy range E > 100 MeV. In the Perseus arm, these quantities are (1.11±0.09) x 10-26 s-1 sr-1 and (2.5±0.9) x 1020 (K km s-1)-1. The decrease of emissivity from the solar circle to the Perseus arm is significant, although the variation of N(H2)/WCO 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 interstellar gas. The derived emissivity in the Perseus arm constrains the amount of molecular gas not detected in CO to be at most one-tenth the amount of atomic hydrogen. A bright, soft spectrum gamma-ray source in this field reported from analysis of COS B data is not seen by EGRET. The spectrum of the extragalactic isotropic emission toward Cepheus is consistent with findings from other regions that have been studied with EGRET data.
Digel Seth William
Grenier Isabelle A.
Heithausen Andreas
Hunter Stan D.
Thaddeus Patrick
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