Large-scale distribution of galactic gamma radiation observed by COS-B
Caraveo, P. A.; Bignami, G. F.; Hermsen, W.; Masnou, J. L.; Lebrun, F.; Bennett, K.; Buccheri, R.; Kanbach, G.; Lichti, G. G.; Mayer-Hasselwander, H. A.; Paul, J. A.; Scarsi, L.; Swanenburg, B. N.; Wills, R. D.; Sacco, B.; Pinkau, K.
Germany, Netherlands, Italy, France
Abstract
A complete survey of the Galaxy in high-energy gamma rays was performed aboard the ESA satellite COS-B. Gamma-ray maps were derived for energy ranges between 70 MeV and 5 GeV from 1975-1979, and are presented as both contour maps and as longitude and latitude profiles for three energy intervals. The spectral ratio is derived in the form of a color index, establishing the approximate constancy of the galactic spectrum as a function of longitude. The average value for the ratio of the intensity observed from 70 to 150 MeV to that from 150 MeV to 5 GeV is 1.09 plus or minus 0.04. The large-scale distribution of gamma-ray emissivity is determined by analysis of the longitude profile, given certain assumptions about the geometrical pattern and z-profile. It is found that the scale height of the gamma ray disk within the solar circle is about 130 pc. Variation of the derived emissivity with galactocentric radius gives an upper limit of a factor of three on the density increase of the interstellar gas between 10 kpc and 5 kpc radius. These results are not in agreement with some recent estimates of the role of molecular hydrogen in the inner galaxy (Solomon and Sanders, 1980, Liszt et al., 1980).