On Gamma Rays from Intergalactic Space.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On Gamma Rays from Intergalactic Space. Shen, S. P. ; Milford, S. N. Abstract Among the possible processes contributing to the gamma-ray back- ground flux above the earth's atmosphere, two are singled out for consideration here: (1) collision of cosmic rays with intergalactic and interstellar matter yielding high-energy gamma rays; (2) decay of intergalactic and interstellar radionuclei (K40 Tl205) yielding monoenergetic gamma rays below 4 Mev. Considering process (1), the presently available observed upper limit for high-energy gamma-ray flux (5 X 10-~ photon cm-2 sec-1 sr-1) yields an upper limit of 10 particles cm-2 sec-1 (order of magnitude) for the omnidirectional flux of intergalactic cosmic rays above 600 Mev, averaged over the visible universe and over the reciprocal of Hubble's constant. Current and future gamma-ray observations, such as from the earth satellite 1961 v, could lower both upper limits. Thus, if the gamma- ray upper limit becomes 5 X 10-~ photon cm-2 sec-1 sr-1, the cosmic-ray upper limit would become 1 particle cm-2 sec-1, etc. Directional observations could single out the interstellar contribution, which, combined with another method (Milford and Shen, Phys. Rev. 122, 1921, 1961), could in principle yield a rough integral energy spectrum for interstellar cosmic rays. The present method can also be reversed to give some information on the density of intergalactic matter, independently of its ionization state. Furthermore, if intergalactic nuclidic abundances are similar to the solar neighborhood values, process (2) would give a primary gamma-ray flux of order 10-~ photon cm-2 sec-1 sr-1, well below the present detection limit but usable, in principle, as a test for the presence or absence of significant amounts of heavy elements in intergalactic space. Finally, nearby large concentrations of diffuse matter (e.g., Orion complex, Magellanic Clouds, M31) should by these two processes alone give local enhancements of gamma-ray flux distinguishable from the surrounding background. Publication: The Astronomical Journal Pub Date: March 1961 DOI: 10.1086/108517 Bibcode: 1961AJ.....66Q.295S full text sources ADS |