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Genesis and Propagation of Cosmic Rays pp 163–174Cite as

Gamma-Ray Views on the Galactic Cosmic-Ray Distribution

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Part of the book series: NATO ASI Series ((ASIC,volume 220))

Abstract

Recent γ-ray studies of the distribution of cosmic rays throughout the Galaxy are discussed based on the observations obtained with the SAS-2 COS-B satellites. For reasons addressed in this paper the picture arising from recent analyses of both data bases differs from the one generally advocated in the past.

  1. (i)

    It seems inevitable to conclude at the moment that the mean density of GeV cosmic-ray nuclei in the galactic plane can vary only weakly — by a factor less than two — from the inner (R ≃ 4 kpc) to the outer (R ≃ 15 — 20 kpc) regions of the Galaxy (R#x2299; ≡10 kpc). The allowed fall-off with increasing galacto-centric distance R is much smaller than the radial gradient in the distribution of the types of objects generally believed to have injected the particles and/or accelerated them either directly or indirectly by producing shocks in the interstellar medium. It is unlikely that this is due to extensive diffusion of the particles into the outer Galaxy. It is suggested here that it might be due to an increase of the mean escape lifetime with increasing galacto-centric distance.

  2. (ii)

    There seems no doubt that the interpretation of the COS-B γ-ray data requires an energy-dependent model, such that the γ-ray intensity spectrum towards the inner Galaxy is softer than for the remainder of the disk. This suggests a softer γ-ray emissivity spectrum in the inner regions, but it is still debatable whether this holds only for the molecular clouds (concentrated in the inner Galaxy) or whether it is a ubiquitous phenomenon (a gradual steepening from the outer towards the inner Galaxy), which could, for instance, be ascribed to a stronger galacto-centric gradient for the CR electrons (<1 GeV) than for the protons. A concentration of steep-spectrum γ-ray sources in the inner Galaxy can also not be excluded.

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Author information

Authors and Affiliations

  1. Astronomy Department, University of California, Berkeley, CA, 94720, USA

    Hans Bloemen

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  1. Hans Bloemen
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Editor information

Editors and Affiliations

  1. Department of Physics and Astronomy, University of Maryland, College Park, Maryland, USA

    Maurice M. Shapiro (Director of the School, Co-Director of the Course) (Director of the School, Co-Director of the Course)

  2. Laboratory for Cosmic-Ray Physics (emeritus), Naval Research Laboratory, Washington, DC, USA

    Maurice M. Shapiro (Director of the School, Co-Director of the Course) (Director of the School, Co-Director of the Course)

  3. 205 Yoakum Parkway, #1720, Alexandria, VA, 22304, USA

    Maurice M. Shapiro (Director of the School, Co-Director of the Course) (Director of the School, Co-Director of the Course)

  4. Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana, 70803-4001, USA

    John P. Wefel (Co-Director of the Course) (Co-Director of the Course)

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© 1988 D. Reidel Publishing Company, Dordrecht, Holland

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Bloemen, H. (1988). Gamma-Ray Views on the Galactic Cosmic-Ray Distribution. In: Shapiro, M.M., Wefel, J.P. (eds) Genesis and Propagation of Cosmic Rays. NATO ASI Series, vol 220. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-4025-3_9

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  • DOI: https://doi.org/10.1007/978-94-009-4025-3_9

  • Publisher Name: Springer, Dordrecht

  • Print ISBN: 978-94-010-8286-0

  • Online ISBN: 978-94-009-4025-3

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