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Diffuse cosmic X- and gamma radiation: The isotropic component

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Summary

A. Spectral features

The ability of the various theories to explain the three main spectral features at 1/4 keV, 60 keV and 1 MeV is summarized in Tables II and III.

Clearly, confirmation of the reality of these features, especially the soft X-ray and γ-ray excesses, is one of the key elements in enabling us to decide between the competing theoretical interpretations.

B. Energy requirements

None of the proposed interpretations are easily explained in terms of the available energy in cosmic rays (except perhaps the Seyfert galaxy proposal, and this runs into difficulties). It seems that one either has to regard normal galaxies at the present epoch as prolific sources of cosmic rays (∼ 1060 erg/galaxy in protons), as is required by the Brecher-Morrison model, or to argue that at early stages in their evolution far more energy is available than at present. One ends up with much the same energy requirement in this approach.

One could conceivably identify such an early phase with the radio galaxy or QSO phenomena: in any event, cosmological evolution plays a major role. Cosmology does ease the energy requirements, but only for the inefficient mechanisms, such as nonthermal bremsstrahlung or π° -production.

It seems that one still needs the metagalactic cosmic ray flux to be ∼ 10-2 of the galactic flux in the diffuse inverse Compton models, and 10-2–10-4 in the nonthermal bremsstrahlung models.

Faced with problems of energetics, one is tempted to turn to the most energetic objects in the Universe, namely Seyfert nuclei and QSO's, to provide the basic energy source, whether directly or indirectly, for the diffuse X-ray background. A direct connection could be more readily investigated when X-ray observations are available of more extra-galactic sources.

C. Angular variations

Another approach, complementary to that of looking for remote discrete sources, is to seek angular fluctuations, or limits on such fluctuations in the diffuse X-ray background.

The best results presently available are those from the X-ray experiment on board OSO 3. Schwartz (1970) reports a limit of δI/I∼four percent on small-scale (∼10°) fluctuations over 10–100 keV over about one-quarter of the sky. If one assumes a astrophysics, namely the origin of cosmic rays, is intimately linked to the origin of the X-ray background.

It may well be that no single mechanism suffices to account for the entire spectrum of isotropic X- and γ-radiation. Nature is sufficiently perverse for there to be a reasonable probability that several different processes are contributing, and considerable ingenuity will be required to ascertain which mechanism, if any, is assigned the dominant role in a given spectral region.

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  1. Joseph Silk

    Present address: Department of Astronomy, University of California, Berkeley, USA

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  1. Princeton University Observatory, Princeton, USA

    Joseph Silk

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Silk, J. Diffuse cosmic X- and gamma radiation: The isotropic component. Space Sci Rev 11, 671–708 (1970). https://doi.org/10.1007/BF00177028

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