We have compared the recent cosmic background γ-ray observations with spectra predicted by various possible cosmic interactions. We find that the observed isotropic γ-rays with energies > 1 MeV can best be explained as being due to the decay of π°-mesons produced in extra-galactic cosmic-ray collisions. This interpretation indicates that extragalactic cosmic-ray sources were more active (or prevalent) in the past and started to form at a redshift of ∼ 100 corresponding to 107–108 years after the ‘big-bang’.

For a present extragalactic gas density of 10−7–10−5 cm−3, the present extragalactic cosmic-ray flux is inferred to be 10−5–10−3 the galactic value.

Calculations are given of K-series X-rays produced by interaction of both low energy cosmic rays and diffuse X-rays above 1 keV with heavier ions present in Hi regions. We further consider electron capture to excited states by cosmic ray nuclei of heavy elements, followed by cascades down to the ground state. It is found that the electron capture process may yield appreciable line intensities in the 1–10 keV range in the galactic plane.

The diffuse X-ray background between 1 keV and 1 MeV is interpreted as non-thermal bremsstrahlung in the intergalactic medium. The observed break in the X-ray spectrum at ∼40 keV yields the heat input to the intergalactic medium, the break being produced by ionization losses of sub-cosmic rays. Proton bremsstrahlung is found not to yield as satisfactory an agreement with observations as electron bremsstrahlung: excessive heating tends to occur. Two alternative models of cosmic ray injection are discussed, one involving continuous injection by evolving sources out to a redshift of about 3, and the other model involving injection by a burst of cosmic rays at a redshift of order 10. The energy density of intergalactic electrons required to produce the observed X-rays is ∼ 10−4 eV/cm3. Assuming a high density (∼ 10−5 cm−3) intergalactic medium, the energy requirement for cosmic ray injection by normal galaxies is ∼ 1058–59ergs/galaxy in sub-cosmic rays. The temperature evolution of the intergalactic medium is discussed, and we find that a similar energy input is also required to explain the observed high degree of ionization (if 3C9 is at a cosmological distance).

We wish to discuss two ways in which non-thermal background X-rays and ultraviolet radiation may interact with intergalactic matter: (i) low energy photons may be intense enough to photoionize the intergalactic medium and maintain it as an Hii region; (ii) the recoil associated with Compton scattering of hard X-rays provides an additional heat input into the medium, as well as causing a characteristic modification of the background radiation spectrum. We shall merely summarize our results here, as fuller details are appearing elsewhere (Rees and Setti, 1969).

Below 1 keV, analyses of X-ray background data are complicated by galactic absorption effects, which cause the received intensity to vary with galactic latitude. Bowyer et al. (1968) observed that the diffuse background did not fall off as rapidly as was expected towards the galactic plane. One plausible interpretation of their data would be to suppose that a significant flux of soft X-rays emanates from the disc itself. I wish to discuss what could be inferred about the latter component from improved observations of its latitude-dependence, and by indirect methods.

Recent developments have made it possible to detect cosmic X-rays with energies as low as several hundred eV. Several measurements of the diffuse X-radiation in this range have been reported (Baxter et al., 1969; Bowyer et al., 1968; Henry et al., 1968). In this note we investigate the possibility that these observers have detected X-radiation emitted by the solar wind. We conclude that they probably have not. However, we also find that bremsstrahlung may be detectable from a region of the sky near the sun. If this measurement is possible, it would represent an important method for determining some characteristics of the solar wind away from the ecliptic plane of the solar system. Even though the mean free path for electron collisions is large compared with the astronomical unit, the collision frequency, electron density, and energy released per encounter are sufficient to yield detectable soft X-radiation for lines of sight close to the sun. We have estimated the expected X-ray intensity in the vicinity of the earth on the basis of two models of the solar wind flow pattern; in the first, the flow is radial in all directions away from the sun, and in the second, the flow is confined to a disc of uniform thickness near the ecliptic. In both cases, we neglect temperature gradients for the electrons and compute the total flux received from interplanetary plasma along the line of sight. Most of the received intensity comes from the segment of the line of sight which is nearest the sun. The results are insensitive with respect to the position of the boundary of the solar cavity. Accordingly, we neglect the boundary and consider an infinitely large solar cavity.

This presentation attempts to describe in very qualitative terms a theory of production of high energy radiation (soft and hard X-rays) in magnetoactive plasmas of astrophysical interest. Special emphasis has been placed on the application of our model to extars and in particular to Sco X-1. More rigorous arguments may be found elsewhere [1] and the interested reader is urged to consult that reference for more details.