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Cosmic rays of the highest energies: I. Evidence for a galactic component

The world data on extensive air showers (EAS) produced by cosmic rays of energy above 1019 eV have been examined in detail, the individual shower characteristics being re-analysed where the basic data are available. The authors consider that their previous claim for an enhancement of intensity in the galactic plane is strengthened, at least in the outer galaxy; in particular they find new evidence for the presence of specific clusters of EAS both near the galactic plane and elsewhere. The clusters may correspond to the presence of specific sources or groups of sources.

The interstellar radiation field: a datum for cosmic ray physics

The imminent launch of the NASA Gamma Ray Observatory has focused on the need to provide up-to-date estimates of the energy density of interstellar radiation away from the galactic plane, this parameter being a prerequisite for calculation of flux of the gamma-rays coming from the inverse Compton interactions of cosmic ray electrons. The authors have made such calculations, using information on stellar distributions and on the extinction properties of dust in the interstellar medium. The ensuing energy densities are probably accurate to about 30%, the uncertainty being due to lack of precise knowledge of the input parameters rather than approximations in the calculations.

Gamma rays from the magellanic clouds and the origin of cosmic rays

Observations with the Compton Gamma Ray Observatory of gamma rays above 100 MeV from the Large and Small Magellanic clouds open up the possibility of testing various hypotheses for the origin of cosmic rays. The authors examine the possibility that the cosmic rays seen at the Earth are of extragalactic origin. The analysis, which relates to a determination of the flux of gamma rays arising from the interaction of cosmic rays with the gas in the MC, shows clearly that this model is not tenable: an upper limit of 11% can be put on the contribution of such extragalactic particles to the observed flux.

The trajectories of cosmic rays at the highest energies. III. Applications of predictions to the results from extensive air shower arrays

For pt. II see ibid., vol. 20, p. 665 (1994). The predicted anisotropies from the calculations reported in the preceding paper, by Dudarewicz et al., are compared with our estimates of the experimental values from a number of EAS arrays: Sydney, Yakutsk, Haverah Park and Chacaltaya. The mass composition of galactic particles is inferred from the analysis and comparison is made with the results from other workers, using different techniques, most notably the Flys Eye results of Bird et al (1993). The results indicate general agreement that the flux of galactic protons falls rapidly with increasing energy and that there is a considerable fraction of heavy nuclei above 1018 eV with a transition to isotropic protons (presumed to be extragalactic) above 1019 eV.

The inverse Compton contribution to medium-latitude Galactic gamma-ray emission

The authors present evidence that the inverse Compton contribution to the diffuse gamma-ray emission above 50 MeV is more important than previously thought. They compute the spectrum of the excess gamma -ray emission above the pi 0 and bremsstrahlung components. The calculation shows that the excess emission is substantial, amounting to about 60% of the total emission at intermediate latitudes. In the Galactic plane, from which most of the Galactic gamma -rays are derived, the excess is relatively much smaller. A model in which there is a cosmic-ray halo of total width 10 kpc and radius 20 kpc yields an inverse Compton contribution which accounts for most of the excess.

Extragalactic gamma rays

An analysis has been made of SAS-II cosmic gamma-ray data leading to a new estimate for the flux of extragalactic gamma rays. The contribution from inverse Compton interactions of electrons escaping from normal galaxies is derived based on calculations for the Galaxy. It is shown that under certain circumstances this contribution may be large. Other possibilities are also considered, involving greater cosmic ray production by galaxies at early epochs.

Cosmic rays above 1017 eV. I. The mass spectrum and the overall anisotropies

The Sydney data on extensive air showers in the energy range 1017 to 1019 eV have been re-examined from the standpoint of endeavouring to determine the energy spectrum of the various masses of particles present in the cosmic ray beam, idealized as a simple mixture of protons and iron nuclei. The analysis suggests that the ratio of iron nuclei to protons increases with energy in the range under consideration, being approximately=1.0 at 1018 eV and approximately=2.0 at 1019 eV.

The sources of cosmic rays

Studies of the mass composition of cosmic rays as a function of energy by the MACRO collaboration support the view that the well known steepening of the overall cosmic ray spectrum at 3*1015 eV is not in fact due to Galactic modulation, as has been commonly assumed. Instead, the authors argue that the Galactic modulation-a reduction in trapping efficiency for the Galaxy as a whole due to the Galactic magnetic field being insufficient-does not become important until a rigidity of about 3*1017 eV. This rigidity is in much better accord with their knowledge of the properties of the Galactic magnetic field-its large-scale topology and its scale length for irregularities (this important scale length having been found to a similar value for other galaxies, too). The consequences are that there must be at least two main Galactic sources for the cosmic radiation in the energy range 109 eV to 1019 eV.

The trajectories of cosmic rays at the highest energies. II. Sensitivity of the anisotropy predictions to model parameters

Further numerical calculations of the trajectories of cosmic-ray protons above 1017 eV have been made following our earlier work. The sensitivity of the derived anisotropies to the parameters of the magnetic field and the nature of the source distribution has been examined. A comparison has been made with the results of other workers.

Cosmic rays above 4*1019 eV

An analysis is made of the distribution with Galactic latitude of cosmic rays of energy above 4*1019 eV from the standpoint of determining whether they are of Galactic or extragalactic origin. Although it is commonly argued that these particles are obviously extragalactic in origin, because of the near isotropy of arrival directions, in fact this could be accommodated by assuming a Galactic origin and a composition of heavy nuclei. However, using new calculations of particle trajectories for a variety of field models the authors argue that the measured latitude distribution is inconsistent with this hypothesis, whatever the charge or (reasonable) magnetic field and source distribution. Thus, these particles are indeed mainly extragalactic and are, presumably, mainly protons, although, as is pointed out, there are some problems even with this hypothesis.