High-energy gamma rays as a probe of cosmic-ray spectral differences throughout the Galaxy. II - A comparison with radio synchrotron emission

Abstract

A spectral analysis of the galactic high-energy γ-ray emission (300 MeV-6 GeV) is presented using the observations obtained with the COS-B satellite. Large-scale spectral variations are found and compared with recent results from a study of the galatic radio-continuum emission at 408 and 1420 MHz. The combination of the radio and γ-ray findings establishes that cosmic-ray spectral variations at GeV energies exist in the Galaxy, amounting to spectral-index differences as large as ~0.5. It is found that toward the outer Galaxy (2nd and 3rd quadrants) both the γ-ray and radio spectra show a flattening with increasing latitude. Toward the inner Galaxy (310^deg^ < l < 50^deg^) the γ-ray data do not show this flattening, which is a strong indication that the findings cannot be attributed to uncertainties in the background level of the γ-ray data. It is not possible to perform a reliable analysis of the inner-Galaxy radio emission, because only northern-hemisphere data are available at 1420 MHz and a large fraction of the available sky coverage is dominated by emission from Loop I. It is clear, however, that the spectra in the plane are stepper than the outer-Galaxy spectra, which is not seen in the high-energy γ-ray data. Possible interpretations are discussed. In addition, it is suggested in this paper that a large fraction of the γ-ray emission at medium latitudes may not originate in the solar vicinity (<~1 kpc), contrary to what has been generally assumed so far. This is ascribed to the thickening, flaring, of the neutral hydrogen layer in the outer Galaxy and to the presence of an ionized medium with a large scale height, as traced by pulsar dispersion measures. The ionized gas does probably not extend far beyond the solar circle and cosmic-ray interactions with this medium may therefore explain the well-known γ-ray excess above the expectation from atomic and molecular gas, seen at medium latitudes toward the inner Galaxy.