The radial gradient of cosmic ray intensity in the Galaxy

The dependence of the cosmic ray intensity on Galactocentric distance is known to be much less rapid than that of the thought-to-be sources: supernova remnants. This is an old problem (‘the radial gradient problem’) which has led to a number of possible ‘scenarios’. Here, we use recent data on the supernova remnant’s radial distribution and correlate it with the measured HII electron temperature (T). We examined two models of cosmic ray injection and acceleration and in both of them the injection efficiency increases with increasing ambient temperature T. The increase is expected to vary as a high power of T   in view of the strong temperature-dependence of the tail of the Maxwell–Boltzmann distribution of particle energies. Writing the efficiency as proportional to TnTn we find n≈8.4n≈8.4. There is thus, yet another possible explanation of the radial gradient problem.