The radial gradient of cosmic ray intensity in the Galaxy

TL;DR

This paper investigates the radial gradient of cosmic ray intensity in the Galaxy, proposing that the efficiency of cosmic ray injection increases sharply with ambient temperature, offering a new explanation for the observed gradient.

Contribution

It introduces a model where cosmic ray injection efficiency depends strongly on ambient temperature, with a power-law index around 8.4, providing a novel explanation for the radial gradient problem.

Findings

Injection efficiency increases with temperature as T^8.4

Correlation between supernova distribution and HII temperature supports the model

Offers a new explanation for the slow radial gradient of cosmic rays

Abstract

The dependence of the cosmic ray intensity on Galactocentric distance is known to be much less rapid than that to be 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's radial distribution and correlate it with the measured HII electron temperature ({\em T}). We examined two models of cosmic ray injection and acceleration and in both of them the injection efficiency increases with increasing ambient temperature {\em T}. The increase is expected to vary as a high power of {\em 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 $T^n$ we find $n\approx 8.4$. There is thus, yet another possible explanation of the radial gradient problem.