A Minimal Model for Understanding Secondary Cosmic Rays

TL;DR

This paper presents a minimal phenomenological model explaining the spectral intensity of secondary cosmic rays, emphasizing matter interactions near sources and during propagation, with implications for gamma-ray observations and source discreteness.

Contribution

It introduces a minimal model that accounts for cosmic-ray interactions near sources and during propagation, providing insights into secondary particle spectra and observational implications.

Findings

Cosmic rays pass through significant matter near sources at GeV energies.

Grammage decreases with energy, becoming negligible beyond 100 GeV.

Most positrons and antiprotons are generated in the interstellar medium.

Abstract

We take a phenomenological approach in a minimal model to understand the spectral intensity of secondary cosmic-ray particles like positrons, antiprotons, Lithium, Beryllium and Boron. Our analysis shows that cosmic rays at $\sim$ GeV energies pass through a significant amount of matter in regions surrounding the sources. This grammage decreases with increasing cosmic-ray energy and becomes negligible beyond $\sim 100$ GeV. During the subsequent propagation in the interstellar medium cosmic rays of all energies up to $\sim 10^5$ GeV/$n$ pass through about 1-2 g cm$^{-2}$ of matter before leaking into the intergalactic medium. It is in the interstellar medium that the bulk of the positrons and antiprotons are generated. Also cosmic-ray nuclei like C, N, and O at all energies generate additional amounts of Li, Be and B nuclei with a spectrum similar to those of C, O etc. The implications of these findings of the minimal model to the observations of gamma rays and also the importance of spatial and temporal discreteness of cosmic-ray sources for modeling cosmic-ray propagation are briefly pointed out.