On the point source approximation of nearby cosmic-ray sources

TL;DR

This paper critically examines the validity of the point source approximation for nearby cosmic-ray sources, revealing its limitations at low energies and differences from energy-dependent escape models, especially for young SNRs.

Contribution

It provides a detailed analysis of the point source approximation's validity for nearby CR sources, highlighting its breakdown at low energies and differences from energy-dependent escape models.

Findings

Point source approximation breaks down at low energies for young nearby sources.

The approximation holds for the combined spectrum of multiple SNRs at Earth.

Results differ significantly from energy-dependent escape models favored by diffusive shock acceleration.

Abstract

In this paper, we check in detail the validity of the widely adopted point source approximation for nearby cosmic-ray (CR) sources. Under an energy independent escape model for CRs from the sources, we show that for young nearby sources, the point source approximation breaks down at lower energies and the CR spectrum depends on the size and the morphology of the source. When applied to the nearby supernova remnants (SNRs), we find that the approximation breaks down for some of the individual remnants like the Vela, but interestingly it still holds good for their combined total spectrum at the Earth. Moreover, we also find that the results obtained under this simple approximation are quite different from those calculated under an energy dependent escape model which is favored by diffusive shock acceleration models inside SNRs. Our study suggests that if SNRs are the main sources of CRs in our Galaxy, then the commonly adopted point source model (with an energy independent escape scenario) appears flawed for CR studies from the nearby SNRs.