Grammage of cosmic rays around Galactic supernova remnants

TL;DR

This paper investigates how cosmic ray grammage near supernova remnants is significantly affected by non-linear wave generation, challenging traditional interpretations of secondary-to-primary ratios for cosmic ray propagation.

Contribution

It demonstrates that near-source non-linear effects cause cosmic rays to spend more time close to sources, impacting grammage estimates and questioning the use of B/C ratios for Galactic propagation analysis.

Findings

CRs up to 10 TeV spend much longer near sources than expected.

Near-source grammage is comparable to total Galactic grammage.

Source trapping contributes an irreducible grammage, but with uncertainties.

Abstract

The residence time of cosmic rays (CRs) in the Galaxy is usually inferred from the measurement of the ratio of secondary-to-primary nuclei, such as the boron (B)/carbon (C) ratio, which provides an estimate of the amount of matter traversed by CRs during their propagation, the so called CR grammage. However, after being released by their parent sources, for instance supernova remnants (SNRs), CRs must cross the disc of the Galaxy, before entering the much lower density halo, in which they are believed to spend most of the time before eventually escaping the Galaxy. In the near-source region, the CR propagation is shown to be dominated by the non-linear self-generation of waves. Here we show that due to this effect, the time that CRs with energies up to $\sim$ 10 TeV spend within a distance $L_{c}\sim 100$ pc from the sources is much larger than naive estimates would suggest. The corresponding grammage is close to current estimates of the total grammage traversed throughout the whole Galaxy. Moreover, there is an irreducible grammage that CRs traverse while trapped downstream of the shock that accelerated them, though this contribution is rather uncertain. We conclude that at energies $\lesssim 1$ TeV, the observed grammage is heavily affected by the near-source non-linear trapping of CRs, and at energies $\gtrsim 1$ TeV it is affected by the source grammage. As a result, the measurement of the B/C ratio should be used very cautiously as an indicator of the propagation of CRs on large Galactic scales.