Self-confinement of low-energy cosmic rays around supernova remnants

TL;DR

This study models how low-energy cosmic rays self-confine around supernova remnants, revealing significant suppression of diffusion and spectral flattening, which impacts cosmic-ray spectrum interpretation.

Contribution

It introduces an extended framework for cosmic-ray transport near supernova remnants, including energy loss effects for energies below 10 GeV.

Findings

Diffusion coefficient can be suppressed by up to 100 times for tens of thousands of years.

Cosmic-ray spectrum flattens below 1 GeV, resembling Voyager observations.

Non-negligible grammage accumulation affects cosmic-ray spectral fitting.

Abstract

Supernova remnants have long been considered as a promising candidate for sources of Galactic cosmic rays. However, modelling cosmic-ray transport around these sources is complicated by the fact that the overdensity of cosmic rays close to their acceleration site can lead to self-confinement, that is the generation of turbulence upon which these particles scatter. Such a highly non-linear problem can be addressed by numerically solving the coupled differential equations describing the evolution in space and time of the escaping particles and of the turbulent plasma waves. In this work, we focus essentially on the propagation of cosmic rays from supernova remnants in the warm ionized and warm neutral phases of the interstellar medium and propose an extended framework to take into account also the effect of energy loss relevant for cosmic rays of energy below 10 GeV. Interestingly, the diffusion coefficient of low-energy cosmic rays could be suppressed by up to 2 orders of magnitude for several tens of kiloyears after the escape from the shock. The cosmic-ray spectrum outside the supernova remnant flattens below 1 GeV at a sufficiently late time reminiscient of the spectral behaviour observed by Voyager. We also find the grammage accumulated around the source to be non-negligible, with important implications for precision fitting of the cosmic-ray spectra.