Supernova remnants in super bubbles as cosmic ray accelerators

TL;DR

This paper investigates the potential of supernova remnants within star clusters to accelerate cosmic rays to PeV energies, combining analytic and numerical methods to explore conditions for such acceleration.

Contribution

It extends previous models by analyzing SNRs in star cluster winds, highlighting the specific conditions needed for PeV cosmic ray acceleration.

Findings

Acceleration to PeV energies is possible but only under extreme conditions.

Most SNRs in star clusters do not reach PeV energies without special circumstances.

Self-generated magnetic turbulence influences maximum particle energies.

Abstract

Supernova remnants (SNRs) are often considered as the main sites of acceleration of cosmic rays in our Galaxy, possibly up to the knee. However, their ability to accelerate particles to reach PeV energies is questionable and lacks observational evidence. Theoretical predictions suggest that only a small subclass of very young SNRs evolving in dense environments could potentially satisfy the necessary conditions to accelerate particles to PeV energies. Most such theoretical investigations are carried out either in the standard interstellar medium or in the wind of the progenitor. Since most core collapse supernovae occur in star clusters, it is important to extend such investigation to SNRs taking place in a star cluster. In this work we focus on a SNR shock propagating in the collective wind of a compact star cluster, and we study the acceleration process as a function time, with special emphasis on the maximum energy of accelerated particles. Using both analytic and numerical approaches we investigate the spectrum of accelerated particles and maximum achievable energy in the case of pre-existing turbulence in the collective wind and self-generated magnetic perturbations. We find that similar to isolated SNRs, acceleration to PeV energies is plausible only for extreme conditions achievable only in a small subset of SNRs.