Can superbubbles accelerate ultra-high energy protons?

TL;DR

This paper evaluates the potential of superbubbles and associated shocks to accelerate protons to ultra-high energies, finding that certain internal shocks can reach PeV energies, but stochastic re-acceleration is less effective than previously thought.

Contribution

It provides a comprehensive analysis of different acceleration mechanisms within superbubbles, identifying the most promising scenarios for reaching ultra-high energy protons.

Findings

Superbubble external shocks can accelerate protons up to 100 TeV.

Internal shocks like supernova remnants can reach 1 PeV and beyond.

Clustered supernovae in magnetized winds can accelerate protons above 10 PeV.

Abstract

We critically assess limits on the maximum energy of protons accelerated within superbubbles around massive stellar clusters, considering a number of different scenarios. In particular, we derive under which circumstances acceleration of protons above peta-electronvolt (PeV) energies can be expected. While the external forward shock of the superbubble may account for acceleration of particles up to 100 TeV, internal primary shocks such as supernova remnants expanding in the low density medium or the collective wind termination shock which forms around a young compact cluster provide more favourable channels to accelerate protons up to 1 PeV, and possibly beyond. Under reasonable conditions, clustered supernovae launching powerful shocks into the magnetised wind of a young and compact massive star cluster are found to be the most promising systems to accelerate protons above 10 PeV. On the other hand, stochastic re-acceleration in the strongly turbulent plasma is found to be much less effective than claimed in previous works, with a maximum proton energy of at most a few hundred TeV.