Microquasar jet-cocoon systems as PeVatrons

TL;DR

This paper proposes that microquasar jet-cocoon systems can accelerate cosmic rays to PeV energies through shear reacceleration, potentially explaining the observed cosmic ray spectrum from Galactic to extragalactic sources.

Contribution

It introduces a novel shear reacceleration model in microquasar jet-cocoon systems as a source of PeV cosmic rays, linking Galactic and extragalactic origins.

Findings

Reacceleration in microquasars explains the proton spectrum shape.

Heavier nuclei contribute to the PeV energy range.

Radio galaxy processes may also accelerate ultrahigh-energy cosmic rays.

Abstract

The origin of Galactic cosmic rays (CRs), particularly around the knee region ($\sim$3 PeV), remains a major unsolved question. Recent observations by LHAAASO suggest that the knee is shaped mainly by protons, with a transition to heavier elements at higher energies. Microquasars -- compact jet-emitting sources -- have emerged as possible PeV CR accelerators, especially after detections of ultrahigh-energy gamma rays from these systems. We propose that the observed proton spectrum (hard below a few PeV, steep beyond) arises from the reacceleration of sub-TeV Galactic CRs via shear acceleration in large-scale microquasar jet-cocoon structures. Our model also naturally explains the observed spectrum of energies around a few tens of PeV by summing up heavier nuclei contributions. Additionally, similar reacceleration processes in radio galaxies can contribute to ultrahigh-energy CRs, bridging Galactic and extragalactic origins. Combined with low-energy CRs from supernova remnants and galaxy clusters around the second knee region, this scenario could provide a unified explanation for CRs across the entire energy spectrum.