Theodore Zhang | The Origin of Cosmic Rays: Microquasar Jet-Cocoon Systems as Natural PeVatrons

Friday 11 Jul 2025, 10:30 → 12:00 Europe/Berlin

SR 05 (Villa)

 The origin of Galactic cosmic rays (CRs), particularly around the knee region (∼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.