Microquasar jet-cocoon systems as PeVatrons

TL;DR

The paper investigates whether shear acceleration in large-scale microquasar jet–cocoon systems can reaccelerate preexisting Galactic cosmic rays to PeV energies, potentially forming the Galactic knee via a hard proton component with a peak near $\sim$PeV. Using a KMZ18-inspired framework and Monte Carlo simulations, it shows that, with representative jet–cocoon parameters, protons can reach $E_{\max}$ around a few PeV and that the escaping spectrum is hard below the peak, while the composition around the knee can be matched by reaccelerating low-energy CRs with AMS-02-informed abundances; diffusion-based propagation with DRAGON then reproduces Earth spectra for a plausible number of microquasars (3–10) with extended jets. The work further proposes a unified Galactic–extragalactic CR scenario, combining SNRs (Gal pop1), MQs (Gal pop2), clusters (ExGal), and radio galaxies (ExGal) to explain CRs from GeV to 100 EeV, including subankle protons and UHECRs via shear, Fe enhancements, and environment-dependent magnetic fields. This framework offers a testable link between Galactic knee physics and extragalactic UHECRs, with predictions for spectra, composition, and anisotropy that can be probed by upcoming CR experiments.

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.

Figures (8)

• Figure 1: A schematic plot of the reacceleration of Galactic low-energy GeV-TeV CRs in the large-scale jet-cocoon system of microquasars.
• Figure 2: The energy spectrum of escaping CRs predicted in the shear acceleration mechanism from Monte Carlo simulations. The parameters are listed in Table \ref{['tab:parameters-default']}. The above plot is multiplied with $\rm exp(-\mathcal{R}/\mathcal{R}_{\rm max})$, where $\mathcal{R}_{\rm max} = 4\times 10^7\rm~GeV$ is the maximum acceleration energy from the microquasar jets.
• Figure 3: Predicted CR intensity on Earth and the mean mass $\langle {\rm ln}(A) \rangle$ distribution from microquasars. The all-particle energy spectra are taken from LHAASO LHAASO:2024knt, HAWC HAWC:2017osk, NUCLEON 2019AdSpR..64.2546G, TIBET TIBETIII:2008qon, TA Matthews:2017rW, and Auger Verzi:2019AO, while the proton spectra are taken from LHAASO LHAASO:2025byy, KASCADE-Grande Arteaga-Velázquez:2017/0, and IceTop IceCube:2019hmk. The measurements of the mean mass $\langle {\rm ln}(A) \rangle$ distribution are taken from LHAASO LHAASO:2024knt and IceTop IceCube:2019hmk.
• Figure 4: An overall picture of the CR spectrum from Galactic to extragalactic components. Gal pop1: CRs from SNRs 2008ARAA..46...89RBlasi:2013rva (dotted curve); Gal pop2: CRs from MQs (dashed curve); ExGal pop1: CRs from extragalactic sources, e.g., CGs Murase:2008yt (dotted-dashed curve); ExGal pop2: UHECRs from RGs Kimura:2017ubz (long dashed curve). The data points are the same as those in Fig. \ref{['fig:knee-mq']}, with the AMS data taken from Refs. AMS:2018qxqAMS:2018cenAMS:2015tnnAMS:2021lxcAMS:2020cai, and the mean mass distribution measured by Auger is taken from Ref. Yushkov:2020nhr.
• Figure 5: The energy spectrum of CRs predicted in the shear acceleration mechanism from Monte Carlo simulations. Three different parameter sets are considered.