arXiv subjects

Zhaodong Shi

Publications and source records attributed to Zhaodong Shi.

6 recordsLinked to original sources

arXiv2026

Superdiffusion of cosmic rays in the vicinity of their accelerators and the resulting $γ$-ray emission

We study the distribution of cosmic rays (CRs) in the vicinity of their accelerators, assuming that the transport of CRs in the interstellar medium surrounding the accelerators is described by the superdiffusion, beyond the normal diffusion. We find that the superdiffusivity, which is characterized by the superdiffusion parameter $α$, impacts significantly the distribution of CRs. For impulsive injection, the CR distribution behaves a constant radial profile, except with a power-law tail for $α< 2$ or a Gaussian tail for $α=2$ at large distance, $r$, from the accelerators. For stationary injection, the radial profile of CR protons tends to being proportional to $r^{α- 3}$, while that of CR electrons can deviate from the $r^{α- 3}$ profile, due to their severe energy losses. We also compute the $γ$-ray emission, produced by the interactions of CRs with ambient gas and radiation fields, within 100 pc regions around the accelerators. We find that by investigating the $γ$-ray morphology, we can distinguish the superdiffusion from the normal diffusion with present and next-generation imaging air Cherenkov telescopes.

astro-ph.HE↗

arXiv2026

Microquasar Cygnus X-3 as the PeVatron powering the Cygnus Bubble

The recent discovery by the LHAASO collaboration of a variable ultra-high-energy (UHE; $E_γ\ge$ 100 TeV) $γ$-ray source associated with the microquasar Cygnus X-3, with a spectrum extending to several PeV, provides compelling evidence for a hadronic super-PeVatron operating within the binary system. Inside the binary, the accelerated protons lose only a small fraction of their energy; upon escaping into the interstellar medium, they propagate diffusively to form a vast gamma-ray ``halo" structure extended to hundreds of parsecs. We argue that this halo has already been detected and corresponds to the Cygnus Bubble, an extended UHE $γ$-ray source reported by the LHAASO collaboration -- which possesses an angular extension of $\approx 6^{\circ}$ and an energy spectrum reaching 1 PeV. While the Cygnus Bubble is generally attributed to the star-forming region Cygnus X (specifically the Cygnus OB2 association at 1.4 kpc), we demonstrate that an association with Cygnus X-3 is physically more natural at energies above 400 TeV. This is supported by the cosmic-ray radial distribution, derived from the $γ$-ray and gas distributions, which points to continuous injection from a point-like source. The energetic requirements of the central accelerator are reasonably affordable and feasible. This reassignment identifies the Cygnus Bubble as a member of the recently discovered population of microquasar UHE $γ$-ray halos.

astro-ph.HE↗

arXiv2026

On the hadronic origin of the very high energy $γ$-ray emission surrounding the young massive stellar cluster Westerlund 1

The Westerlund 1 (Wd 1) is the most massive known young star cluster in the Galaxy, and an extended $γ$-ray source HESS J1646-458 surrounding it has been detected up to 80 TeV in the very high energy, implying that cosmic rays (CRs) are accelerated effectively in the region. However, the dominant radiation process contributing to the $γ$-ray emission is not well constrained. In the present work, we develop a model of CR acceleration at the termination shock in the superbubble inflated by the interaction of the cluster wind from the Wd 1 with the surrounding interstellar medium. We then calculate the flux and radial profile of $γ$ rays produced by the inelastic collisions of the hadronic CRs with the ambient gas. Our results with reasonable parameters can explain well the spectrum and radial profile of the $γ$-ray emission of HESS J1646-458, and consequently the $γ$-ray emission of HESS J1646-458 is likely to be of hadronic origin.

astro-ph.HE↗

arXiv2024

Nuclear de-excitation line emissions from giant molecular clouds

Understanding how cosmic rays (CRs) propagate within the giant molecular clouds (GMCs) is critical for studying the dynamics and chemical processes inside the clouds. The flux of low-energy CRs inside the dense cores of GMCs strongly affects the heating and ionization of the gases and further influences the star-forming process. We analytically calculated the CR distribution inside GMCs assuming different diffusion coefficients, and estimated the corresponding nuclear de-excitation line emission and the ionization rate resulting from the interaction between the penetrating CRs and gases. We find that future MeV observations can be used as a unique probe to measure the low-energy CR density in situ and test different CR propagation scenario inside GMCs.

astro-ph.HE↗

arXiv2024

Low-Energy Cosmic Rays and Associated MeV Gamma-Ray Emissions in the Protoplanetary System

Low-energy cosmic rays (LECRs) play a crucial role in the formation of planetary systems, and detecting and reconstructing the properties of early LECRs is essential for understanding the mechanisms of planetary system formation. Given that LECRs interact with the surrounding medium to produce nuclear de-excitation line emissions, which are gamma-ray emissions with energy mainly within 0.1--10 MeV and are unaffected by stellar wind modulation, these emissions can accurately reflect the properties of LECRs. This study introduces an innovative method for using gamma-ray emissions to infer LECR properties. We employed the Parker transport equation to simulate the propagation and spectral evolution of LECRs in a protoplanetary disk and calculated the characteristic gamma-ray emissions resulting from interactions between LECRs and disk material. These gamma-ray emissions encapsulate the spectral information of LECRs, providing a powerful tool to reconstruct the cosmic ray environment at that time. This method, supported by further theoretical developments and observations, will fundamentally enhance our understanding of the impact of CRs on the origin and evolution of planetary systems and address significant scientific questions regarding the cosmic ray environment at the origin of life.

astro-ph.HE↗

arXiv2022

Proton and Helium Heating by Cascading Turbulence in a Low-beta Plasma

How ions are energized and heated is a fundamental problem in the study of energy dissipation in magnetized plasmas. In particular, the heating of heavy ions (including ${}^{4}\mathrm{He}^{2+}$, ${}^{3}\mathrm{He}^{2+}$ and others) has been a constant concern for understanding the microphysics of impulsive solar flares. In this article, via two-dimensional hybrid-kinetic Particle-in-Cell simulations, we study the heating of Helium ions (${}^{4}\mathrm{He}^{2+}$) by turbulence driven by cascading waves launched at large scales from the left-handed polarized Helium ion cyclotron wave branch of a multi-ion plasma composed of electrons, protons, and Helium ions. We find significant parallel (to the background magnetic field) heating for both Helium ions and protons due to the formation of beams and plateaus in their velocity distribution functions along the background magnetic field. The heating of Helium ions in the direction perpendicular to the magnetic field starts with a lower rate than that in the parallel direction, but overtakes the parallel heating after a few hundreds of the proton gyro-periods due to cyclotron resonances with mainly obliquely propagating waves induced by the cascade of injected Helium ion cyclotron waves at large scales. There is however little evidence for proton heating in the perpendicular direction due to the absence of left-handed polarized cyclotron waves near the proton cyclotron frequency. Our results are useful for understanding the preferential heating of ${}^{3}\mathrm{He}$ and other heavy ions in the ${}^{3}\mathrm{He}$-rich solar energetic particle events, in which Helium ions play a crucial role as a species of background ions regulating the kinetic plasma behavior.

astro-ph.SR↗