Huan Yu’s research while affiliated with Kunming Medical University and other places

With the advancement of detector technology, significant progress has been made in understanding Pulsar Wind Nebulae (PWNe) through multi-wavelength observations, particularly in the X-ray and TeV γ -ray bands. While young PWNe have been extensively studied, PWNe with ages around 10 kyr remain relatively underexplored. In this study, we investigate the radiative properties of four selected PWNe associated with the γ -ray sources HESS J1420-607, HESS J1418-609, HESS J1427-608, and HESS J1303-631 using a time-dependent electron population model. High-energy electrons and positrons are injected into the nebula, producing multi-wavelength non-thermal emission through synchrotron radiation and inverse Compton scattering. Based on comparisons with previous studies, we assume that four sources have not yet been compressed by the reverse shock, with estimated ages around 7–8 kyr. The injected particles follow a broken power-law distribution, with spectral indices consistent with previous studies. We find that the four nebulae are currently particle-dominated systems with magnetic field strengths ranging from a few μ G to 10 μ G, in agreement with theoretical expectations for PWNe at similar evolutionary stages. Our results support the scenario that all four sources originate with PWNe, as their multi-wavelength nonthermal emission can be reproduced with reasonable parameters.

HESS J1420−607 is a γ -ray emitting source associated with the pulsar wind nebula (PWN) powered by the energetic pulsar PSR J1420−6048. Based on 14 yr of data obtained with the Fermi Large Area Telescope, we re-analyzed its GeV γ -ray radiative properties, resulting in detailed spectra obtained within the band 10–200 GeV. Moreover, we use a one-zone time-dependent model for the multiband nonthermal emission from pulsar wind nebulae to investigate the radiative properties of the nebula associated with HESS J1420−607. Assuming that the electrons/positrons are injected into the PWN with a broken power law spectrum with indexes of ∼1.6 and ∼2.7, as well as a break Lorentz factor of ∼5 × 10 ⁶ , the results indicate that the multi-wavelength spectral energy distribution is consistent with the detected fluxes in both X-rays and γ -rays. The results support that the γ -rays detected can be produced via inverse Compton scattering of the energetic electrons/positrons within the PWN.

Ultra-high-energy (UHE, E>0.1PeV) γ-ray sources emitting detected γ-rays with energies above 100 TeV in the Galaxy are potential candidates for the origin of the cosmic rays with energies up to 1 PeV. However, it is usually in debate whether the γ-rays are produced by protons/nuclei or electrons/positrons. Recently, photons with energies up to ∼0.27 PeV from LHAASO J2018＋ 3651 have been detected with the Large High Altitude Air Shower Observatory (LHAASO). We investigate the origin of the γ-rays associated with the UHE source, and the detected fluxes can be reproduced via inverse Compton scattering of the electrons/positrons in the pulsar wind nebula (PWN) G75.2＋ 0.1. The electrons/positrons which have a spectrum of a power-law with cut-off energy powered due to spin-down of the pulsar are continuously injected into the nebula. Our results support that the UHE source originates from the PWN, and a detailed γ-ray spectrum in the range 10 TeV – 1 PeV from future observations can give more insight the cutoff pattern in the energy distribution of the particles injected into the nebula.

In addition to accelerating electrons and protons, non-relativistic quasi-parallel shocks are expected to possess the ability to accelerate heavy ions. The shocks in supernova remnants are generally supposed to be accelerators of Galactic cosmic rays, which consist of many species of particles. We investigate the diffusive shock acceleration of electrons, protons and helium ions in a non-relativistic quasi-parallel shock through a 1D particle-in-cell simulation with a helium-to-proton number density ratio of 0.1, which is relevant for Galactic cosmic rays. The simulation indicates that waves can be excited by the flow of energetic protons and helium ions upstream of a non-relativistic quasi-parallel shock with a sonic Mach number of 14 and an Alfvén Mach number of 19.5 in the shock rest frame, and that the charged particles are scattered by the self-generated waves and accelerated gradually. Moreover, the spectra of the charged particles downstream of the shock are thermal with a non-thermal tail, and the acceleration is efficient, with about $7{{\ \rm per\ cent}}$ and $5.4{{\ \rm per\ cent}}$ of the bulk kinetic energy transferred into the non-thermal protons and helium ions, respectively, in the near downstream region by the end of the simulation.

In addition to electrons and protons, nonrelativistic quasiparallel shocks are expected to possess the ability to accelerate heavy ions. The shocks in supernova remnants are generally supposed to be accelerators of the Galactic cosmic rays, which consist of many species of particles. We investigate diffusive shock acceleration (DSA) of electrons, protons and helium ions in a nonrelativistic quasiparallel shock through 1D particle-in-cell (PIC) simulation with a helium-to-proton number density ratio of 0.1, which is relevant for the Galactic cosmic rays. The simulation indicates that waves can be excited by the flow of the energetic protons and helium ions upstream of the nonrelativistic quasiparallel shock with a sonic Mach number of 18 and an alfv\'{e}n Mach number of 16.5 in the shock rest frame, and the charged particles are scattered by the self-generated waves and accelerated gradually. Moreover, the spectra of the charged particles downstream of the shock are thermal plus a nonthermal tail, and the acceleration is efficient with about $7\%$ and $5.4\%$ of the bulk kinetic energy transferred into the nonthermal protons and helium ions in the near downstream region at the end of the simulation, respectively.

The Crab nebula is a prominent pulsar wind nebula detected in multiband observations ranging from radio to very high-energy γ -rays. Recently, γ -rays with energies above 1 PeV have been detected by the Large High Altitude Air Shower Observatory, and the energy of the most energetic particles in the nebula can be constrained. In this paper, we investigate the broadest spectral energy distribution of the Crab nebula and the energy distribution of the electrons emitting the multiwavelength nonthermal emission based on a one-zone time-dependent model. The nebula is powered by the pulsar, and high-energy electrons/positrons with a broken power-law spectrum are continually injected in the nebula as the pulsar spins down. Multiwavelength nonthermal emission is generated by the leptons through synchrotron radiation and inverse Compton scattering. Using appropriate parameters, the detected fluxes for the nebula can be well reproduced, especially for the γ -rays from 10 ² MeV to 1 PeV. The results show that the detected γ -rays can be produced by the leptons via the inverse Compton scattering, and the lower limit of the Lorentz factor of the most energetic leptons is ∼ 8.5 × 10 ⁹ . It can be concluded that there exist electrons/positrons with energies higher than 4.3 PeV in the Crab nebula.

The Crab nebula is a prominent pulsar wind nebula (PWN) detected in multiband observations ranging from radio to very high-energy (VHE) $\gamma$-rays. Recently, $\gamma$-rays with energies above $1 \mathrm{PeV}$ had been detected by the Large High Altitude Air Shower Observatory (LHAASO), and the energy of the most energetic particles in the nebula can be constrained. In this paper, we investigate the broadest spectral energy distribution of the Crab nebula and the energy distribution of the electrons emitting the multiwavelength nonthermal emission based on a one-zone time-dependent model. The nebula is powered by the pulsar, and high-energy electrons/positrons with a broken power-law spectrum are continually injected in the nebula as the pulsar spins down. Multiwavelength nonthermal emission is generated by the leptons through synchrotron radiation and inverse Compton scattering. Using appropriate parameters, the detected fluxes for the nebula can be well reproduced, especially for the $\gamma$-rays from $10^2\,\mathrm{MeV}$ to $1\,\mathrm{PeV}$. The results show that the detected $\gamma$-rays can be produced by the leptons via the inverse Compton scattering, and the lower limit of the Lorentz factor of the most energetic leptons is $\sim 8.5\times10^{9}$. It can be concluded that there are electrons/positrons with energies higher than 4.3\,PeV in the Crab nebula.

The supernova remnant (SNR) G106.3 + 2.7 has a comet-shaped morphology in radio with a head and an elongated tail, and a pulsar PSR J2229 + 6114 is located in the northern boundary of the head. Recently, γ-rays with energies range from GeV to 500 TeV spatially coincident with the tail region have been detected. We investigate whether the γ-rays can be produced by the high-energy electrons/positrons in a nebula powered by PSR J2229 + 6114. Using a one-zone model for the multiband emission from the nebula, the detected radio, X-ray and γ-ray fluxes can be well reproduced with the assumption that a fraction (∼0.12) of the spin-down luminosity is used to power the leptons in the emission zone. The particle spectrum has a break at the lorentz factor γ>108 mainly due to the radiative loss of synchrotron radiation, and the magnetic field strength in the emission zone can be constrained to be ∼4μ G. The result shows that the detected γ-ray spectrum can be well produced via IC scattering of the high-energy electrons/positrons in the nebula which is in the tail region of the remnant.

eHWC J2019+368 is one of the sources emitting γ-rays with energies higher than 100 TeV based on the recent measurement with the High Altitude Water Cherenkov Observatory (HAWC), and the origin is still in debate. The pulsar PSR J2021+3651 is spatially coincident with the TeV source. We investigate theoretically whether the multiband non-thermal emission of eHWC J2019+368 can originate from the pulsar wind nebula (PWN) G75.2+0.1 powered by PSR J2021+3651. In the model, the spin-down power of the pulsar is transferred to high-energy particles and magnetic field in the nebula. As the particles with an energy distribution of either a broken power law or a power law continually injected into the nebula, the multiband non-thermal emission is produced via synchrotron radiation and inverse Compton scattering. The spectral energy distribution of the nebula from the model with the reasonable parameters is generally consistent with the detected radio, X-ray, and TeV γ-ray fluxes. Our study supports that the PWN has the ability to produce the TeV γ-rays of eHWC J2019+368, and the most energetic particles in the nebula have energies up to about 0.4 PeV.

eHWC J2019+368 is one of the sources emitting $\gamma$-rays with energies higher than 100 TeV based on the recent measurement with the High Altitude Water Cherenkov Observatory (HAWC), and the origin is still in debate. The pulsar PSR J2021+3651 is spatially coincident with the TeV source. We investigate theoretically whether the multiband nonthermal emission of eHWC J2019+368 can originate from the pulsar wind nebula (PWN) G75.2+0.1 powered by PSR J2021+3651. In the model, the spin-down power of the pulsar is transferred to high-energy particles and magnetic field in the nebula. As the particles with an energy distribution of either a broken power-law or a power-law continually injected into the nebula, the multiband nonthermal emission is produced via synchrotron radiation and inverse Compton scattering. The spectral energy distribution of the nebula from the model with the reasonable parameters is generally consistent with the detected radio, X-ray and TeV $\gamma$-ray fluxes. Our study supports that the PWN has the ability to produce the TeV $\gamma$-rays of eHWC J2019+368, and the most energetic particles in the nebula have energies up to about 0.4 PeV.