High-energy photons from the Crab Nebula: The Crab Nebula contains a pulsar that excites the surrounding gas to emit high-energy radiation. The combination of the pulsar's youth and nearby location makes the nebula the brightest gamma-ray source in the sky. The LHAASO Collaboration report observations of this source at energies of tera– to peta–electron volts, extending the spectrum of this prototypical object. They combine these data with observations at lower energies to model the physics of the emission process. The multiwave-length data can be explained by a combination of synchrotron radiation and inverse Compton scattering. Science , abg5137, this issue p. 425

Supernova remnants (SNRs) have been considered to be the dominant contributors to Galactic cosmic rays. However, the relation between high-energy particles trapped in SNRs and cosmic rays observed at the Earth remains obscure. In this paper, we fit the spectral energy distributions of 35 SNRs with a simple one-zone emission model and analyze correlations of model parameters to uncover the evolution of high-energy particle distribution in SNRs. We find that (1) the particle distribution in general can be described by a broken power-law function with a high-energy cutoff for all SNRs; (2) the low-energy spectrum becomes harder and the break energy decreases with aging of SNRs, (3) for most middle-age SNRs, the energy loss timescale of electrons at the high-energy cutoff is approximately equal to the age of the corresponding remnant implying quenching of very-high-energy electron acceleration; for young SNRs, this energy loss timescale is shorter than the age of SNRs implying continuous electron acceleration at the cutoff energy; and for a few old age SNRs, the energy loss timescale is longer than the corresponding age, which may suggest escaping of higher energy particles from SNRs. Finally, we comment on the implications of these results on the SNR origin of Galactic cosmic rays.

We investigate the physical properties of BL Lac objects (BL Lacs) jets by modelling the quasi-simultaneous spectral energy distributions (SEDs) of 22 Fermi BL Lacs in the frame of a simple one-zone synchrotron self-Compton (SSC) model. We obtained the best-fitting model parameters and their uncertainties for each BL Lac through the χ²-minimization procedure and discussed their implications on the physical processes. The modelling results show that the one-zone SSC model can successfully fit the SEDs of high-synchrotron-peaked BL Lacs (HBLs) and intermediate-synchrotron-peaked BL Lacs (IBLs), but fails to explain the SEDs of low-synchrotron-peaked BL Lacs. The statistical analysis results for model parameters are summarized as follows. (1) No correlation is found between magnetic field (B) and the broken energy of relativistic electrons distribution ( $\gamma ^{\prime }_{\rm b}$ ) for HBLs and IBLs, but there are inverse correlations between $\gamma ^{\prime }_{\rm b}$ and the radius of emitting blob ( $R^{\prime }_{\rm b}$ ) as well as the electrons number $K^{\prime }_{\rm e}$ for HBLs and IBLs. It is therefore concluded that the variation of $\gamma ^{\prime }_{\rm b}$ is mainly caused by that of $R^{\prime }_{\rm b}$ rather than B for HBLs and IBLs. (2) The Poynting flux in jets cannot account for the observed radiations since the power in Poynting flux is smaller than the radiative power, and the cold protons could be the primary energy carrier in the jets.

We report the detection of GeV γ-ray emission from the very-high-energy γ-ray source VER J2227+608 associated with the “tail” region of supernova remnant (SNR) G106.3+2.7. The GeV γ-ray emission is extended and spatially coincident with molecular clouds traced by CO emission. The broadband GeV to TeV emission of VER J2227+608 can be well fitted by a single power-law function with an index of 1.90 ± 0.04, without obvious indication of spectral cutoff toward high energies. The pure leptonic model for the γ-ray emission can be marginally ruled out by the X-ray and TeV data. In the hadronic model, the low energy content of CRs and the hard γ-ray spectrum, in combination with the center bright source structure, suggest that VER J2227+608 may be powered by the Pulsar wind nebula instead of shocks of the SNR. And the cutoff energy of the proton distribution needs to be higher than ∼400 TeV, which makes it an attractive PeVatron candidate. Future observations by the upcoming Large High Altitude Air Shower Observatory and the Cherenkov Telescope Array in the north could distinguish these models and constrain the maximum energy of cosmic rays in SNRs.

We report the discovery of an ultrahigh-energy (UHE) gamma-ray source, LHAASO J2108+5157, by analyzing the LHAASO-KM2A data of 308.33 live days. A significant excess of gamma ray–induced showers is observed in both energy bands of 25−100 and >100 TeV with 9.5σ and 8.5σ, respectively. This source is not significantly favored as an extended source with an angular extension smaller than the point-spread function of KM2A. The measured energy spectrum from 20 to 200 TeV can be approximately described by a power-law function with an index of −2.83 ± 0.18_stat. A harder spectrum is demanded at lower energies considering the flux upper limit set by Fermi-LAT observations. The position of the gamma-ray emission is correlated with a giant molecular cloud, which favors a hadronic origin. No obvious counterparts have been found, and deeper multiwavelength observations will help to cast new light on this intriguing UHE source.