Evidence for extreme PeV cosmic ray acceleration from LHAASO

Abstract

It is well-known that cosmic rays (CRs) as charged high-energy particles need to be accelerated by electric fields induced by magnetic fields in varieties of astrophysical environments. Due to the highly conductive nature of astrophysical plasmas, large scale electric fields can only exist in some peculiar circumstances, such as perpendicular shocks, jets, and spinning misaligned magnetic dipoles. Observational evidence for CR acceleration by such large scale electric fields is obscure since its characteristics haven't been explored extensively. The first results from the Large High Altitude Air Shower Observatory (LHAASO) imply that although PeV particles may be accelerated in varieties of ultra-high-energy (UHE) γ-ray sources, they likely escape from their sites of acceleration effectively giving rise to UHE γ-ray spectra softer than that of the diffuse γ-rays from the galactic plane. The all-particle energy spectrum and mean of the logarithmic mass of PeV CRs also suggest a new hard CR spectral component near the spectral knee that can be associated with CRs escaping from their site of acceleration and producing the diffuse γ-ray emission. The drift of high-energy particles along large scale electric fields may account for the acceleration of PeV CRs explaining these LHAASO results.