摘要
Over the last two decades,we have seen two revolutions in y-ray astronomy-at GeV and TeV energies.The detection of thousands of GeV and hundreds of TeV y-ray emitters representing more than ten galactic and extragalactic source populations indicates that cosmic ray(CR)factories are distributed throughout the Universe in a wide diversity of forms and scales(see,e.g.,ref.[1]and references therein).These days,we are witnessing a new revolution,this time in the PeV band.Recently,the LHAASO Collaboration[2]reported detection of a dozen ultra-high-energy(UHE;E≥0.1 PeV)y-ray sources,thus opening a new window in the electromagnetic spectrum.UHE y-rays,produced in interactions of charged particles(protons and electrons)with an order of magnitude higher energies,serve as direct messengers of CR PeV atrons.
