Peta-electron volt gamma-ray emission from the Crab Nebula

Abstract

The Crab pulsar and the surrounding nebula powered by the pulsar's rotational energy through the formation and termination of a relativistic electron-positron wind is a bright source of gamma-rays carrying crucial information about this complex conglomerate. We report the detection of \(\gamma\)-rays with a spectrum showing gradual steepening over three energy decades, from \(5\times 10^{-4}\) to \(1.1\) petaelectronvolt (PeV). The ultra-high-energy photons exhibit the presence of a PeV electron accelerator (a pevatron) with an acceleration rate exceeding 15% of the absolute theoretical limit. Assuming that unpulsed \(\gamma\)-rays are produced at the termination of the pulsar's wind, we constrain the pevatron's size, between \(0.025\) and \(0.1\) pc, and the magnetic field \(\approx 110 \mu\)G. The production rate of PeV electrons, \(2.5 \times 10^{36}\) erg \(\rm s^{-1}\), constitutes 0.5% of the pulsar's spin-down luminosity, although we do not exclude a non-negligible contribution of PeV protons to the production of the highest energy \(\gamma\)-rays.