On the Potential of Bright, Young Pulsars to Power Ultrahigh Gamma-Ray Sources

The recent discovery of a new population of ultrahigh-energy gamma-ray sources with spectra extending beyond 100 TeV revealed the presence of Galactic PeVatrons—cosmic-ray factories accelerating particles to PeV energies. These sources, except for the one associated with the Crab Nebula, are not yet identified. With an extension of 1° or more, most of them contain several potential counterparts, including supernova remnants, young stellar clusters, and pulsar wind nebulae (PWNe), which can perform as PeVatrons and thus power the surrounding diffuse ultrahigh-energy gamma-ray structures. In the case of PWNe, gamma-rays are produced by electrons, accelerated at the pulsar wind termination shock, through the inverse Compton scattering of 2.7 K cosmic microwave background (CMB)radiation. The high conversion efficiency of pulsar rotational power to relativistic electrons, combined with the short cooling timescales, allow gamma-ray luminosities up to the level of ${L}_{\gamma }\sim 0.1\dot{E}$. The pulsar spin-down luminosity, $\dot{E}$, also determines the absolute maximum energy of individual photons: ${E}_{\gamma ,max}\approx 0.9{\dot{E}}_{36}^{0.65}\,\,{\rm{PeV}}$. This fundamental constraint dominates over the condition set by synchrotron energy losses of electrons for young PWNe with typical magnetic field of ≈100 μG with $\dot{E}\lesssim {10}^{37}\ \mathrm{erg}\,{{\rm{s}}}^{-1}$. We discuss the implications of Eγ,max by comparing it with the highest-energy photons reported by LHAASO from a dozen of ultrahigh-energy sources. Whenever a PWN origin of the emission is possible, we use the LHAASO measurements to set upper limits on the nebular magnetic field.