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

Abstract

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 γ ∼ 0.1 E ̇ . The pulsar spin-down luminosity, E ̇ , also determines the absolute maximum energy of individual photons: E γ , max ≈ 0.9 E ̇ 36 0.65 PeV . This fundamental constraint dominates over the condition set by synchrotron energy losses of electrons for young PWNe with typical magnetic field of ≈100 μ G with E ̇ ≲ 10 37 erg s − 1 . We discuss the implications of E γ ,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.