Ultra-high Energy Inverse Compton Emission from Galactic Electron Accelerators

Abstract

The High-Altitude Water Cherenkov Observatory (HAWC) and the Large High Altitude Air Shower Observatory (LHAASO) are dramatically improving our view of the gamma-ray sky above 100 TeV energies. It is generally held that emission from astrophysical sources at such high energies will preferentially trace the presence of PeV protons or nuclei, due to the unavoidable Klein–Nishina suppression of inverse Compton (IC) emission from electrons. However, if the spectrum of accelerated electrons hardens in the Klein-Nishina regime, significant IC emission >100 TeV is possible. Such equilibrium spectra are known to result in IC cooling dominated environments. On large scales, the necessary environmental conditions are shown to occur only at large Galactic radii and above/below the Galactic disk, locations unlikely to coincide with powerful accelerators. The situation is different in localised regions in the Galactic disk, notably in the spiral arms, and in particular in/close to regions of enhanced star formation activity. These are also the natural locations of powerful young pulsars, the most promising electron accelerators. This suggests, that future searches will detect many ultra-high energy sources, and that 100 TeV sources recently discovered by HAWC and LHAASO may be naturally understood as being produced by powerful pulsars.