Towards Searching for UHE Photons from Galactic PeVatrons: Estimating Fluxes and Constraining Spectral Parameters

The recent discoveries of PeV photons from galactic sources by gamma-ray observatories were unexpected and exciting. Some source spectra reveal no visible cutoff, posing the question how far the photon emission extends into the ultra-high-energy (UHE, here beyond $10\,\text{PeV}$) regime. We evaluate if and under which conditions giant air shower observatories like the Pierre Auger Observatory can contribute to test the UHE luminosity of the detected PeV $\gamma$-sources. Based on our previous analysis, we include the latest observations of V4641 Sgr (by HAWC) and of LHAASO J1819-2541 (by LHAASO). Comparing extrapolated photon fluxes to existing directional upper limits from the Pierre Auger Observatory in the energy range 200$-$3000~PeV, the parameter range when extrapolating the flux from V4641 Sgr North can be constrained already now. We conclude that the constantly improving sensitivity of air shower arrays will deliver relevant information on the UHE luminosity of PeV $\gamma$-sources, particularly if the energy threshold can be lowered to reduce the gap between gamma-ray and air shower observatories.