Search for primary photons at tens of PeV with the Pierre Auger Observatory

The observation of primary photons with energies around $10^{16}$ eV would be particularly interesting after the discovery of Galactic gamma-ray sources with spectra extending into the PeV range. Since photons are connected to the acceleration of charged particles, searches for photons enhance the multi-messenger understanding of cosmic-ray sources as well as of transient astrophysical phenomena, while offering wealthy connections to neutrino astronomy and dark matter. Additionally, diffuse photon fluxes are expected from cosmic-ray interactions with Galactic matter and background radiation fields. Previously, the energy domain between $1$ and $200$ PeV was only explored from the Northern Hemisphere. The Pierre Auger Observatory is the largest astroparticle experiment in operation and, thanks to its location, has a sizable exposure to the Southern sky, including the Galactic center region. In this contribution, we present the first search for photons from the Southern hemisphere between $50$ and $200$ PeV exploiting the Auger data acquired during $\sim4$ yr of operation. We describe the method to discriminate photons against the dominating hadronic background; it is based on the measurements of air showers taken with the low-energy extension of the Pierre Auger Observatory composed by $19$ water-Cherenkov detectors spanning $\sim2$ km$^2$ and an Underground Muon Detector. The search for a diffuse flux of photons is presented and its results are interpreted according to theoretical model predictions. This study extends the range of Auger photon searches to almost four decades in energy.

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