Limits on ultra-high energy photons with the Pierre Auger Observatory
2019 September 05
The Pierre Auger Observatory is the most sensitive air-shower detector for primary photons with energies above $\sim 0.2$ EeV. Using the Surface Detector array (SD), photons in the zenith angle range from 30$^\circ$ to 60$^\circ$ can be identified through the broad time-structure of the signals expected to be induced in the SD stations. An additional signature for photon-induced air showers is the steeper lateral distribution of secondary particles at the ground with respect to nucleonic showers. Stringent limits are set to the diffuse flux of ultra-high energy (UHE) photons above 10 EeV, using SD data collected between 2004 and mid-2018 with an exposure of $40\,000$ km$^2\,$sr$\,$yr. Below 1 EeV, unprecedented separation power between primary photons and hadrons can be achieved by combining observables from the low-energy enhancements of the Pierre Auger Observatory, namely three upwards-pointing fluorescence telescopes, which can directly measure the atmospheric depth of the shower maximum, and the 750 m SD array, which again accesses the steeper lateral distribution of photon-induced air showers. For the first time, limits on the integral photon flux below 1 EeV are presented using data collected by the Pierre Auger Observatory between mid-2010 and 2015. This extends the range of photon searches at the Pierre Auger Observatory to about three decades in energy.