Study of downward Terrestrial Gamma-ray Flashes with the surface detector of the Pierre Auger Observatory

ROBERTA COLALILLO; PIERRE AUGER COLLABORATION; M. GABRIELA NICORA

Conferencia; 38th International Cosmic Ray Conference (ICRC2023); 2023

The surface detector (SD) of the Pierre Auger Observatory, consisting of 1660 water-Cherenkovdetectors (WCDs), covers 3000 km2in the Argentinian pampa. Thanks to the high efficiency ofWCDs in detecting gamma rays, it represents a unique instrument for studying downward TerrestrialGamma-ray Flashes (TGFs) over a large area. Peculiar events, likely related to downward TGFs,were detected at the Auger Observatory. Their experimental signature and time evolution are verydifferent from those of a shower produced by an ultrahigh-energy cosmic ray. They happen incoincidence with low thunderclouds and lightning, and their large deposited energy at the groundis compatible with that of a standard downward TGF with the source a few kilometers above theground. A new trigger algorithm to increase the TGF-like event statistics was installed in thewhole array. The study of the performance of the new trigger system during the lightning seasonis ongoing and will provide a handle to develop improved algorithms to implement in the Augerupgraded electronic boards. The available data sample, even if small, can give important cluesabout the TGF production models, in particular, the shape of WCD signals. Moreover, the SDallows us to observe more than one point in the TGF beam, providing information on the emissionangle.