BOLT: Imaging Lightning and Terrestrial Gamma-ray Flashes at the Pierre Auger Observatory

The Pierre Auger Observatory has detected downward terrestrial gamma-ray flashes (TGFs) with its water-Cherenkov detectors. Understanding this high-energy radiation occurring during thunderstorms requires combining such measurements with observations of lightning processes in their earliest stages. To meet this challenge, the Broadband Observatory of Lightning and TGFs (BOLT) is currently under construction to image lightning propagation in three dimensions with high time resolution using radio interferometry, extending the unique multi-detector capabilities of the Pierre Auger Observatory. BOLT is based on eleven modified Auger Engineering Radio Array (AERA) stations operating in the 30–80 MHz bandwidth and deployed at strategic locations within the Auger array. While the AERA stations by themselves already provide the necessary spatial and timing resolution, a key modification for BOLT is the implementation of a long buffer readout. This capability enables the reconstruction of lightning development and the correlation of radio emissions with TGF-related signals observed by the Observatory’s water-Cherenkov detectors.This contribution presents recent hardware developments, including the long buffer readout, progress toward selective triggering and precision timing, and first field data analyzed using insights from previous AERA measurements, illustrating the growing capability of BOLT for combined lightning and TGF studies. Together with the existing detector systems of the Pierre Auger Observatory, BOLT establishes a powerful experimental framework for advancing our understanding of lightning physics and associated high-energy atmospheric phenomena.