Pushing the VHE Frontier: LST-1’s Inaugural Detection of the Distant Quasar OP 313

In December 2023, the Flat Spectrum Radio Quasar OP 313 experienced an extraordinary very-high-energy (VHE, E > 100 GeV) gamma-ray flare, reaching an integral flux of 0.3 Crab Units above 100 GeV. This event marked the first VHE detection of OP 313 by the first Large-Sized Telescope~(LST-1) at the Northern site of the Cherenkov Telescope Array Observatory, delivering its inaugural scientific result and establishing OP 313 as the most distant blazar detected in this energy regime (z = 0.997). Coordinated observations with LST-1, the MAGIC telescopes, and \textit{Fermi}-LAT enabled us to capture the detailed spectral and temporal evolution of the flare, which we compared with a low-emission state observed in January 2024. A complementary multi-wavelength campaign — from radio through X-rays — enabled us to construct and model the broad-band spectral energy distribution within a two-zone leptonic framework. In this scenario, synchrotron and external Compton processes, involving seed photons from the dusty torus and broad-line region, account for the observed emission, although several combinations of photon fields remain plausible. Furthermore, the broad energy coverage provided by our observations allowed us to probe the attenuation of VHE gamma-rays by the Extragalactic Background Light, yielding competitive upper limits on its intensity. This work not only demonstrates the breakthrough capabilities of LST-1 in VHE gamma-ray astronomy but also provides fresh insights into the complex radiative mechanisms of high-redshift blazars, paving the way for future studies of extreme extragalactic sources.