Discovery of the Very High Energy emission from the distant FSRQ OP 313 with the Large-Sized Telescope prototype.

In December 2023 the Large-Sized Telescope prototype (LST-1) of the Cherenkov Telescope Array Observatory (CTAO) observed the distant (z=0.997) Flat Spectrum Radio Quasar (FSRQ) OP 313 following an alert of increased activity by the Fermi-LAT Space Telescope. Thanks to its very low energy threshold and large effective area LST-1 detected very-high-energy (VHE, E<100 GeV) emission from OP 313. OP 313 is the tenth FSRQ ever detected in the VHE regime and the furthest blazar ever detected so far at these energies. This result represents the first scientific discovery of the LST-1, showing the exceptional capabilities of CTAO, whose LST northern array is currently under construction in La Palma (Canary Islands, Spain).