Detection of the distant quasar OP 313 with the first Large-Sized Telescope of CTAO

Context . Flat spectrum radio quasars (FSRQs) often remain undetected at very-high-energy (VHE, E > 100 GeV) γ rays due to their typically soft spectra and large distance. In December 2023, the Large-Sized Telescope prototype (LST-1) detected for the first time VHE γ -ray emission from the FSRQ OP 313 during an extraordinarily bright flare, becoming the furthest blazar ever observed at VHE with z = 0.997. Aims . We aim to characterize the γ -ray emission of OP 313 during this flare, comparing it with its average emission state in order to understand the processes leading to this detection. Its remarkable distance also enables studies on the extragalactic background light (EBL), with the goal of evaluating the attenuation of VHE γ -ray photons. Thanks to an intensive multiwavelength campaign, we also study in detail the broadband emission of the source. Methods . We characterize the γ -ray emission during the flare in December 2023 and the low emission state observed in January 2024 thanks to the LST-1 and MAGIC data and quasi-simultaneous Fermi -LAT observations. This dataset also enables us to evaluate the EBL attenuation by systematically exploring the EBL intensity over the γ -ray spectrum.. Finally, we study the multiwavelength emission and interpret the broadband spectral energy distribution (SED) within blazar radiative models including the thermal contributions from the accretion disk, dusty torus and broad line region. Results . We measured significant VHE γ -ray emission during the December 2023 flare at a level of 0.3 Crab units above 100 GeV. We also characterize the flare brightness in the high-energy (HE, E > 100 MeV) γ -ray band, which was found to be a factor of 50 above the average emission seen by Fermi -LAT. The HE and VHE observations allow us to set constrains on the EBL density, resulting in an upper limit at 0.6 μm of λI λ < 8.74 nW m 2 sr −1 with LST-1 data, and λ I λ < 14.7 nW m 2 sr −1 with MAGIC data, including systematic uncertainties. Finally, thanks to the extensive multiwavelength campaign organized, we are able to construct and model the broadband SED of OP 313 within the framework of a two-zone leptonic model where the γ -ray emission is produced via inverseCompton scattering of the broad line region, accretion disk, and dusty torus photon fields. However, the dominant external photon field remains unknown, as several combinations are able to successfully explain the γ -ray emission observed.

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