Optical spectral characterization of OP 313

Context. The flat spectrum radio quasar (FSRQ) OP 313 was discovered in December 2023 in very-high-energy γ rays above 100 GeV, enabling for the first time a complete broadband characterization of its emission. However, the lack of updated measurements of its accretion disc, broad line region, and dusty torus hampers a detailed interpretation of the role of accretion in the observed γ-ray production. Aims. We intended to characterize, during high-activity states, the brightness of the external photon fields contributing to the infrared-to-ultraviolet emission–namely the dusty torus, broad line region, and accretion disc–as well as to investigate potential variability and blurring effects on the optical broad emission lines. We also aimed to constrain the particle population responsible for the continuum non-thermal synchrotron emission. Methods. We present new spectroscopic observations of OP 313 with the NOT and TNG telescopes in order to characterize its optical spectrum and variability with respect to archival observations from SDSS performed during a low emission state. We measured the luminosity of different broad emission lines, evaluating possible changes in the broad line region luminosity. These measurements also enabled an updated characterization of the broad line region, accretion disc, and dusty torus properties. Results. We measured the Mg <ns0:sc>II</ns0:sc> emission line, detectable in six of the seven spectra, with an average flux of FMg <ns0:sc>II</ns0:sc> = (0.85 ± 0.11) × 10−14 erg cm−2 s−1. Its equivalent width and luminosity are consistent with a constant emission line with a variable non-thermal continuum that buries other lines (Hδ and Hγ). From the stable Mg <ns0:sc>II</ns0:sc> line we derived a constant luminosity of the thermal components, finding log(LBLR [erg s−1]) = 44.91 ± 0.19, log(Ldisc [erg s−1]) = 45.91 ± 0.19, and log(Ltorus [erg s−1]) = 44.70 ± 0.16, and an estimated black hole mass of log(MBH/M⊙) = 8.36 ± 0.18. These estimates are in line with with those derived from the C <ns0:sc>III</ns0:sc>] emission line, detected in five of the seven spectra. The characteristics derived from the emission line and the indicator of the accretion rate from the disc/Eddington luminosity ratio λ = LAD/LEdd = 0.23 ± 0.10, along with a high Compton dominance observed by previous studies, favour a FSRQ-like nature of OP 313, contrary to the argued changing-look nature of the source. Under this scenario, the γ-ray variability is fully attributed to changes in the particle population, rather than variations in the actual thermal structure.