Optical spectral characterization of OP 313. Constraining the contribution of thermal and non-thermal optical emission

The quasar OP 313 was discovered in December 2023 in very-high-energy $γ$ rays above 100 GeV, enabling for the first time a complete characterization of its emission. However, the lack of updated measurements of its accretion disk, broad line region and dusty torus hampers a detailed interpretation of the role of accretion in the observed $γ$-ray production. We intend to characterize, during high-activity states, the external photon fields contributing to the IR-to-UV emission$-$namely dusty torus, broad line region and accretion disk$-$and investigate potential variability and blurring effects on the broad emission lines. We present new spectroscopic observations of OP 313 with the NOT and TNG telescopes to characterize its optical spectrum and variability with respect to archival observations from SDSS. We measure the luminosity of different broad emission lines, characterizing the broad line region, accretion disk and dusty torus. We measure the Mg II emission line, with an average flux of $F_{\mathrm{Mg \ II}} = (0.85 \pm 0.11)\times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$. Its equivalent width and luminosity are consistent with a constant line with a variable non-thermal continuum. From the stable Mg II line we derive a constant luminosity of the thermal components, $\log(L_{\mathrm{BLR}} \ \mathrm{[erg \ s^{-1}]}) = 44.91 \pm 0.19$, $\log(L_{\mathrm{disk}} \ \mathrm{[erg \ s^{-1}]}) = 45.91 \pm 0.19$ and $\log(L_{\mathrm{torus}} \ \mathrm{[erg \ s^{-1}]}) = 44.70 \pm 0.16$, and estimated a BH mass of $\log(M_{BH}/M_{\odot})=8.36 \pm 0.18$, in line with with that derived from the C III] line. These characteristics and the indicator of the accretion rate from the disk/Eddington luminosity ratio $λ=L_{AD}/L_{Edd} = 0.23 \pm 0.10$, along with a high Compton dominance, favour a FSRQ-like nature, contrary to the argued changing-look nature.

💡 Research Summary

The paper presents a comprehensive optical spectroscopic study of the flat‑spectrum radio quasar OP 313, the first FSRQ detected at very‑high‑energy (VHE) γ‑rays (>100 GeV) in December 2023. The authors obtained seven new optical spectra during 2024 using the 2.5‑m Nordic Optical Telescope (NOT) with the ALFOSC instrument and the 3.58‑m Telescopio Nazionale Galileo (TNG) with the DOLORES spectrograph. These data were taken during a period of strong γ‑ray and optical flaring, and were compared with an archival low‑state spectrum from the Sloan Digital Sky Survey (SDSS) obtained in 2006.

Data reduction was performed with the PYPEIT pipeline, including bias subtraction, flat‑fielding, optimal extraction, and flux calibration using contemporaneous spectrophotometric standard stars. Absolute fluxes were cross‑checked against simultaneous g, r, i photometry. The spectral coverage spans 3200–10150 Å, allowing the authors to search for broad emission lines across the UV‑optical range.

Only two broad lines are reliably detected in all high‑state spectra: Mg II λ2798 and C III] λ1909. Other lines such as Hδ and Hγ are present but heavily blended with the bright non‑thermal synchrotron continuum, rendering them unusable for quantitative analysis. The authors model each line with Gaussian components superimposed on a linear continuum (chosen over a power‑law because the fitting window is narrow). For Mg II, a two‑Gaussian fit (broad + narrow) is used for the low‑state SDSS spectrum, while a single Gaussian suffices for the high‑state data where the broad component is largely buried under the continuum. C III] is fitted with a single Gaussian in all epochs.

The Mg II equivalent width (EW) shows dramatic variability: in the low‑state SDSS spectrum EW ≈ –33 Å, while during the 2024 flares the continuum brightens by a factor >10, reducing EW to <5 Å and in some cases making the line virtually undetectable. Despite this, the line centroid and full‑width at half‑maximum (FWHM ≈ 3000 km s⁻¹) remain stable, and the line flux varies by only ~40 %. This behavior indicates that the line emission itself is essentially constant, and the apparent changes are caused by dilution from the variable synchrotron continuum. A similar pattern is observed for C III].

Using the Mg II line luminosity and standard scaling relations, the authors derive the broad‑line region (BLR) luminosity L_BLR = 10^{44.91 ± 0.19} erg s⁻¹. From established BLR‑disk and BLR‑torus correlations they infer a thermal accretion‑disk luminosity L_disk = 10^{45.91 ± 0.19} erg s⁻¹ and a dusty‑torus infrared luminosity L_torus = 10^{44.70 ± 0.16} erg s⁻¹. The line widths combined with the line luminosities yield a black‑hole mass M_BH = 10^{8.36 ± 0.18} M_⊙, consistent with an independent estimate from the C III] line.

The ratio of disk luminosity to Eddington luminosity, λ = L_disk / L_Edd = 0.23 ± 0.10, places OP 313 in the regime of moderately efficient accretion typical of FSRQs. Prior multi‑wavelength studies have reported a high Compton dominance (the ratio of inverse‑Compton to synchrotron peak luminosities), suggesting that external Compton (EC) scattering off photons from the BLR, disk, and torus dominates the γ‑ray output. The present optical results reinforce this picture: the external photon fields appear stable, while the γ‑ray variability correlates with changes in the jet’s electron population rather than with variations in the thermal components.

In summary, the paper demonstrates that OP 313’s optical spectrum is dominated by a highly variable non‑thermal synchrotron continuum that can mask broad emission lines during flares, but the underlying thermal components (disk, BLR, torus) remain essentially unchanged. This constancy, together with the measured black‑hole mass and accretion rate, supports a classification of OP 313 as a classic FSRQ rather than a “changing‑look” quasar. The work provides a clear methodology for disentangling thermal and non‑thermal contributions in blazars, emphasizing the importance of multi‑epoch spectroscopy for interpreting high‑energy variability in VHE‑detected sources.