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

TL;DR

OP 313, a high-energy FSRQ at $z\approx0.997$, is used to quantify thermal external photon fields (BLR, disc, torus) and the non-thermal optical continuum during a major $\gamma$-ray flare. New NOT and TNG spectroscopy, complemented by SDSS data, yields consistent Mg II and C III] line measurements and constrains the thermal luminosities, BLR and torus radii, a BH mass of $\log(M_{BH}/M_\odot)\approx 8.36$, and an Eddington ratio $\lambda\approx0.23$. The Mg II line remains effectively constant while the non-thermal continuum varies by factors up to ~10, implying line dilution rather than intrinsic BLR changes and arguing against a changing-look nature. The steep optical continuum ($\alpha\approx1.3$–$1.6$) implies a cooled electron distribution ($p\approx3.6$–$4.1$) and, together with a Compton dominance $>1$, supports an FSRQ with external photon fields shaping the $\gamma$-ray emission.

Abstract

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.

Figures (13)

• Figure 1: Optical spectra of OP 313 observed with NOT and TNG telescopes. Left: Blue spectra. Right: Red spectra. The NOT spectrum from May 1 and June 18 are represented in blue and magenta, respectively. The TNG spectra from May 28 and July 3 are represented in orange and green, respectively. The TNG spectra represented here correspond to the co-added spectra of both exposures of each night. The SDSS spectrum taken on March 25, 2006, is also shown in black. The positions of the brightest emission lines reported in vandenberk2001 with a relative flux to the $Ly\alpha$ line $100 \times F/F_{Ly\alpha} \geq 1$ and within the spectral range of the observations presented in this work are highlighted with vertical dashed grey lines and indicated in the figure.
• Figure 2: Modelling of the Mg II line profile for the spectrum taken by the SDSS with two Gaussian components plus a linear function to model the synchrotron continuum. The bottom panel shows the residuals of the fit.
• Figure 3: EW measured for each spectrum obtained for OP 313 with respect to the relation between the EW of a constant Mg II emission line and the continuum flux evaluated at 5300 Å. The dashed lines represent the 2$\sigma$ confidence level of the derived relation.
• Figure 4: Constant fit to the measured flux of the Mg II and C III] emission lines over time for the different spectra analysed here. Top: Mg II line. Bottom: C III] line. The black points correspond to the observational measurements. The black dashed line is the best fit constant value obtained and the blue and green contours represents the 2$\sigma$ confidence interval of the fit in each case.
• Figure 5: Historical optical $V$-band light curve of OP 313. Different markers represent data from the different databases used, as shown in the legend. Blue vertical lines highlight the dates of the spectroscopic observations. The top panel shows the complete light curve while the bottom left and right panels show the zoomed-in light curves around the dates of the SDSS and NOT/TNG spectra, respectively.