The Relationship Between Emission Line and Continuum Luminosity and the Baldwin Effect in Blazars. I. The Case of the Mg II λ2798 Å Emission Line

TL;DR

The paper addresses how Mg II λ2798 Å line luminosity scales with the 3000 Å continuum in vast samples of radio-quiet quasars and FSRQ blazars, and whether the Baldwin Effect arises as a natural consequence of this relation. By accounting for AGN variability with binning and using a robust RQ control sample alongside a thoroughly vetted FSRQ set, the authors reveal statistically significant differences in the line–continuum slopes and BE between RQ and blazar populations, implying jet contributions or intrinsic disk-spectral differences in FSRQs. They introduce and apply the Non-Thermal Dominance (NTD) diagnostic to separate disk- and jet-dominated continua, finding substantial fractions of both samples with NTD < 1 and showing that BE can be explained as a mathematical consequence of the line–continuum relation, without invoking additional physics. The work highlights jet-driven BLR ionization in blazars and suggests a range of physical mechanisms in radio-quiet systems, while providing a framework for extending the analysis to other lines (e.g., Hβ, C IV) in future studies with potential implications for AGN unification and BLR physics.

Abstract

Aims. This study investigates the relationship between the Mg II λ2798 Å emission line and the 3000 Å continuum luminosity, as well as the Baldwin Effect, in a sample of 40,685 radio-quiet (RQ) quasars and 441 Flat Spectrum Radio Quasars (FSRQs). Methods. We perform a comprehensive re-evaluation of the Mg II-3000 Å correlation, explicitly accounting for dispersion introduced by AGN variability. After excluding >3000 radio-loud sources, we employ a binning technique to mitigate variability effects, yielding a refined empirical relation. We also further examine the Non-Thermal Dominance (NTD) parameter, to investigate the dominant source of the continuum. Results. Our analysis reveals statistically significant differences in the slopes of the line-continuum luminosity relation between RQ quasars and FSRQs, with a parallel discrepancy in the Baldwin Effect. These findings imply either (1) intrinsic differences in the accretion disk spectra of RQ AGNs and FSRQs or (2) jet-induced continuum emission in FSRQs contributing to Broad Line Region (BLR) ionization. We also found that a substantial fraction of both RQ quasars (43.8\%) and blazars (55.5\%) exhibit NTD < 1. For blazars, this suggests that the accretion disk alone cannot fully explain BLR ionization; while we interpret NTD < 1 in radio-quiet quasars as a signature of several physical mechanisms: anomalies in the BLR structure (such as outflow or inflows), time lags between continuum and line variations, and the suppression of the UV continuum by a strong corona that diverts accretion power. Finally, we demonstrate that the Baldwin Effect emerges naturally from the line-continuum luminosity relationship, requiring no additional physical mechanism to explain its origin

Figures (14)

• Figure 1: Example of the fitting procedure for the spectra of the FSRQ sample. The source is 5BZQ J1520+0732 (SDSS J152045.54+073230.5). Top panel: The observed spectra (blue), the final fit (black), and the power-law continuum (red). Middle panel: Spectral components fitted (excluding the continuum), including the broad Gaussian component (green), the narrow Gaussian component (orange), and the fitted Fe II emission (blue). Bottom panel: Residuals from the fit.
• Figure 2: Top: The median of the emission line components (blue), with red vertical lines indicating the integration limits. Bottom: Comparison of the emission line fluxes obtained by integrating the spectra (after subtracting the continuum and Fe II emission) versus integrating the fitted Gaussians. The blue line represents the $y = x$ equality.
• Figure 3: Comparison between the measurements (for the common sources) performed in this work and in S11, for the 3000 Å continuum luminosity (left panel), the Mg II $\lambda 2798$ Å emission line luminosity (middle panel), and the Mg II $\lambda 2798$ Å equivalent width (right panel). The dashed line represents $y = x$.
• Figure 4: The relationship between the 3000 Å continuum luminosity and the Mg II $\lambda$2798 Å emission line luminosity for the RQ control sample is shown, with the unbinned data in the left panel and the binned data in the right panel. The red solid line represents the fitted relation for each data set. In the right panel, the fitted line corresponds to the relationship presented in Eq. \ref{['eqlumirelRQ']}.
• Figure 5: Relationship between the 3000 Å continuum luminosity and the equivalent width of the Mg II $\lambda$2798 Å emission line for the RQ control sample, with the unbinned data shown in the left panel and the binned data in the right panel. The solid red line in both panels represents the fitted relation obtained for each dataset.