The relationship between the emission line, continuum luminosity, and Baldwin effect in blazars

I. The case of the Mg II λ2798 Å emission line

¹ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro #1, Tonantzintla, Puebla 72840, Mexico
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, Köln, Germany
⁴ Byurakan Astrophysical Observatory after V.A. Ambartsumian, Aragatsotn Province 378433, Armenia
⁵ Astrophysical Research Laboratory of Physics Institute, Yerevan State University, 1 Alek Manukyan St., Yerevan, Armenia
⁶ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, CDMX 04510, Mexico

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 May 2025
Accepted: 10 November 2025

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 performed a comprehensive re-evaluation of the Mg II-3000 Å correlation, explicitly accounting for dispersion introduced by active galactic nucleus (AGN) variability. After excluding > 3000 radio-loud sources, we employed a binning technique to mitigate variability effects, yielding a refined empirical relation. We also further examined the nonthermal 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 find 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. On the other hand, 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 naturally emerges from the line-continuum luminosity relationship, requiring no additional physical mechanism to explain its origin.