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

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