Quest for a Coherent Definition of Weak-line Quasars and Its Physical Implications

Weak-line quasars (WLQs) represent a subset of type 1 quasars distinguished by remarkably weak high-ionization broad emission lines, yet exhibiting normal optical/UV continua. This study establishes a physically motivated definition of WLQs using 371,091 quasars from the Sloan Digital Sky Survey Data Release 16 catalog. By analyzing outliers in three key relations, the L 1350 Å –C iv blueshift relation, the Baldwin effect, and the log L 2500 Å − α ox relation, we identify two critical thresholds in C iv equivalent width (EW): 8.9 ± 0.2 Å and 19.3 ± 0.3 Å. Quasars with EW(C iv ) < 8.9 ± 0.2 Å are defined as WLQs, exhibiting enhanced C iv blueshifts, significant deviations from the Baldwin effect, and a high fraction of X-ray weak objects (nearly half of this population). Quasars with EW(C iv ) > 19.3 ± 0.3 Å show normal quasar properties, while objects with intermediate C iv EW (8.9–19.3 Å) are defined as “bridge quasars,” showing transitional behaviors. Systematic analysis of emission-line attenuation of WLQs reveals a clear positive correlation between the attenuation factor and ionization energy, with high-ionization lines (HILs; e.g., He ii , C iv ) suppressed by factors of ∼3 σ –4 σ compared to low-ionization lines (LILs; e.g., Mg ii , O i ). This ionization-stratified attenuation supports the shielding gas model, where geometrically thick inner accretion disk obscures high-energy photons, suppressing HIL emission, while the LILs remain less affected. Based on this accretion-disk geometry, we argue that WLQs and normal quasars defined in our framework generally correspond to the slim disk and standard thin disk regimes, respectively, while the bridge quasars represent a transitional phase between the two states. This work establishes a unified observational criterion for WLQs and discusses their physical implications, highlighting the role of accretion-driven shielding gas in shaping their unique spectral features.