Questing a Coherent Definition of Weak-line Quasars and its Physical Implications

Weak-line quasars (WLQs) are a subset of type 1 quasars with remarkably weak high-ionization broad emission lines but normal optical/UV continua. Using 371,091 quasars from SDSS DR16, we define WLQs by analyzing outliers in three relations: the L1350-CIV blueshift, the Baldwin effect, and the logL2500-alpha_ox. We find two CIV EW thresholds: $8.9\pm0.2${\AA} and $19.3\pm0.3${\AA}. WLQs (EW(CIV)<$8.9\pm0.2${\AA}) have enhanced CIV blueshifts, deviate from the Baldwin effect, and include many X-ray weak objects (nearly half). Normal quasars (EW(CIV)>$19.3\pm0.3${\AA}) show typical properties, while bridge quasars (intermediate EW) are transitional. WLQs show a positive correlation between line attenuation and ionization energy: high-ionization lines (e.g., HeII, CIV) are suppressed by ~3-4{\sigma} compared to low-ionization lines (e.g., MgII, OI). This supports the shielding gas model, where a thick inner accretion disk obscures high-energy photons, suppressing high-ionization lines, while low-ionization lines are less affected. We suggest that WLQs and normal quasars correspond to slim and thin disk regimes, respectively, with bridge quasars as a transitional phase. This work provides a unified criterion for WLQs and highlights the role of accretion-driven shielding gas in their spectral features.