2026ApJ...998..311L

The radius-luminosity (R_BLR-L₅₁₀₀) relation is fundamental to active galactic nucleus (AGN) studies, enabling supermassive black hole (SMBH) mass estimates and AGN-based cosmology applications. However, its high-luminosity end remains poorly calibrated due to insufficient reliable reverberation mapping (RM) data. We present a four-year RM campaign of the luminous quasar E1821+643 using the Lijiang 2.4 m telescope, supplemented by archival multiwavelength data. E1821+643 is the most luminous AGN with an Hβ RM measurement to date. The measured time lag of $83.{2}_{-18.7}^{+17.5}$ days is a factor of 5.6 shorter than predicted by the canonical R_BLR-L₅₁₀₀ relation. By compiling the full Hβ RM sample, we find that such deviation defines a lower envelope (0.2R_BLR) of measured lags across the entire luminosity range, while the upper envelope lies near 2R_BLR, implying that the scatter for individual AGNs can reach 1 dex. Spectral decomposition reveals two distinct Hβ components: a core component with a lag of $267.{0}_{-17.6}^{+16.6}$ days closer to the R_BLR-L₅₁₀₀ relation, and a redshifted tail with a much shorter lag of -49.0${}_{-34.5}^{+50.5}$ days. The short-lag component not only accounts for the significantly shortened overall lag but also leads to an opposite interpretation of the intrinsic broad-line region (BLR) kinematics. These effects can introduce systematic uncertainties in black hole mass estimates by factors of up to tens. Our findings demonstrate that shortened lags in high-accretion-rate AGNs arise from multicomponent BLR structures, posing substantial challenges to single-epoch mass estimates and impacting SMBH demographics and cosmological applications.