The Most Luminous H <i>β</i> Reverberation Mapping of E1821+643 Indicates the Lower Boundary of the Radius–Luminosity Relation

Abstract The radius–luminosity ( R BLR –L 5100 ) 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 H β 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 – L 5100 relation. By compiling the full H β RM sample, we find that such deviation defines a lower envelope (0.2 R BLR ) of measured lags across the entire luminosity range, while the upper envelope lies near 2 R BLR , implying that the scatter for individual AGNs can reach 1 dex. Spectral decomposition reveals two distinct H β components: a core component with a lag of 267 . 0 − 17.6 + 16.6 days closer to the R BLR – L 5100 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.