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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>83</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>18.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>17.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>267</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>17.6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>16.6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> days closer to the R BLR – L 5100 relation, and a redshifted tail with a much shorter lag of −49.0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>34.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>50.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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.