The Broad-Line Region Size-Luminosity Relation in AGN Constrained by the Viewing Angle with the FRADO Model

Abstract

We investigate the H$β$ broad-line region (BLR) size-luminosity ($R_{\rm BLR}$-$L_{5100}$) relation of active galactic nuclei (AGN) using a Bayesian viewing angle inference framework based on the Failed Radiatively Accelerated Dusty Outflow (FRADO) model. First, we update the FRADO model by introducing a phase factor to account for the anisotropic visibility of the illuminated cloud surface. Second, we construct a Bayesian framework to constrain the viewing angle by matching the FRADO-based $R_{\rm BLR}$ to the measured H$β$ lags of 151 AGN from Wang & Woo (2024). We successfully reproduce the slope and intrinsic scatter of the observed $R_{\rm BLR}$-$L_{5100}$ relation, finding that the inferred viewing angles are broadly consistent with the estimates from dynamical modeling of velocity-resolved reverberation data, albeit with substantial scatter. As the observed H$β$ time lag and line velocity depend on the viewing angle, the black hole mass ($M_{\rm BH}$) calibrated from our work is typically larger than the reverberation-mapping estimates using an average virial factor. Our results provide a physically motivated interpretation of the $R_{\rm BLR}$-$L_{5100}$ relation and the systematic uncertainties in reverberation and single-epoch $M_{\rm BH}$ estimates.