Correcting Black Hole Masses Biased by Size Inertia in Broad Line Regions

Active Galactic Nuclei (AGN) with broad emission line regions (BLRs) contain rapidly growing supermassive black holes. Their masses are mostly derived from BLR gas motions inferred from spectra and BLR sizes estimated from the AGN luminosity. The "breathing" BLR model predicts that spectral line width changes when luminosity varies, keeping mass estimates constant. Here, we present observations of an AGN sample at z<0.1 with two epochs separated by ~20 years. We find that AGN luminosity and mass estimates vary on average by a factor of 2 and show a long-term mean-reversion pattern, while the line widths remain constant on average. Variability without a breathing BLR inflates the scatter in size-luminosity relations and offers a natural explanation for why the relations depend on instantaneously measured Eddington ratios. Finally, more precise masses are obtained when the BLR size is estimated from the narrow-line luminosity L _[OIII] .