Strong long-term variability in active galactic nuclei affects virial black hole mass measurements

Active galactic nuclei (AGN) with broad emission line regions (BLRs) contain rapidly growing supermassive black holes. Their masses are typically derived from BLR gas motions inferred from spectra and BLR sizes estimated from the AGN luminosity. The breathing BLR model predicts that the spectral line width should change as luminosity varies, keeping mass estimates constant. Here, we present observations of an AGN sample at redshift <0.1 with two epochs separated by 20 years. On average, we find that luminosity and mass estimates vary by a factor of 2, showing a long-term mean-reversion pattern, while the line widths remain constant. A constant line width suggests that full BLRs do not change size during continuum or line flux variability, naturally explaining why scaling relations depend on instantaneously measured Eddington ratios. As a result, we find that single-epoch masses with the best repeatability after 20 years are obtained when sizes are estimated from the narrow [OIII] λ5007Å emission line luminosity.