Broad absorption line outflows are common in low Eddington ratio active galactic nuclei

Abstract

Context. Broad absorption line (BAL) quasars exhibit significant outflows, offering insights into active galactic nucleus (AGN) feedback. While typically associated with high Eddington ratios, BAL quasars also occur in low Eddington ratio regimes, which remain poorly understood. The aim of this study is to compare BAL properties and variability across these regimes. Aims. We investigate the occurrence rates, absorption characteristics, and variability of BAL quasars at low and high Eddington ratios. Methods. Using the SDSS DR16 quasar catalog, we selected a redshift-matched control sample to compare low and high Eddington ratio BAL quasar sources. We first examined the BAL fraction as a function of Eddington ratio. Key absorption parameters (equivalent width, absorption line width, velocity range, and depth) were analyzed, and a multi-epoch variability study was conducted using repeat spectra, followed by a comparison of parameter distributions between the two samples. Results. For the first time, we report an increase in the BAL fraction toward low Eddington ratios, in addition to the previously known trend of high BAL fraction at high Eddington ratios. While high Eddington sources show extreme absorption features, overall distributions are statistically similar except for maximum outflow velocity. No significant variability differences were observed. The correlation between outflow velocity, Eddington ratio, and luminosity supports the role of radiation pressure in driving quasar outflows. For low Eddington ratios, additional mechanisms, such as softer SEDs, larger outflow distances, and thickened accretion disks from radiatively inefficient processes, likely drive outflow formation.