A Timescale-Resolved Analysis of the Breathing Effect in Quasar Broad Line Regions

TL;DR

This study investigates the breathing effect in quasar broad emission lines across a large dataset to evaluate its impact on single-epoch black hole mass estimates, revealing complex, timescale-dependent behaviors that vary among different lines.

Contribution

It provides the first large-scale analysis of the breathing effect beyond reverberation-mapped AGNs, assessing its variability and implications for SMBH mass estimation accuracy.

Findings

No significant breathing in Ha, Hb, MgII lines.

CIV line shows a significant anti-breathing trend at intermediate timescales.

Breathing signals are intermittent and depend on timescale and line type.

Abstract

The single-epoch virial method is a fundamental tool for estimating supermassive black hole (SMBH) masses in large samples of AGNs and has been extensively employed in studies of SMBH-galaxy co-evolution across cosmic time. However, since this method is calibrated using reverberation-mapped AGNs, its validity across the entire AGN population remains uncertain. We aim to examine the breathing effect-the variability of emission line widths with continuum luminosity-beyond reverberation-mapped AGNs, to assess the validity and estimate potential systematic uncertainties of single-epoch virial black hole mass estimates. We construct an unprecedentedly large multi-epoch spectroscopic dataset of quasars from SDSS DR16, focusing on four key broad emission lines (Ha, Hb, MgII, and CIV). We assess how breathing behavior evolves with the rest-frame time interval between observations. We detect no significant breathing signal in Ha, Hb, or MgII at any observed timescale. In contrast, CIV exhibits a statistically significant anti-breathing trend, most prominent at intermediate timescales. Notably, for Hb, which has shown breathing in previous reverberation-mapped samples, we recover the effect only in the small subset of quasars with clearly detected BLR lags and only during the epochs when such lags are measurable-suggesting that both the lag and breathing signals are intermittent, possibly due to a weak correlation between optical and ionizing continua. These results highlight the complex, variable, and timescale-dependent nature of line profile variability and underscore its implications for single-epoch black hole mass estimates.