More variable quasars have stronger emission lines

TL;DR

This study finds that quasars with greater UV/optical variability, likely caused by accretion disc turbulence, also exhibit stronger emission lines, suggesting a causal link between disc turbulence and emission line strength.

Contribution

It demonstrates, for the first time, a robust positive correlation between UV/optical variability amplitude and emission line equivalent widths in quasars, indicating a causal connection.

Findings

Positive correlation between variability amplitude and emission line strengths.

Correlations remain after controlling for luminosity, black hole mass, Eddington ratio, and redshift.

Suggests disc turbulence influences emission line production mechanisms.

Abstract

The UV/optical variation, likely driven by accretion disc turbulence, is a defining characteristic of type 1 active galactic nuclei (AGNs) and quasars. In this work we investigate an interesting consequence of such turbulence using quasars in SDSS Stripe 82 for which the measurements of the UV/optical variability amplitude are available from $\sim$ 10 years long light curves. We discover positive correlations between UV/optical variability amplitude $\sigma_{rms}$ and equivalent widths of CIV, Mg II and [OIII]5007 emission lines. Such correlations remain statistically robust through partial correlation analyses, i.e., after controlling the effects of other variables including bolometric luminosity, central supermassive black hole mass, Eddington ratio and redshift. This, for the first time, indicates a causal link between disc turbulence and emission line production. We propose two potential underlying mechanisms both of which may be involved: 1) quasars with stronger disc turbulence have on average bluer/harder broadband SED, an expected effect of the disc thermal fluctuation model; 2) stronger disc turbulence could lead to launch of emission line regions with larger covering factors.