On the variability of quasars: a link between Eddington ratio and optical variability?

TL;DR

This study investigates the relationship between black hole properties and optical variability in quasars, revealing that the Eddington ratio influences variability independently of black hole mass and luminosity.

Contribution

It demonstrates that quasar variability is primarily driven by the Eddington ratio, providing new insights into the physical mechanisms behind optical variability.

Findings

Anticorrelation between luminosity and variability confirmed.

Variability correlates with black hole mass.

Eddington ratio identified as a key driver of variability.

Abstract

Repeat scans by the Sloan Digital Sky Survey (SDSS) of a 278 square degree stripe along the Celestial equator have yielded an average of over 10 observations each for nearly 8,000 spectroscopically confirmed quasars. Over 2500 of these quasars are in the redshift range such that the CIV emission line is visible in the SDSS spectrum. Utilising the width of these CIV lines and the luminosity of the nearby continuum, we estimate black hole masses for these objects. In an effort to isolate the effects of black hole mass and luminosity on the photometric variability of our dataset, we create several subsamples by binning in these two physical parameters. By comparing the ensemble structure functions of the quasars in these bins, we are able to reproduce the well-known anticorrelation between luminosity and variability, now showing that this anticorrelation is independent of the black hole mass. In addition, we find a correlation between variability and the mass of the central black hole. By combining these two relations, we identify the Eddington ratio as a possible driver of quasar variability, most likely due to differences in accretion efficiency.