Black hole mass and variability in quasars

TL;DR

This study finds a significant positive correlation between black hole mass and variability amplitude in quasars, suggesting that more massive black holes exhibit larger flux variations, likely due to differences in accretion processes.

Contribution

It provides the first clear observational evidence linking black hole mass to quasar variability amplitude, using a sample of about 100 quasars with robust mass and variability measurements.

Findings

Black hole mass correlates with variability amplitude at 99% significance.

More massive black holes show approximately 0.2 mag larger flux variations.

The correlation is most evident for time lags of a few months to 2 years.

Abstract

We report on a study that finds a positive correlation between black hole mass and variability amplitude in quasars. Roughly 100 quasars at z<0.75 were selected by matching objects from the QUEST1 Variability Survey with broad-lined objects from the Sloan Digital Sky Survey. Black hole masses were estimated with the virial method using the broad Hbeta line, and variability was characterized from the QUEST1 light curves. The correlation between black hole mass and variability amplitude is significant at the 99% level or better and does not appear to be caused by obvious selection effects inherent to flux-limited samples. It is most evident for rest frame time lags of the order a few months up to the QUEST1 maximum temporal resolution of about 2 years. The correlation between black hole mass and variability amplitude means that the more massive black holes have larger percentage flux variations. Over 2-3 orders of magnitude in black hole mass, the amplitude increases by approximately 0.2 mag. A likely explanation for the correlation is that the more massive black holes are starving and produce larger flux variations because they do not have a steady inflow of gaseous fuel. Assuming that the variability arises from changes in the accretion rate Li & Cao [8] show that flux variations similar to those observed are expected as a consequence of the more massive black holes having cooler accretion disks.