Evolution of Quasar Stochastic Variability along Its Main Sequence

Abstract

We explore the evolution of the time variability (in the optical g -band and on timescales of weeks to years) of Sloan Digital Sky Survey Stripe 82 quasars along the quasar main sequence. A parent sample of 1004 quasars within 0.5 ≤  z  ≤ 0.89 was used for our statistical studies; we then made subsamples from our parent sample: a subsample of 246 quasars with similar luminosities, and a subsample of 399 quasars with similar (i.e., the ratio of the equivalent width of Fe ii within 4435–4685 Å to that of H β ). We find the variability amplitude decreases with luminosity ( L bol ). The anticorrelation between the variability amplitude and is weak but statistically significant. The characteristic timescale, τ , correlates mostly with quasar luminosity; its dependence on is statistically insignificant. After controlling luminosity and , the high- and low-FWHM samples have similar structure functions. These results support the framework that is governed by Eddington ratio and the FWHM of H β is mostly determined by orientation. We then provide new empirical relations between variability parameters and quasar properties (i.e., luminosity and ). Our new relations are consistent with the scenario that quasar variability is driven by thermal fluctuations in the accretion disk; τ seems to correspond to the thermal timescale. From our new relations, we find that the short-term variability is mostly sensitive to L bol . Based on this we propose that quasar short-term (a few months) variability might be a new type of “Standard Candle” and can be adopted to probe cosmology.