2014MNRAS.437.1976W

We present a two-epoch Sloan Digital Sky Survey and Gemini/GMOS+William Herschel Telescope/ISIS variability study of 50 broad absorption line (BAL) quasars of redshift range 1.9 < z < 4.2, containing 38 Siiv and 59 Civ BALs and spanning rest-frame time intervals of ~10 months to 3.7 years. We find that 35/50 quasars exhibit one or more variable BALs, with 58 percent of Siiv and 46 percent of Civ BALs showing variability across the entire sample. On average, Siiv BALs show larger fractional change in BAL pseudo-equivalent width than Civ BALs, as referenced to an unabsorbed continuum+emission line spectrum constructed using non-negative matrix factorization. No correlation is found between BAL variability and quasar luminosity, suggesting that ionizing continuum changes do not play a significant role in BAL variability (assuming the gas is in photoionization equilibrium with the ionizing continuum). A subset of 14 quasars have one variable BAL from each of Siiv and Civ with significant overlap in velocity space and for which variations are in the same sense (strengthening or weakening) and which appear to be correlated (98 percent confidence). We find examples of both appearing and disappearing BALs in weaker/shallower lines with disappearance rates of 2.3 percent for Civ and 5.3 percent for Siiv, suggesting average lifetimes of 142 and 43 years, respectively. We identify five objects in which the BAL is coincident with the broad emission line, but appears to cover only the continuum source. Assuming a clumpy inhomogeneous absorber model and a typical size for the continuum source, we infer a maximum cloud radius of 10¹³ to 10¹⁴ cm, assuming Eddington limited accretion.