THE EVOLUTION OF QUASAR C IV AND Si IV BROAD ABSORPTION LINES OVER MULTI-YEAR TIMESCALES

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Abstract
We investigate the variability of C IV lambda 1549 broad absorption line (BAL) troughs over rest-frame timescales of up to approximate to 7 yr in 14 quasars at redshifts z greater than or similar to 2.1. For nine sources at sufficiently high redshift, we also compare the C IV and Si IV lambda 1400 absorption variation. We compare shorter and longer term variability using spectra from up to four different epochs per source and find complex patterns of variation in the sample overall. The scatter in the change of absorption equivalent width (EW), Delta EW, increases with the time between observations. BALs do not, in general, strengthen or weaken monotonically, and variation observed over shorter (less than or similar to months) timescales is not predictive of multi-year variation. We find no evidence for asymmetry in the distribution of Delta EW that would indicate that BALs form and decay on different timescales, and we constrain the typical BAL lifetime to be greater than or similar to 30 yr. The BAL absorption for one source, LBQS 0022+0150, has weakened and may now be classified as a mini-BAL. Another source, 1235+1453, shows evidence of variable, blue continuum emission that is relatively unabsorbed by the BAL outflow. C IV and Si IV BAL shape changes are related in at least some sources. Given their high velocities, BAL outflows apparently traverse large spatial regions and may interact with parsec-scale structures such as an obscuring torus. Assuming BAL outflows are launched from a rotating accretion disk, notable azimuthal symmetry is required in the outflow to explain the relatively small changes observed in velocity structure over times up to 7 yr.
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