Time variability of ultra-fast BAL outflows using SALT: C iv absorption depth based analysis

We probe the small-scale absorption line variability using absorption depth based analysis of a sample of 64 ultra-fast outflow (UFO) C iv broad absorption line (BAL) quasars monitored using the Southern African Large Telescope. We confirm the strong monotonic increase in the strength of variability with increasing outflow velocity. We identify regions inside the BAL trough for each source where the normalized flux difference between two epochs is >0.1 for a velocity width >= 500 km s(-1) (called 'variable regions'). We find that the total number of variable regions increases with the time interval probed and the number of BALs showing variable regions almost doubles from short (<2 yr) to long (>2 yr) time-scales. We study the distributions of variable region properties such as its velocity width, depth, and location. These regions typically occupy a few-tenths of the entire width of the BAL. Their widths are found to increase with increasing time-scales having typical widths of similar to 2000 km s(-1) for Delta t > 2 yr. However, their absolute velocity with respect to z(em) and their relative position within the BAL profile remain random irrespective of the time-scale probed. The equivalent width variations of the BALs are strongly dependent on the size and depth of the variable regions but are little dependent on their total number. Finally, we find that similar to 17 per cent of the UFO BALs show uncorrelated variability within the BAL trough.