We investigate broad absorption line ( BAL ) disappearance and emergence using a 470 BAL-quasar sample over \leq 0.10–5.25 rest-frame years with at least three spectroscopic epochs for each quasar from the Sloan Digital Sky Survey .
We identify 14 disappearing BALs over \leq 1.73–4.62 rest-frame years and 18 emerging BALs over \leq 1.46–3.66 rest-frame years associated with the C iv \lambda \lambda 1548,1550 and/or Si iv \lambda \lambda 1393,1402 doublets , and report on their variability behavior .
BAL quasars in our dataset exhibit disappearing/emerging C iv BALs at a rate of 2.3 ^ { +0.9 } _ { -0.7 } and 3.0 ^ { +1.0 } _ { -0.8 } per cent , respectively , and the frequency for BAL to non-BAL quasar transitions is 1.7 ^ { +0.8 } _ { -0.6 } per cent .
We detect four re-emerging BALs over \leq 3.88 rest-frame years on average and three re-disappearing BALs over \leq 4.15 rest-frame years on average , the first reported cases of these types .
We infer BAL lifetimes along the line of sight to be nominally \la 100–1000 yr using disappearing C iv BALs in our sample .
Interpretations of ( re- ) emerging and ( re- ) disappearing BALs reveal evidence that collectively supports both transverse-motion and ionization-change scenarios to explain BAL variations .
We constrain a nominal C iv /Si iv BAL outflow location of \la 100 pc from the central source and a radial size of \ga 1 \times 10 ^ { -7 } pc ( 0.02 au ) using the ionization-change scenario , and constrain a nominal outflow location of \la 0.5 pc and a transverse size of \sim 0.01 pc using the transverse-motion scenario .
Our findings are consistent with previous work , and provide evidence in support of BALs tracing compact flow geometries with small filling factors .